Maker Pro
Maker Pro

Looking for opinions on LED dimmer circuit

J

Joel Moore

Jan 1, 1970
0
Let me start by giving credit where due. I found this attractive LED
dimmer design on the following site:

http://www.e-f-w.com/community/modules.php?name=News&file=article&sid=10

I'm not at all familiar with MOSFETs (and to be honest my general
electronics skills are very limited...which is why I'm here) so I was a
little uncertain about how to proceed in adapting that design for 24VDC.

So I did some reading on MOSFETs and after getting even more confused
about them I simply changed the layout of the LED array to work with 24V
(6 LEDs and a 120 ohm current-limiting resistor in series) and plugged it
in. No problem despite the fact that the MOSFET gate is now seeing
nearly 24V rather than 12V. Then I connected 11 more of these LED
strings in parallel and ran that. Still no problems.

Here's where I'm a little uncertain. According to the article one IRF510
MOSFET should be able to handle up to around 16 LEDs before you have to
get something more powerful. But in my case I have 72 LEDs running
through a single IRF510. The chip doesn't even feel warm. Should I not
be surprised that I can support so many more LEDs simply by moving to a
24V design or is the author of that article either mistaken or being
overly cautious about his MOSFET?

Is there anyplace I can read a simple description of how to design with
MOSFETs? The back of the MOSFET package has a long list of parameters,
none of which mean anything to me. What's important to consider when
working with these things?

And can anyone see a possible flaw in this design? It almost seems too
simple to me. I don't want to implement this if it's likely to fail a
few months down the road. Commercial LED dimmers can cost hundreds of
dollars yet here we have a circuit for less than $10 (including a 555
timer which I'm not using in my case) which seems pretty effective to me.
So what's wrong with it?

Here's an ASCII version of my circuit:

24VDC -----------
| |
| |
10K LED ARRAY
| |
| |
| Drain
| |
-- Gate IRF510
| |
| Source
| |
470K |
| |
| |
-----------
|
Sinking
PWM
Output
|
GND---------

Thanks for any input on this.

Joel Moore
 
M

Mac

Jan 1, 1970
0
Let me start by giving credit where due. I found this attractive LED
dimmer design on the following site:

http://www.e-f-w.com/community/modules.php?name=News&file=article&sid=10

I'm not at all familiar with MOSFETs (and to be honest my general
electronics skills are very limited...which is why I'm here) so I was a
little uncertain about how to proceed in adapting that design for 24VDC.

So I did some reading on MOSFETs and after getting even more confused
about them I simply changed the layout of the LED array to work with 24V
(6 LEDs and a 120 ohm current-limiting resistor in series) and plugged it
in. No problem despite the fact that the MOSFET gate is now seeing
nearly 24V rather than 12V. Then I connected 11 more of these LED
strings in parallel and ran that. Still no problems.

Here's where I'm a little uncertain. According to the article one IRF510
MOSFET should be able to handle up to around 16 LEDs before you have to
get something more powerful. But in my case I have 72 LEDs running
through a single IRF510. The chip doesn't even feel warm. Should I not
be surprised that I can support so many more LEDs simply by moving to a
24V design or is the author of that article either mistaken or being
overly cautious about his MOSFET?

Is there anyplace I can read a simple description of how to design with
MOSFETs? The back of the MOSFET package has a long list of parameters,
none of which mean anything to me. What's important to consider when
working with these things?

And can anyone see a possible flaw in this design? It almost seems too
simple to me. I don't want to implement this if it's likely to fail a
few months down the road. Commercial LED dimmers can cost hundreds of
dollars yet here we have a circuit for less than $10 (including a 555
timer which I'm not using in my case) which seems pretty effective to me.
So what's wrong with it?

Here's an ASCII version of my circuit:

24VDC -----------
| |
| |
10K LED ARRAY
| |
| |
| Drain
| |
-- Gate IRF510
| |
| Source
| |
470K |
| |
| |
-----------
|
Sinking
PWM
Output
|
GND---------

Thanks for any input on this.

Joel Moore

Joel,

One very important rule with MOSFET's such as the IRF510 which you appear
to be breaking is that the Voltage from gate to source must not exceed
some critical value. This is usually 20 Volts. If you are really applying
almost 24 Volts to the gate when the source is grounded, then you are
playing with fire. The datasheet or package will call this Vgs max or
something similar. You can fix this by tweaking resistor values so that
the gate voltage is lower. For example, change both resistors to 10 k,
then the maximum gate voltage will be around 12 V which should be fine for
you. How fast is your PWM going, by the way? If it is really fast, then
you might use smaller resistors than 10 k, but if it is slow, you could
use larger resistors.

