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jfet as current limiter

  • Thread starter Marco Trapanese
  • Start date
M

Marco Trapanese

Jan 1, 1970
0
Il 15/10/2012 16:27, John S ha scritto:
Try it with a slight modification. Still not really good, but better.


Well well well, the simulation is very nice. Tomorrow I'll change the
pcb to try this version.

Thank you
Marco
 
R

Rocky

Jan 1, 1970
0
Il 15/10/2012 14:49, Rocky ha scritto:


I don't know exactly, but I thinks a couple of V should be tolerated
without any problems. The supply voltage of the current generator is
+24V and it seems it will work until its output reaches 16-17V.
I tested the LM317L constant current source circuit using a 51ohm
resistor and mine limited at 24mA, changing by less than 0.1mA from 3V
to 24V with no visible temp sensitivity. Only downside is that as Phil
Hobbs pointed out it drops about 2.9V just before going into current
limit. At 10mA the voltage drop is 2 volts.
 
J

John S

Jan 1, 1970
0
Il 15/10/2012 16:27, John S ha scritto:



Well well well, the simulation is very nice. Tomorrow I'll change the
pcb to try this version.

Thank you
Marco

I think whit3rd's version is much better. His will not drift with
temperature noticeably. I would use the LM431. See the data sheet for
applications and you will also see whit3rd's version there.
 
M

Marco Trapanese

Jan 1, 1970
0
Il 15/10/2012 21:25, Rocky ha scritto:
I tested the LM317L constant current source circuit using a 51ohm
resistor and mine limited at 24mA, changing by less than 0.1mA from 3V
to 24V with no visible temp sensitivity. Only downside is that as Phil
Hobbs pointed out it drops about 2.9V just before going into current
limit. At 10mA the voltage drop is 2 volts.


The problem is not the voltage drop, but it should work down to few
mAmps without affecting in any way the linearity. That is if the current
source sources 6.84 mA or 7.53 mA I must get 6.84 and 7.53 mA at the
output. This over the whole 4-20 mA range.

Marco
 
M

Marco Trapanese

Jan 1, 1970
0
Il 15/10/2012 22:03, John S ha scritto:
I think whit3rd's version is much better. His will not drift with
temperature noticeably. I would use the LM431. See the data sheet for
applications and you will also see whit3rd's version there.


But his solution needs a third connection. Unfortunately, I should rely
only on the two terminals (series connection).

I just need to limit the current below 30 mA and above 20 mA. If the
real world knows the result of the spice simulations of your suggestion
it should work fine ;)

Marco
 
J

John S

Jan 1, 1970
0
Il 15/10/2012 22:03, John S ha scritto:



But his solution needs a third connection. Unfortunately, I should rely
only on the two terminals (series connection).

I just need to limit the current below 30 mA and above 20 mA. If the
real world knows the result of the spice simulations of your suggestion
it should work fine ;)

Marco

I don't understand, Marco. Where is the third connection?

John
 
J

John S

Jan 1, 1970
0
Il 15/10/2012 22:03, John S ha scritto:



But his solution needs a third connection. Unfortunately, I should rely
only on the two terminals (series connection).

I just need to limit the current below 30 mA and above 20 mA. If the
real world knows the result of the spice simulations of your suggestion
it should work fine ;)

Marco

Oh! I just realized you were looking at a different post by whit3rd. His
10/13/2012 3:12 PM post is the one I was referring to. Hopefully I have
his permission to partially re-post the important part:

*****************************************************************
There's a classical answer, using TL431 programmable zener

(warning: bad ASCII art, use monospace font)


(+)
|
+-------+
| /
| /
R1 |
+----|
| |>
| \
| |
| |
| |
/---/ |
/ \ |
/ \-----+
--+-- R2
| |
+-------+
|
(-)

I_limit = 2.5V /R2 + (minor correction for zener bias)

which can be redone with an LM4041 for smaller Vref, using a PNP
transistor...
the range of 23... 30 mA should be easy to hit with 2V or so of voltage
burden.
 
