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dual gate n-chan MOSFETs

L

Leon Sorokin

Jan 1, 1970
0
i'm making a lighting system where i need lights to be independently turned
on and off, or at the flip of a switch, have all of them constant on. will
an N-channel dual-gate MOSFET do the trick? if so, does anyone know of one
that can support approx 500ma from drain to source?

otherwise i need to use OR gates, which is an extra IC that i would rather
not use, since i have limited board space. so i am hoping that the dual
gates on the MOSFET are isolated from each other, otherwise it wouldnt make
much sense, right? and i'm also hoping i can find one that can support the
500ma current. well, it'll probably be a 450-500ma 3Hz 3.4V pulse. not a
constant on.

thanks.
Leon
 
W

Winfield Hill

Jan 1, 1970
0
Leon Sorokin wrote...
i'm making a lighting system where i need lights to be independently turned
on and off, or at the flip of a switch, have all of them constant on. will
an N-channel dual-gate MOSFET do the trick? if so, does anyone know of one
that can support approx 500ma from drain to source?

otherwise i need to use OR gates, which is an extra IC that i would rather
not use, since i have limited board space. so i am hoping that the dual
gates on the MOSFET are isolated from each other, otherwise it wouldnt make
much sense, right? and i'm also hoping i can find one that can support the
500ma current. well, it'll probably be a 450-500ma 3Hz 3.4V pulse. not a
constant on.

What are you switching, exactly? You can use two mosfets in series.
 
L

Leon Sorokin

Jan 1, 1970
0
they're LED clusters, i'm using a 4017 BCD counter to cycle through them,
but i also need the option of having all of them on at the same time through
a switch - there's a flashing mode, and a constant on mode, so i need
isolated gates, so all the gate1s stay separate but all gate2s are joined,
but isolated from all the gate1s. forget the 3hz pulse and 500 ma, i'm
mixing projects : ). the current is around 100ma tho cause i'm using 5 leds
per cluster in parallel and their typical Vf is 3.6V@30ma, so it does need
to support maybe at least 150ma draind source current.
 
J

John Fields

Jan 1, 1970
0
they're LED clusters, i'm using a 4017 BCD counter to cycle through them,
but i also need the option of having all of them on at the same time through
a switch - there's a flashing mode, and a constant on mode, so i need
isolated gates, so all the gate1s stay separate but all gate2s are joined,
but isolated from all the gate1s. forget the 3hz pulse and 500 ma, i'm
mixing projects : ). the current is around 100ma tho cause i'm using 5 leds
per cluster in parallel and their typical Vf is 3.6V@30ma, so it does need
to support maybe at least 150ma draind source current.
---
Why not use diodes?

+V>----------------+------+------+------+------+
| | | | |
[R] [R] [R] [R] [R]
| | | | |
[LED] [LED] [LED] [LED] [LED]
| | | | |
+------+------+------+------+
|
D
FLASH>--->|--+---G
| S
ON------->|--+ |
| |
[R] |
| |
GND>---------+-----+
 
T

Tam/WB2TT

Jan 1, 1970
0
Leon Sorokin said:
i'm making a lighting system where i need lights to be independently
turned on and off, or at the flip of a switch, have all of them constant
on. will an N-channel dual-gate MOSFET do the trick? if so, does anyone
know of one that can support approx 500ma from drain to source?

otherwise i need to use OR gates, which is an extra IC that i would rather
not use, since i have limited board space. so i am hoping that the dual
gates on the MOSFET are isolated from each other, otherwise it wouldnt
make much sense, right? and i'm also hoping i can find one that can
support the 500ma current. well, it'll probably be a 450-500ma 3Hz 3.4V
pulse. not a constant on.

thanks.
Leon
Two resistors feeding into the base of a bipolar transistor will form an OR
gate, although it is nor clear that an OR is what you want.

Tam
 
J

John Larkin

Jan 1, 1970
0
they're LED clusters, i'm using a 4017 BCD counter to cycle through them,
but i also need the option of having all of them on at the same time through
a switch - there's a flashing mode, and a constant on mode, so i need
isolated gates, so all the gate1s stay separate but all gate2s are joined,
but isolated from all the gate1s. forget the 3hz pulse and 500 ma, i'm
mixing projects : ). the current is around 100ma tho cause i'm using 5 leds
per cluster in parallel and their typical Vf is 3.6V@30ma, so it does need
to support maybe at least 150ma draind source current.
---
Why not use diodes?