But more importantly, I don't think your circuit makes any sense. The
MOSFET is nearly useless, if you really built the circuit as drawn above,
because when the PWM output is low, the MOSFET will be on, but the PWM
will be sinking all the current that flows through the MOSFET, and when
the PWM output is high, there will be no potential from the drain to the
source, and hence no current flowing. So you could just as easily remove
the MOSFET from your circuit above, and replace it with a short circuit
and there would be no change in behavior. The whole point of the MOSFET is
to handle large currents, or possibly to act as a variable resistor
capable of handling some power dissipation. In a PWM circuit, I would
expect the former.

This leads me to believe that you didn't really wire it that way. You
probably have the PWM output going to the gate of the MOSFET, and the
source of the MOSFET is directly grounded?

The LED array should also contain some kind of current limiting resistor
setup, as mentioned in the article. It might be helpful if you drew out
the details of the LED array, since they matter.

Also, is your PWM circuit getting hot at all? If you DID really wire the
circuit as shown, I imagine there is a lot of current going through it,
and I would expect it to get warm.

Anyway, if you use the MOSFET as a switch, you can put a lot of current
through it (two amps maybe for the IRF510? I don't know the exact number
off the top of my head, but it would be called Id max). As long as you
are not exceeding Id max, then you can keep connecting LED's until the
FET feels warm. It may be possible in some situations to exceed Id max
without the MOSFET getting warm, so you want to do some calculations to
verify that you are not doing that. (To do this, we would need to know
details of the LED array box.)

--Mac
 
F

Fred Bloggs

Jan 1, 1970
0
Joel said:
Let me start by giving credit where due. I found this attractive LED
dimmer design on the following site:

http://www.e-f-w.com/community/modules.php?name=News&file=article&sid=10

I'm not at all familiar with MOSFETs (and to be honest my general
electronics skills are very limited...which is why I'm here) so I was a
little uncertain about how to proceed in adapting that design for 24VDC.

So I did some reading on MOSFETs and after getting even more confused
about them I simply changed the layout of the LED array to work with 24V
(6 LEDs and a 120 ohm current-limiting resistor in series) and plugged it
in. No problem despite the fact that the MOSFET gate is now seeing
nearly 24V rather than 12V. Then I connected 11 more of these LED
strings in parallel and ran that. Still no problems.

Here's where I'm a little uncertain. According to the article one IRF510
MOSFET should be able to handle up to around 16 LEDs before you have to
get something more powerful. But in my case I have 72 LEDs running
through a single IRF510. The chip doesn't even feel warm. Should I not
be surprised that I can support so many more LEDs simply by moving to a
24V design or is the author of that article either mistaken or being
overly cautious about his MOSFET?

Is there anyplace I can read a simple description of how to design with
MOSFETs? The back of the MOSFET package has a long list of parameters,
none of which mean anything to me. What's important to consider when
working with these things?

And can anyone see a possible flaw in this design? It almost seems too
simple to me. I don't want to implement this if it's likely to fail a
few months down the road. Commercial LED dimmers can cost hundreds of
dollars yet here we have a circuit for less than $10 (including a 555
timer which I'm not using in my case) which seems pretty effective to me.
So what's wrong with it?

Here's an ASCII version of my circuit:

24VDC -----------
| |
| |
10K LED ARRAY
| |
| |
| Drain
| |
-- Gate IRF510
| |
| Source
| |
470K |
| |
| |
-----------
|
Sinking
PWM
Output
|
GND---------

Thanks for any input on this.

Joel Moore

The Radio Shack datasheet for the IRF510 is:
• Mfr. #IRF510 • Case Type: TO-220 • Channel Type: N • V(DS): 60 •
V(GS): +/-20 • I(D): 3.0 Amps • Power Dissipation: 20W

So you are on reprieve running V(GS) at 20V -at least it is operating in
the deep depths of its resistive region. Your white LEDs have a
forward drop of about 3.3V at the 20-30mA level so that six in series
gives 6x3.3=19.3V and this leaves 24-19.3=4.7 across the MOSFET +
current limiting resistor. With 120 ohm current limiting and an ohm or
two from the MOSFET, you end up with somewhat less than 40mA through the
LEDs. The 12x of these strings is something like 0.5A which is well
within the MOSFET rating. So your circuit is okay except for the V(GS)
drive and the slightly excess LED current. You should change it around
like so:

View in a fixed-width font such as Courier.
 