J

Jamie

Jan 1, 1970
0
Marco said:
Il 12/10/2012 05:12, Jamie ha scritto:




Well, one good news and one bad news.

The good one is there is almost no difference among single items, so
this issue is fixed.

The bad one is it depends too much on temperature. At room temperature
with 12k + 12k the current is limited at 25 mA. But after a couple of
seconds it rises up to 30 mA. Heating the bjt about 50 °C the current is
over 35 mA.

I need to guarantee the current never goes above 30 mA over the whole
commercial temperature range (0-70 °C).

I'm going to add a 1n4148 in series of ("of"? is it right?) the zener.

Two questions:

- do I need to add also a resistor on the emitter?
- in order to work fine I'm afraid the diode should be thermally
connected to the bjt. Otherwise they share only the ambient temperature,
but when the bjt heats due to his own current the diode doesn't
compensate at all.

Any thoughts?

Marco
It's strange that you experienced that much of a change in Beta in the
short time, I've never experienced that much of a change, but then
again, you did add that extra R that I didn't put there.

All is not lost however. Below is a circuit that was posted by Phil H
which is something of an old relic but it seems to work nicely with out
the use of a reference. It uses the Vf of the base and also with the way
it's configured it'll counter balance the beta while self heating takes
place.

As Q1 self heats, the Vf (base) will drop and thus Beta will increase.
When this happens however, current limit will increase, but it'll also
force Q2 to pull R1 down, causing a - feed back effect.

R1 should be selected for lowest Loop supply, in this case I picked 5
volts but you can do what you wish. R2 is the base line of the limited
value. You can replace Q1 with a low Vgs(th) NMOS and still maintain a
desired value with out the worries of the NMOS process variations. I
have sims for both..


5..24
+-------------------------+
| +|
| +--+++
+ .-. |
--- R1 10k | | +
- | | |
| '-' +
| + |/
=== Q1 ++-| 2N2222
GND | |>
| +
| |
+ |
| |
\| |
2N2222 |+-+ Q2
<| |
+ .-.
| R2| | Current R
| | 31
| '-' R = 0.68 / 0.020mA
| |
+----+
|
===
GND
(created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)


Jamie

Version 4
SHEET 1 880 680
WIRE 80 48 32 48
WIRE 224 48 160 48
WIRE 304 48 224 48
WIRE 224 64 224 48
WIRE 32 96 32 48
WIRE 304 96 304 48
WIRE 240 144 224 144
WIRE 224 192 224 144
WIRE 32 208 32 176
WIRE 304 240 304 192
WIRE 304 240 288 240
WIRE 304 256 304 240
WIRE 224 336 224 288
WIRE 256 336 224 336
WIRE 304 336 256 336
WIRE 256 368 256 336
FLAG 32 208 0
FLAG 256 368 0
SYMBOL voltage 32 80 R0
WINDOW 123 0 0 Left 2
WINDOW 39 0 0 Left 2
SYMATTR InstName V1
SYMATTR Value 6
SYMBOL npn 240 96 R0
SYMATTR InstName Q1
SYMATTR Value 2N2222
SYMBOL res 208 48 R0
SYMATTR InstName R1
SYMATTR Value 12k
SYMBOL npn 288 192 M0
SYMATTR InstName Q2
SYMATTR Value 2N2222
SYMBOL res 288 240 R0
SYMATTR InstName R2
SYMATTR Value 30
SYMBOL current 80 48 R270
WINDOW 0 32 40 VTop 2
WINDOW 3 126 153 VBottom 2
WINDOW 123 0 0 Left 2
SYMATTR InstName I1
SYMATTR Value PULSE(5m 100m 0 1m 1m 10m 20m)
SYMATTR SpiceLine load
TEXT 299 -18 Left 2 !;tran 0 1 0 100u
TEXT -200 -24 Left 2 !.dc I1 5m 30m 500uA
 
R

Rocky

Jan 1, 1970
0
Il 15/10/2012 21:25, Rocky ha scritto:


The problem is not the voltage drop, but it should work down to few
mAmps without affecting in any way the linearity. That is if the current
source sources 6.84 mA or 7.53 mA I must get 6.84 and 7.53 mA at the
output. This over the whole 4-20 mA range.