+V>----------------+------+------+------+------+
| | | | |
[R] [R] [R] [R] [R]
| | | | |
[LED] [LED] [LED] [LED] [LED]
| | | | |
+------+------+------+------+
|
D
FLASH>--->|--+---G
| S
ON------->|--+ |
| |
[R] |
| |
GND>---------+-----+


And why not put some of those LEDs in series and save a lot of power?

John
 
L

Leon Sorokin

Jan 1, 1970
0
this is for a design we had a while ago, i just saw it and thought maybe i
could revise it. i think our problem was that if we used diodes, we would be
applying 12-16v to the 4017 outputs and were afraid of damaging them. so we
thought maybe using another diode to protect the outputs like this as well
as isolate the gates from each other for flashing mode, seemed great in
theory.



GND
|
S
4017 output--|>|---|>|----G
| D----LED1---+3.3V
|
|
| GND
| |
| S
4017 output--|>|---|>|----G
| D----LED2---+3.3V
|
-- -/o- --+16
on/off

but then we tested and found out that the diode that was protecting the
outputs was also preventing the gates from going to a low state, so we had a
constant glow from the gate capacitance not being discharged because of the
diode. we could have thrown a resistor in but we didnt want a high RDSon
because the gate was not sufficiently saturated if we were going to drop the
input voltage from 16 to something that would not damage the 4017 outputs.
and we were using a National semi switching voltage reg and were already
underpowering the LEDs by 0.3V.

each led has to flash independently from the outputs, as well as being
constant on simultaniusly with the rest of them in mode2. so you need to
isolate the 2 nets somehow, that 2 gate nchannel mosfet looked like the
ticket. if it could take 150ma DS current.

John Fields said:
they're LED clusters, i'm using a 4017 BCD counter to cycle through them,
but i also need the option of having all of them on at the same time
through
a switch - there's a flashing mode, and a constant on mode, so i need
isolated gates, so all the gate1s stay separate but all gate2s are joined,
but isolated from all the gate1s. forget the 3hz pulse and 500 ma, i'm
mixing projects : ). the current is around 100ma tho cause i'm using 5
leds
per cluster in parallel and their typical Vf is 3.6V@30ma, so it does need
to support maybe at least 150ma draind source current.
---
Why not use diodes?

+V>----------------+------+------+------+------+
| | | | |
[R] [R] [R] [R] [R]
| | | | |
[LED] [LED] [LED] [LED] [LED]
| | | | |
+------+------+------+------+
|
D
FLASH>--->|--+---G
| S
ON------->|--+ |
| |
[R] |
| |
GND>---------+-----+
 
K

Ken Smith

Jan 1, 1970
0
they're LED clusters, i'm using a 4017 BCD counter to cycle through them,
but i also need the option of having all of them on at the same time through
a switch - there's a flashing mode, and a constant on mode, so i need
isolated gates, so all the gate1s stay separate but all gate2s are joined,
but isolated from all the gate1s. forget the 3hz pulse and 500 ma, i'm
mixing projects : ). the current is around 100ma tho cause i'm using 5 leds
per cluster in parallel and their typical Vf is 3.6V@30ma, so it does need
to support maybe at least 150ma draind source current.

Suggestions:

Drop the 4017 and use a 22V10 and program all the different modes into it.

Use a PIC.

For all LEDs on, connect the Vss of the 4017 to the VCC so that all
outputs are high. For all off ground both. For flashing alternate
between the two.

Use an 8051 to do that and play music while it happens.
 
R

Rich Grise

Jan 1, 1970
0
i'm making a lighting system where i need lights to be independently
turned on and off, or at the flip of a switch, have all of them constant
on. will an N-channel dual-gate MOSFET do the trick? if so, does anyone
know of one that can support approx 500ma from drain to source?

otherwise i need to use OR gates, which is an extra IC that i would rather
not use, since i have limited board space. so i am hoping that the dual
gates on the MOSFET are isolated from each other, otherwise it wouldnt
make much sense, right? and i'm also hoping i can find one that can
support the 500ma current. well, it'll probably be a 450-500ma 3Hz 3.4V
pulse. not a constant on.