F

Fred Bloggs

Jan 1, 1970
0
Fred said:
The Radio Shack datasheet for the IRF510 is:
• Mfr. #IRF510 • Case Type: TO-220 • Channel Type: N • V(DS): 60 •
V(GS): +/-20 • I(D): 3.0 Amps • Power Dissipation: 20W

So you are on reprieve running V(GS) at 20V -at least it is operating in
the deep depths of its resistive region. Your white LEDs have a forward
drop of about 3.3V at the 20-30mA level so that six in series gives
6x3.3=19.3V and this leaves 24-19.3=4.7 across the MOSFET + current
limiting resistor. With 120 ohm current limiting and an ohm or two from
the MOSFET, you end up with somewhat less than 40mA through the LEDs.
The 12x of these strings is something like 0.5A which is well within the
MOSFET rating. So your circuit is okay except for the V(GS) drive and
the slightly excess LED current. You should change it around like so:

View in a fixed-width font such as Courier.

.
.
. 24VDC ----+-------------------------------------+
. | |
. | |
. | |
. 1.2k |
. | |
. +----+------+ |
. | | | |
. ___/ |0.47u | LED ARRAY
. 12V // \ --- | |
. zener --- --- |555 osc |
. | | +---+ Drain
. | | | |out |
. | | | |-------+--------Gate IRF510
. | | | | | |
. | | | | | Source
. | | +-+-+ | |
. | | | 10K |
. | | | | |
. | | | | |
. GND----+----+------+---------+---------------+
.
.
.
.
.
. LED ARRAY
.
.
. 220
. +--|>|-|>|-|>|-|>|-|>|-|>|---/\/\-+
. | |
. | 220 |
. +--|>|-|>|-|>|-|>|-|>|-|>|---/\/\-+
. | |
. | 220 |
. 24Vdc ---+--|>|-|>|-|>|-|>|-|>|-|>|---/\/\-+--> TO MOSFET
. | | DRAIN
. | 220 |
. +--|>|-|>|-|>|-|>|-|>|-|>|---/\/\-+
. | |
. | 220 |
. +--|>|-|>|-|>|-|>|-|>|-|>|---/\/\-+
. | |
. | 220 |
. +--|>|-|>|-|>|-|>|-|>|-|>|---/\/\-+
.
.
.

You said 72 LEDs so- add a 100u 35WVDC cap from 24VDC to GND right at
the LED array :
View in a fixed-width font such as Courier.

.. LED ARRAY
 
F

Fred Bloggs

Jan 1, 1970
0
Let me start by giving credit where due. I found this attractive LED
dimmer design on the following site:

http://www.e-f-w.com/community/modules.php?name=News&file=article&sid=10

I'm not at all familiar with MOSFETs (and to be honest my general
electronics skills are very limited...which is why I'm here) so I was a
little uncertain about how to proceed in adapting that design for 24VDC.

So I did some reading on MOSFETs and after getting even more confused
about them I simply changed the layout of the LED array to work with 24V
(6 LEDs and a 120 ohm current-limiting resistor in series) and plugged it
in. No problem despite the fact that the MOSFET gate is now seeing
nearly 24V rather than 12V. Then I connected 11 more of these LED
strings in parallel and ran that. Still no problems.

Here's where I'm a little uncertain. According to the article one IRF510
MOSFET should be able to handle up to around 16 LEDs before you have to
get something more powerful. But in my case I have 72 LEDs running
through a single IRF510. The chip doesn't even feel warm. Should I not
be surprised that I can support so many more LEDs simply by moving to a
24V design or is the author of that article either mistaken or being
overly cautious about his MOSFET?

Is there anyplace I can read a simple description of how to design with
MOSFETs? The back of the MOSFET package has a long list of parameters,
none of which mean anything to me. What's important to consider when
working with these things?

And can anyone see a possible flaw in this design? It almost seems too
simple to me. I don't want to implement this if it's likely to fail a
few months down the road. Commercial LED dimmers can cost hundreds of
dollars yet here we have a circuit for less than $10 (including a 555
timer which I'm not using in my case) which seems pretty effective to me.
So what's wrong with it?

Here's an ASCII version of my circuit:

24VDC -----------
| |
| |
10K LED ARRAY
| |
| |
| Drain
| |
-- Gate IRF510
| |
| Source
| |
470K |
| |
| |
-----------
|
Sinking
PWM
Output
|
GND---------

The bipolar 555 draws something like 8mA so a little more bias for the
zener is called for- a first cut would look like this:
View in a fixed-width font such as Courier.

..
 
F

Fred Bloggs

Jan 1, 1970
0
Fred said:
The bipolar 555 draws something like 8mA so a little more bias for the
zener is called for- a first cut would look like this:
View in a fixed-width font such as Courier.