Marco

Well, its 2 components, easy to build and cheap - maybe hard to
simulate if you don't have the LM317L model, but being 2 terminal it
won't affect the current from a current source. For 'guaranteed'
stability I put a 100nF cap across the circuit.
 
M

Marco Trapanese

Jan 1, 1970
0
Il 15/10/2012 23:13, John S ha scritto:
Oh! I just realized you were looking at a different post by whit3rd. His
10/13/2012 3:12 PM post is the one I was referring to. Hopefully I have
his permission to partially re-post the important part:


Yep, I saw the wrong post :)
Sorry!

*****************************************************************
There's a classical answer, using TL431 programmable zener

(warning: bad ASCII art, use monospace font)


(+)
|
+-------+
| /
| /
R1 |
+----|
| |>
| \
| |
| |
| |
/---/ |
/ \ |
/ \-----+
--+-- R2
| |
+-------+
|
(-)

I_limit = 2.5V /R2 + (minor correction for zener bias)

which can be redone with an LM4041 for smaller Vref, using a PNP
transistor...
the range of 23... 30 mA should be easy to hit with 2V or so of voltage
burden.


Thank you!
Marco
 
J

josephkk

Jan 1, 1970
0
Not sure as to what you mean by that last statement? The operation of
that circuit does depend on a bjt characteristics. The Vbe and
Vce(sat) combined will give much less over loss than what you would get
with a jFet or linear regulators

Vce(Sat) should be ~170mV and Vbe being around 0.7v to a sum of 0.870v
loss in this type of circuit.

R1 supplies minimum current in series with R3 to achieve ~ 20mA at
around 6 volts in the loop supply. WHen loop supply is up to 24 volts,
the 2.5V ref will clamp that and attempt to give a steady reference to
the base via R3.

bjt's work in the same manner as fets do with the exception of how
they are biased of course. With a fet, it's all about the Vgs and the
V(Th) with the Vgd that turns it into a constant current device, but the
problem his is, you normally have to get up in the Vg(th) region for it
to start working, that could be higher than the losses you wish to
sacrifice.

With bjt's, it's always current biased instead of voltage and the
voltage here starts ~0.7v and is repeatable within reason between batches.

Jamie
Tech, not engineer; but then again you admit that don't you.

?-)
 
J

josephkk

Jan 1, 1970
0
Il 15/10/2012 21:25, Rocky ha scritto:



The problem is not the voltage drop, but it should work down to few
mAmps without affecting in any way the linearity. That is if the current
source sources 6.84 mA or 7.53 mA I must get 6.84 and 7.53 mA at the
output. This over the whole 4-20 mA range.

Marco

Ahhh. Then what you want is a 4-20 mA transmitter IC. Readily available.

?-)
 
M

Marco Trapanese

Jan 1, 1970
0
Il 16/10/2012 07:09, josephkk ha scritto:
Ahhh. Then what you want is a 4-20 mA transmitter IC. Readily available.


Nope.
I want just what I asked :)
A bipole which limits the current over 20 mA and which is completely
transparent for currents below that.

Marco
 
M

Marco Trapanese

Jan 1, 1970
0
Il 15/10/2012 18:10, Marco Trapanese ha scritto:
Well well well, the simulation is very nice. Tomorrow I'll change the
pcb to try this version.


It works like a charm up to 75 °C! The current changes only a little:
from 24.1 mA @ 18 °C to 26.8 mA @ 75 °C.

This time the correspondence between the simulation and the real circuit
is quite impressive!

Power of the emitter resistor.

Marco
 
M

Marco Trapanese

Jan 1, 1970
0
Il 16/10/2012 02:22, Jamie ha scritto:
It's strange that you experienced that much of a change in Beta in the
short time, I've never experienced that much of a change, but then
again, you did add that extra R that I didn't put there.