You don't need any electronics at all. Put one side of all of the switches
to ground. Use isolation diodes. For the "all on" switch, put a diode from
each LED cathode to the one switch's other contact. For the rest, put a
diode from the LED cathode to each LED's individual switch. Open-collector
outputs would work also.

So there's two diodes at each LED string, one going to the "all on" switch,
(along with the rest of the "all on" diodes), the other going to the
corresponding switch for that LED string.

R R R R
| | | |
L L L L
| | | |
-+- -+- -+- -+-
| | | | | | | |
D D D D D D D D -- > etc.
| | | | | | | |
S | S | S | S | Individual
| | | | | | | |
| +----------+-----------+----------+
| | | | |
| | | | S All
| | | | |
G G G G G

Plus on top, all diode and LED anodes up.

(I hope you appreciate this diagram - it took me nearly an hour to
write! ;-) )

Cheers!
Rich
 
K

Kevin Aylward

Jan 1, 1970
0
John said:
they're LED clusters, i'm using a 4017 BCD counter to cycle through
them, but i also need the option of having all of them on at the
same time through a switch - there's a flashing mode, and a
constant on mode, so i need isolated gates, so all the gate1s stay
separate but all gate2s are joined, but isolated from all the
gate1s. forget the 3hz pulse and 500 ma, i'm mixing projects : ).
the current is around 100ma tho cause i'm using 5 leds per cluster
in parallel and their typical Vf is 3.6V@30ma, so it does need to
support maybe at least 150ma draind source current.
---
Why not use diodes?

+V>----------------+------+------+------+------+
| | | | |
[R] [R] [R] [R] [R]
| | | | |
[LED] [LED] [LED] [LED] [LED]
| | | | |
+------+------+------+------+
|
D
FLASH>--->|--+---G
| S
ON------->|--+ |
| |
[R] |
| |
---------+-----+


And why not put some of those LEDs in series and save a lot of power?

John

Ahmmm.. Care to look at that statement again:)

Hint: "Theres no such thing as a free lunch".

Kevin Aylward
[email protected]
http://www.anasoft.co.uk
SuperSpice, a very affordable Mixed-Mode
Windows Simulator with Schematic Capture,
Waveform Display, FFT's and Filter Design.
 
J

John Fields

Jan 1, 1970
0
John said:
they're LED clusters, i'm using a 4017 BCD counter to cycle through
them, but i also need the option of having all of them on at the
same time through a switch - there's a flashing mode, and a
constant on mode, so i need isolated gates, so all the gate1s stay
separate but all gate2s are joined, but isolated from all the
gate1s. forget the 3hz pulse and 500 ma, i'm mixing projects : ).
the current is around 100ma tho cause i'm using 5 leds per cluster
in parallel and their typical Vf is 3.6V@30ma, so it does need to
support maybe at least 150ma draind source current.
---
Why not use diodes?

+V>----------------+------+------+------+------+
| | | | |
[R] [R] [R] [R] [R]
| | | | |
[LED] [LED] [LED] [LED] [LED]
| | | | |
+------+------+------+------+
|
D
FLASH>--->|--+---G
| S
ON------->|--+ |
| |
[R] |
| |
---------+-----+


And why not put some of those LEDs in series and save a lot of power?

John

Ahmmm.. Care to look at that statement again:)

Hint: "Theres no such thing as a free lunch".

---

| 2V |
10V>--[LED>]--[LED>]--[LED>]--[LED>]--[LED>]--+
|
0V>-------------------------------------------+
<--20mA

Ps = IE = 0.02A * 10V = 0.2W




0V<----------------------------+
<--100mA |
|
|
|<-- 10V --->| 20mA--> |
10V>--+-[R]---[LED>]-----------+
| |
| |
|<-- 10V --->| 20mA--> |
+-[R]---[LED>]-----------+
| |
| |
|<-- 10V --->| 20mA--> |
+-[R]---[LED>]-----------+
| |
| |
|<-- 10V --->| 20mA--> |
+-[R]---[LED>]-----------+
| |
| |
|<-- 10V --->| 20mA--> |
+-[R]---[LED>]-----------+