.
.
. 24VDC ----+---------------------------------+-------+
. | | |
. +----+ | |
. | | | |
. 1.2k 1.2k | |
. | | | |
. +----+------+ | |
. | | | | |
. ___/ |0.47u | LED ARRAY |+
. 12V // \ --- | | ---
. zener --- --- |555 osc | ---
. | | +---+ Drain |100u
. | | | |out | |
. | | | |----100---+-Gate IRF510 |
. | | | | | | |
. | | | | | Source |
. | | +-+-+ | | |
. | | | 10K | |
. | | | | | |
. | | | | | |
. GND----+----+------+------------+--------+-------+
.
.
.
.
.
. LED ARRAY
.
.
. 220
. 24Vdc ---+--|>|-|>|-|>|-|>|-|>|-|>|---/\/\-+--> TO MOSFET
. | | DRAIN
. | 220 |
. +--|>|-|>|-|>|-|>|-|>|-|>|---/\/\-+
. | |
. | 220 |
. +--|>|-|>|-|>|-|>|-|>|-|>|---/\/\-+
. | |
. | 220 |
. +--|>|-|>|-|>|-|>|-|>|-|>|---/\/\-+
. | |
. | 220 |
. +--|>|-|>|-|>|-|>|-|>|-|>|---/\/\-+
. | |
. | 220 |
. +--|>|-|>|-|>|-|>|-|>|-|>|---/\/\-+
. | |
. | 220 |
. +--|>|-|>|-|>|-|>|-|>|-|>|---/\/\-+
. | |
. | 220 |
. +--|>|-|>|-|>|-|>|-|>|-|>|---/\/\-+
. | |
. | 220 |
. +--|>|-|>|-|>|-|>|-|>|-|>|---/\/\-+
. | |
. | 220 |
. +--|>|-|>|-|>|-|>|-|>|-|>|---/\/\-+
. | |
. | 220 |
. +--|>|-|>|-|>|-|>|-|>|-|>|---/\/\-+
. | |
. | 220 |
. +--|>|-|>|-|>|-|>|-|>|-|>|---/\/\-+
.
.

And in that 555 dimmer circuit shown at
http://www.e-f-w.com/community/modules.php?name=News&file=article&sid=10
replace the 1K resistor from the top of the 1M pot to the power supply
with a 10K resistor.
 
J

Joel Moore

Jan 1, 1970
0
Mac said:
One very important rule with MOSFET's such as the IRF510 which you
appear to be breaking is that the Voltage from gate to source must not
exceed some critical value. This is usually 20 Volts. If you are
really applying almost 24 Volts to the gate when the source is
grounded, then you are playing with fire. The datasheet or package
will call this Vgs max or something similar. You can fix this by
tweaking resistor values so that the gate voltage is lower. For
example, change both resistors to 10 k, then the maximum gate voltage
will be around 12 V which should be fine for you.

That's good to know and easy enough to fix.
How fast is your PWM going, by the way? If it is really fast, then you
might use smaller resistors than 10 k, but if it is slow, you could
use larger resistors.

I have it running at 5 KHz right now. The controller that's outputting
the PWM gives me several choices ranging from 32 Hz to 32 KHz. I'm
using the light to illuminate something for viewing through an camera
and it seemed if I went lower than 5 KHz I noticed some slight
"wavering" in the brightness.
But more importantly, I don't think your circuit makes any sense. The
MOSFET is nearly useless, if you really built the circuit as drawn
above, because when the PWM output is low, the MOSFET will be on, but
the PWM will be sinking all the current that flows through the MOSFET,
and when the PWM output is high, there will be no potential from the
drain to the source, and hence no current flowing. So you could just
as easily remove the MOSFET from your circuit above, and replace it
with a short circuit and there would be no change in behavior. The
whole point of the MOSFET is to handle large currents, or possibly to
act as a variable resistor capable of handling some power dissipation.
In a PWM circuit, I would expect the former.
This leads me to believe that you didn't really wire it that way. You
probably have the PWM output going to the gate of the MOSFET, and the
source of the MOSFET is directly grounded?

I don't have the circuit in front of me right now but I'm pretty certain
it's wired as shown in my ASCII diagram. There's only a few components
so it would be hard to make that mistake (hard, but not impossible --
I'll verify it when I head back to work tomorrow).

Anyway, I think I see what you're saying about the MOSFET being
pointless. However, at one point I tried wiring the LED array directly
to the PWM's output and it wasn't shining as bright as when the MOSFET
was involved. The controller's specs (http://www.tri-
plc.com/t100md1616.htm) claim the output is capable of sinking 1A so I
would have thought it was plenty capable but it looked like it wasn't
getting enough current. I may have to re-experiment with that to verify
it.
The LED array should also contain some kind of current limiting
resistor setup, as mentioned in the article. It might be helpful if
you drew out the details of the LED array, since they matter.

I did descibe it somewhere in my OP. It's 12 parallel strings of LEDs
where each string consists of 6 LEDs (Vf = 3.6) and one 120 ohm
resistor.
Also, is your PWM circuit getting hot at all? If you DID really wire
the circuit as shown, I imagine there is a lot of current going
through it, and I would expect it to get warm.

Hmmm. I haven't checked it yet. I'll do that tomorrow. For my own
education, what tells you there's a lot of current going through there?
The LED array should require around 240 mA. What's happening between
the gate and the source when the MOSFET is conducting? Even if that
looked like a short there wouldn't that branch of the circuit only draw
a couple of mA through the 10K resistor?

Thanks for your help.
 
J

Joel Moore

Jan 1, 1970
0
Fred Bloggs said:
The Radio Shack datasheet for the IRF510 is:
• Mfr. #IRF510 • Case Type: TO-220 • Channel Type: N • V(DS): 60 •
V(GS): +/-20 • I(D): 3.0 Amps • Power Dissipation: 20W

So you are on reprieve running V(GS) at 20V -at least it is operating
in
the deep depths of its resistive region. Your white LEDs have a
forward drop of about 3.3V at the 20-30mA level so that six in series
gives 6x3.3=19.3V and this leaves 24-19.3=4.7 across the MOSFET +
current limiting resistor. With 120 ohm current limiting and an ohm or
two from the MOSFET, you end up with somewhat less than 40mA through
the LEDs. The 12x of these strings is something like 0.5A which is
well within the MOSFET rating. So your circuit is okay except for the
V(GS) drive and the slightly excess LED current. You should change it
around like so:

Actually, the white LEDs are spec'd with a forward drop of 3.6 V which is
what I based my calculations on. So there should (in theory) only be
around 20 mA of current running through them.

But yeah, I'll fix my circuit to lower the voltage at the gate.

Thanks.
 
F

Fred Bloggs

Jan 1, 1970
0
Joel said:
Actually, the white LEDs are spec'd with a forward drop of 3.6 V which is
what I based my calculations on. So there should (in theory) only be
around 20 mA of current running through them.

But yeah, I'll fix my circuit to lower the voltage at the gate.

Thanks.

Why don't you measure it- that number is some kind of worst case and
seems high.
 
T

Terry Pinnell

Jan 1, 1970
0
Joel Moore said:
That's good to know and easy enough to fix.


I have it running at 5 KHz right now. The controller that's outputting
the PWM gives me several choices ranging from 32 Hz to 32 KHz. I'm
using the light to illuminate something for viewing through an camera
and it seemed if I went lower than 5 KHz I noticed some slight
"wavering" in the brightness.



I don't have the circuit in front of me right now but I'm pretty certain
it's wired as shown in my ASCII diagram. There's only a few components
so it would be hard to make that mistake (hard, but not impossible --
I'll verify it when I head back to work tomorrow).

Anyway, I think I see what you're saying about the MOSFET being
pointless. However, at one point I tried wiring the LED array directly
to the PWM's output and it wasn't shining as bright as when the MOSFET
was involved. The controller's specs (http://www.tri-
plc.com/t100md1616.htm) claim the output is capable of sinking 1A so I
would have thought it was plenty capable but it looked like it wasn't
getting enough current. I may have to re-experiment with that to verify
it.


I did descibe it somewhere in my OP. It's 12 parallel strings of LEDs
where each string consists of 6 LEDs (Vf = 3.6) and one 120 ohm
resistor.


Hmmm. I haven't checked it yet. I'll do that tomorrow. For my own
education, what tells you there's a lot of current going through there?
The LED array should require around 240 mA. What's happening between
the gate and the source when the MOSFET is conducting? Even if that
looked like a short there wouldn't that branch of the circuit only draw
a couple of mA through the 10K resistor?

Thanks for your help.

Although I too know little about MOSFETs, like Mac I'm puzzled as to
what purpose the IRF510 serves in your circuit as drawn?

Also, simulating it in the state when the MOSFET source is grounded
looks like this:
http://www.terrypin.dial.pipex.com/Images/LED-driver1.gif
From which you can see that the voltage drop across individual LEDs is
1.6V, and your gate is safely under the 20V max specified.

And didn't you also say that you're not using the 555 PWM section of
the original circuit? So how do you achieve the dimming?

Maybe I've just misunderstood...?
 
M

Mac

Jan 1, 1970
0
I have it running at 5 KHz right now. The controller that's outputting
the PWM gives me several choices ranging from 32 Hz to 32 KHz. I'm
using the light to illuminate something for viewing through an camera
and it seemed if I went lower than 5 KHz I noticed some slight
"wavering" in the brightness.

Without doing any calculations, I'll just say stick with 10 k or
thereabouts resistors, then.
I don't have the circuit in front of me right now but I'm pretty certain
it's wired as shown in my ASCII diagram. There's only a few components
so it would be hard to make that mistake (hard, but not impossible --
I'll verify it when I head back to work tomorrow).