Pp = IE = 0.1A * 10V = 1.0W


Ps = 0.2Pp, = 0.8 watts saved
 
J

John Fields

Jan 1, 1970
0
John Fields said:
they're LED clusters, i'm using a 4017 BCD counter to cycle through them,
but i also need the option of having all of them on at the same time
through
a switch - there's a flashing mode, and a constant on mode, so i need
isolated gates, so all the gate1s stay separate but all gate2s are joined,
but isolated from all the gate1s. forget the 3hz pulse and 500 ma, i'm
mixing projects : ). the current is around 100ma tho cause i'm using 5
leds
per cluster in parallel and their typical Vf is 3.6V@30ma, so it does need
to support maybe at least 150ma draind source current.
---
Why not use diodes?

+V>----------------+------+------+------+------+
| | | | |
[R] [R] [R] [R] [R]
| | | | |
[LED] [LED] [LED] [LED] [LED]
| | | | |
+------+------+------+------+
|
D
FLASH>--->|--+---G
| S
ON------->|--+ |
| |
[R] |
| |
GND>---------+-----+


this is for a design we had a while ago, i just saw it and thought maybe i
could revise it. i think our problem was that if we used diodes, we would be
applying 12-16v to the 4017 outputs and were afraid of damaging them. so we
thought maybe using another diode to protect the outputs like this as well
as isolate the gates from each other for flashing mode, seemed great in
theory.



GND
|
S
4017 output--|>|---|>|----G
| D----LED1---+3.3V
|
|
| GND
| |
| S
4017 output--|>|---|>|----G
| D----LED2---+3.3V
|
-- -/o- --+16
on/off

but then we tested and found out that the diode that was protecting the
outputs was also preventing the gates from going to a low state, so we had a
constant glow from the gate capacitance not being discharged because of the
diode. we could have thrown a resistor in but we didnt want a high RDSon
because the gate was not sufficiently saturated if we were going to drop the
input voltage from 16 to something that would not damage the 4017 outputs.
and we were using a National semi switching voltage reg and were already
underpowering the LEDs by 0.3V.

each led has to flash independently from the outputs, as well as being
constant on simultaniusly with the rest of them in mode2. so you need to
isolate the 2 nets somehow, that 2 gate nchannel mosfet looked like the
ticket. if it could take 150ma DS current.


---
1. Please don't top post.

2. You shouldn't be using the MOSFETs as source followers.

3. To assure the gates return to 0V when they're being driven through
diodes, they (the gates) need to be returned to ground through
resistors.


+V \
| |
+------+------+------+------+ |
| | | | | |
[R] [R] [R] [R] [R] |
| | | | | |
[LED] [LED] [LED] [LED] [LED] |
| | | | | |
+------+------+------+------+ |
| > ARRAY "0"
D |
4017 Q0>------->|--+---G |
| S |
ALL ON>-----+-->|--+ | |
| | | |
| [R] | |
| | | |
GND>-----+--|------+----+ |
| | /
| |
. .
. .
. . \
| | +V |
| | | |
| | +------+------+------+------+ |
| | | | | | | |
| | [R] [R] [R] [R] [R] |
| | | | | | | |
| | [LED] [LED] [LED] [LED] [LED] |
| | | | | | | |
| | +------+------+------+------+ |
| | | > ARRAY "n"
| | D |
4017 Qn>------->|--+---G |
| | | S |
| +-->|--+ | |
| | | |
| [R] | |
| | | |
+---------+----+ |
/

Also, to conserve power (as John Larkin suggested) you should look
into driving your LEDs in series.
 
K

Ken Smith

Jan 1, 1970
0
John Fields said:
2. You shouldn't be using the MOSFETs as source followers.

Also, the driver section doesn't need more than:

3 Resistors
2 Capacitors
1 CD4011
1 CD4017
1 2 Pole 4 position switch

With this you can get the gate drive needed for the following modes:

1 All off
2 All on
3 Blink in turn
4 All blink together.


If you run the CD40XX stuff from a >9V supply, the gate drive will be
enough that you don't need "logic level" MOSFETs and can use the lower
cost ones.
 
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