Anyway, I think I see what you're saying about the MOSFET being
pointless. However, at one point I tried wiring the LED array directly
to the PWM's output and it wasn't shining as bright as when the MOSFET
was involved. The controller's specs (http://www.tri-
plc.com/t100md1616.htm) claim the output is capable of sinking 1A so I
would have thought it was capable but it looked like it wasn't getting
enough current. I may have to re-experiment with that to verify it.


I did descibe it somewhere in my OP. It's 12 parallel strings of LEDs
where each string consists of 6 LEDs (Vf = 3.6) and one 120 ohm resistor.

OK, I see it now. Although you didn't mention the Vf in your post, I don't
think.
Hmmm. I haven't checked it yet. I'll do that tomorrow. For my own
education, what tells you there's a lot of current going through there?
The LED array should require around 240 mA. What's happening between
the gate and the source when the MOSFET is conducting?

No, the short would be from drain to source. The gate to source is like a
capacitor, with the catch that the gate to source voltage controls the
drain to source resistance. So when the PWM sinks current, the transistor
gate will be at close to 24 Volts, the source will be at GND, and the
drain to source will exhibit a very low resistance. Since the source is in
series with the PWM (in your diagram) all the 240 mA would go through the
PWM.
Even if that
looked like a short there wouldn't that branch of the circuit only draw
a couple of mA through the 10K resistor?

Well, that branch wouldn't contribute much due to the 470 k resistor in
series with the 10 k resistor. (No DC current flows into or out of the
gate of a MOSFET.) But as noted above, the drain to source path would be
carrying the full 240 mA.
Thanks for your help.

240 mA qualifies as a lot of current. But this is very light duty for
that particular FET (I see where Fred dug up the datasheet). Also, in this
application, it looks like you plan to always keep the FET either off or
on all the way, so you probably don't need to worry about heat
dissipation. In the article you referred to, the FET was being used in a
mode where it was in between off and on. In this mode, FET's sometimes
dissipate a lot of heat, that's why the guy worried about how many LED's
were attached.

What duty cycle are you PWM'ing at? Right now you are sort of inverting.
That is, when the PWM is high, your LED's are off, and when the PWM is
low, your LED's are on. But here is how you WANT to hook it up: (since
you have a whole Ampere of drive current available we'll use low value
resistors):



100
PWM-----/\/\/\---+-----Gate
|
/
\ 220
/
\
|
|
GND


With this arrangement, when the PWM is high, the LED's will be on, and
when the PWM is low, the LED's will be off. If the duty cycle is not 50%,
this will make a difference. At high duty cycles the LED's will be
brighter, and at low duty cycles, they will be dimmer. But with your other
circuit, it is just the opposite.

--Mac

By the way, it looks like Fred's posts are pretty good, too, though he is
talking about the 555, not the tri-plc board.
 
F

Fred Bloggs

Jan 1, 1970
0
Terry said:
Also, simulating it in the state when the MOSFET source is grounded
looks like this:
http://www.terrypin.dial.pipex.com/Images/LED-driver1.gif
From which you can see that the voltage drop across individual LEDs is
1.6V, and your gate is safely under the 20V max specified.

He is using Nichia white LEDs with Vf=3.6V for If=20mA at 20oC. The Vf
drifts at about -10mV/oC, and the heat conduction is not so good, so
that the OP may be initializing at 20mA nominal, but the current will
definitely drift upwards at a rate 60mV/(oC x 120Ohm)=0.5mA/oC in his
array. My guess is he will end up with 27mA. And he is using a $330 PLC
controller with open-collector NPN PWM output rated for 350mA
continuous, 1A peak. He doesn't need the MOSFET at all.
 
M

Mac

Jan 1, 1970
0
On Sat, 27 Aug 2005 14:43:47 +0000, Fred Bloggs wrote:
[snip]
he is using a $330 PLC
controller with open-collector NPN PWM output rated for 350mA
^^^^^^^^^^^^^^^^^^^^^^

Ooops. I didn't catch that. Then he better not wire the circuit the way I
showed it, with the PWM directly connected to the gate with no pullup!

I just skimmed through the datasheet and saw that it said 24 V 1 amp
output for the digital outputs. I didn't notice that it was open-collector
output.

--Mac
 
M

Mac

Jan 1, 1970
0
Joel Moore said:
I don't have the circuit in front of me right now but I'm pretty certain
it's wired as shown in my ASCII diagram. There's only a few components
so it would be hard to make that mistake (hard, but not impossible --
I'll verify it when I head back to work tomorrow).

Anyway, I think I see what you're saying about the MOSFET being
pointless. However, at one point I tried wiring the LED array directly
to the PWM's output and it wasn't shining as bright as when the MOSFET
was involved. The controller's specs (http://www.tri-
plc.com/t100md1616.htm) claim the output is capable of sinking 1A so I
would have thought it was plenty capable but it looked like it wasn't
getting enough current. I may have to re-experiment with that to verify
it.
[snip]

Although I too know little about MOSFETs, like Mac I'm puzzled as to
what purpose the IRF510 serves in your circuit as drawn?

Also, simulating it in the state when the MOSFET source is grounded
looks like this:
http://www.terrypin.dial.pipex.com/Images/LED-driver1.gif

No. In your gif you've got the 10 K and 470 K resistors in series with
the LED array. That is not how the OP drew it originally. The 10 K was
connected between 24 VDC and the gate. The 470 K was connected between the
gate and source, and the source was connected to the PWM output.
From which you can see that the voltage drop across individual LEDs is
1.6V, and your gate is safely under the 20V max specified.

When the PWM sinks current and goes almost to GND, the 10 K and 470 K form
a divider with 24 V across it. So Vgs is 24 * (470/(470+10)) = 23.5 V. At
least that is the way I am reading it.
And didn't you also say that you're not using the 555 PWM section of
the original circuit? So how do you achieve the dimming?

[snip]

--Mac
 
F

Fred Bloggs

Jan 1, 1970
0
Mac said:
On Sat, 27 Aug 2005 14:43:47 +0000, Fred Bloggs wrote:
[snip]

he is using a $330 PLC
controller with open-collector NPN PWM output rated for 350mA

^^^^^^^^^^^^^^^^^^^^^^

Ooops. I didn't catch that. Then he better not wire the circuit the way I
showed it, with the PWM directly connected to the gate with no pullup!

I just skimmed through the datasheet and saw that it said 24 V 1 amp
output for the digital outputs. I didn't notice that it was open-collector
output.

--Mac

Yeah- it's a sloppy manual- mentions Darlington NPN. They're just using
some Allegro part.
 
T

Terry Pinnell

Jan 1, 1970
0
Mac said:
Joel Moore said:
But more importantly, I don't think your circuit makes any sense. The
MOSFET is nearly useless, if you really built the circuit as drawn
above, because when the PWM output is low, the MOSFET will be on, but
the PWM will be sinking all the current that flows through the MOSFET,
and when the PWM output is high, there will be no potential from the
drain to the source, and hence no current flowing. So you could just
as easily remove the MOSFET from your circuit above, and replace it
with a short circuit and there would be no change in behavior. The
whole point of the MOSFET is to handle large currents, or possibly to
act as a variable resistor capable of handling some power dissipation.
In a PWM circuit, I would expect the former.


This leads me to believe that you didn't really wire it that way. You
probably have the PWM output going to the gate of the MOSFET, and the
source of the MOSFET is directly grounded?

I don't have the circuit in front of me right now but I'm pretty certain
it's wired as shown in my ASCII diagram. There's only a few components
so it would be hard to make that mistake (hard, but not impossible --
I'll verify it when I head back to work tomorrow).

Anyway, I think I see what you're saying about the MOSFET being
pointless. However, at one point I tried wiring the LED array directly
to the PWM's output and it wasn't shining as bright as when the MOSFET
was involved. The controller's specs (http://www.tri-
plc.com/t100md1616.htm) claim the output is capable of sinking 1A so I
would have thought it was plenty capable but it looked like it wasn't
getting enough current. I may have to re-experiment with that to verify
it.
[snip]

Although I too know little about MOSFETs, like Mac I'm puzzled as to
what purpose the IRF510 serves in your circuit as drawn?

Also, simulating it in the state when the MOSFET source is grounded
looks like this:
http://www.terrypin.dial.pipex.com/Images/LED-driver1.gif

No. In your gif you've got the 10 K and 470 K resistors in series with
the LED array. That is not how the OP drew it originally. The 10 K was
connected between 24 VDC and the gate. The 470 K was connected between the
gate and source, and the source was connected to the PWM output.
From which you can see that the voltage drop across individual LEDs is
1.6V, and your gate is safely under the 20V max specified.

When the PWM sinks current and goes almost to GND, the 10 K and 470 K form
a divider with 24 V across it. So Vgs is 24 * (470/(470+10)) = 23.5 V. At
least that is the way I am reading it.
And didn't you also say that you're not using the 555 PWM section of
the original circuit? So how do you achieve the dimming?

[snip]

--Mac

You're right of course - careless drawing slip, sorry. I've corrected
it. I've also included LEDs closer to the spec the OP and Fred
described. (Best CircuitMaker can do is Vf = 3.4V @ 40mA.) So these
are now dropping 3V, and Vgs is above recommended level.
http://www.terrypin.dial.pipex.com/Images/LED-driver1.gif
 
F

Fred Bloggs

Jan 1, 1970
0
Terry said:
But more importantly, I don't think your circuit makes any sense. The
MOSFET is nearly useless, if you really built the circuit as drawn
above, because when the PWM output is low, the MOSFET will be on, but
the PWM will be sinking all the current that flows through the MOSFET,
and when the PWM output is high, there will be no potential from the
drain to the source, and hence no current flowing. So you could just
as easily remove the MOSFET from your circuit above, and replace it
with a short circuit and there would be no change in behavior. The
whole point of the MOSFET is to handle large currents, or possibly to
act as a variable resistor capable of handling some power dissipation.
In a PWM circuit, I would expect the former.

This leads me to believe that you didn't really wire it that way. You
probably have the PWM output going to the gate of the MOSFET, and the
source of the MOSFET is directly grounded?

I don't have the circuit in front of me right now but I'm pretty certain
it's wired as shown in my ASCII diagram. There's only a few components
so it would be hard to make that mistake (hard, but not impossible --
I'll verify it when I head back to work tomorrow).

Anyway, I think I see what you're saying about the MOSFET being
pointless. However, at one point I tried wiring the LED array directly
to the PWM's output and it wasn't shining as bright as when the MOSFET
was involved. The controller's specs (http://www.tri-
plc.com/t100md1616.htm) claim the output is capable of sinking 1A so I
would have thought it was plenty capable but it looked like it wasn't
getting enough current. I may have to re-experiment with that to verify
it.

[snip]

Although I too know little about MOSFETs, like Mac I'm puzzled as to
what purpose the IRF510 serves in your circuit as drawn?

Also, simulating it in the state when the MOSFET source is grounded
looks like this:
http://www.terrypin.dial.pipex.com/Images/LED-driver1.gif

No. In your gif you've got the 10 K and 470 K resistors in series with
the LED array. That is not how the OP drew it originally. The 10 K was
connected between 24 VDC and the gate. The 470 K was connected between the
gate and source, and the source was connected to the PWM output.

From which you can see that the voltage drop across individual LEDs is
1.6V, and your gate is safely under the 20V max specified.

When the PWM sinks current and goes almost to GND, the 10 K and 470 K form
a divider with 24 V across it. So Vgs is 24 * (470/(470+10)) = 23.5 V. At
least that is the way I am reading it.

And didn't you also say that you're not using the 555 PWM section of
the original circuit? So how do you achieve the dimming?

[snip]

--Mac


You're right of course - careless drawing slip, sorry. I've corrected
it. I've also included LEDs closer to the spec the OP and Fred
described. (Best CircuitMaker can do is Vf = 3.4V @ 40mA.) So these
are now dropping 3V, and Vgs is above recommended level.
http://www.terrypin.dial.pipex.com/Images/LED-driver1.gif

Check http://www.nichia.com/specification/led_lamp/NSPL500S.pdf
 
J

Joel Moore

Jan 1, 1970
0

I should have posted a part number a long time ago. Sorry.

The LED I'm using is from Agilent, part # HLMP-CW30-PS000. Here's a
(long) link:

http://www.home.agilent.com/cgi-bin/pub/agilent/Product/cp_Product.jsp?
NAV_ID=-25328.536894664.00&LANGUAGE_CODE=eng&CONTENT_KEY=HLMP-CW30-PS000
&ID=HLMP-CW30-PS000&COUNTRY_CODE=US

Anyway, I just got into work so I'm going to try a couple of the things
suggested here (not using the MOSFET, fixing the resistors) and I'll post
again with more info and some responses to points raised elsewhere in the
thread.

Also, I confirmed that my circuit is wired as I described in my OP.

Joel
 
J

Joel Moore

Jan 1, 1970
0
He is using Nichia white LEDs with Vf=3.6V for If=20mA at 20oC. The Vf
drifts at about -10mV/oC, and the heat conduction is not so good, so
that the OP may be initializing at 20mA nominal, but the current will
definitely drift upwards at a rate 60mV/(oC x 120Ohm)=0.5mA/oC in his
array. My guess is he will end up with 27mA. And he is using a $330 PLC
controller with open-collector NPN PWM output rated for 350mA
continuous, 1A peak. He doesn't need the MOSFET at all.

Ok, I feel a little silly because I just tried connecting my LED array
directly to the PLC's output and it works fine. You're absolutely right
that the MOSFET isn't necessary in my application. I sort of recognized
that before but when I tried it without the MOSFET I saw a significant
reduction in the brightness. There was probably some other factor that
caused this (maybe the PWM was running at 50% or something) but I managed
to convince myself that the MOSFET was helping in some way.

Anyway, the effort was not a waste because I have started comprehending
how MOSFETs work (pawing through some old college textbooks last night
also helped in that regard) and I still plan on trying to produce the
circuit using a 555 timer for use when the PLC isn't available.

Thanks everyone for your help (and for validating the worth of venerable
USENET once again).

Joel Moore
 
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