If you're referring to the R in the bottom is the simulation of the load.

All is not lost however. Below is a circuit that was posted by Phil H
which is something of an old relic but it seems to work nicely with out
the use of a reference. It uses the Vf of the base and also with the way
it's configured it'll counter balance the beta while self heating takes
place.

As Q1 self heats, the Vf (base) will drop and thus Beta will increase.
When this happens however, current limit will increase, but it'll also
force Q2 to pull R1 down, causing a - feed back effect.


If you add ".step temp 0 80 202 to the spice's directives you will see
how much change the Ilimit w/ temperature (about 5 mA).
Using the circuit proposed by JohnS the current changes about 1 mA only.

It makes the things easier to match the constraints dealing with
tolerances and differences among batches.

Bye
Marco
 
J

Jamie

Jan 1, 1970
0
Marco said:
Il 16/10/2012 02:22, Jamie ha scritto:




If you're referring to the R in the bottom is the simulation of the load.





If you add ".step temp 0 80 202 to the spice's directives you will see
how much change the Ilimit w/ temperature (about 5 mA).
Using the circuit proposed by JohnS the current changes about 1 mA only.

It makes the things easier to match the constraints dealing with
tolerances and differences among batches.

Bye
Marco
That last circuit is not a simulated run, that current limited is a
classic and i've used it many times over the years. I don't know why
you're having such wandering current problems like that, it does not
happen in real life when I use them and they get used in many places
that we employ control units that have no current limiting.

I beginning to think you are using different parts and different
scenarios..

Use what ever works for you,.

Jamie
 
J

josephkk

Jan 1, 1970
0
Il 16/10/2012 07:09, josephkk ha scritto:



Nope.
I want just what I asked :)
A bipole which limits the current over 20 mA and which is completely
transparent for currents below that.

Marco
It has volts of overhead but it will do just that. Just setup the inputs
for full scale (20 mA).

?-)
 
J

Jamie

Jan 1, 1970
0
josephkk said:
It has volts of overhead but it will do just that. Just setup the inputs
for full scale (20 mA).

?-)

I put together the 2 btj version here at home base using 2222's, 10k
and 31 ohm I r.

I did this early this evening, I applied enough to generate 100mA and
it started at 20.6mA using a 15 volt supply. It has been 1.5 hours and I
just looked at the amp meter, it's reading 20.7ma and flickering back
down to 20.6 mA. that maybe no more than 100uA at most..

When I first tested this, I put my hot air wand on it for a short time
to get the btj's hot to the point of near destruction, I saw no more
than 2mA increase. The - feed back (not voltage but effect) seems to
work nicely as expected.

I just can't see how a few here are seeing outrageous readings as they
are? Makes me wonder at times.

I think maybe some ot to go back to the actual proto testing and stop
believing what the sim states. The sim is not always correct.

Its like saying it has to be true if it's on the internet..

I only did this for my own curiosity because we have used that type
of current limiter for many things over the years and still have them
operating in circuits made years ago. I don't remember ever doing a
temperature test, ambient or junction, on this limiter.

I am satisfied with it, the rest can do as they please :)

Btw, this was the one that Phil H originally posted if I remember..
old relic. The circuit, not him :)


Jamie
 
M

Marco Trapanese

Jan 1, 1970
0
Il 17/10/2012 00:20, Jamie ha scritto:
That last circuit is not a simulated run, that current limited is a
classic and i've used it many times over the years. I don't know why
you're having such wandering current problems like that, it does not
happen in real life when I use them and they get used in many places
that we employ control units that have no current limiting.

I beginning to think you are using different parts and different
scenarios..


Perhaps we miserunderstood something :)
I built your first circuit (bjt + zener) and it changed too much the
current over temperature span.
Then, I modified it with the suggestions of John S and now it doesn't
change the current but 1-2 mA. In both cases the measures are very close
to the sim outputs.

I didn't build your second circuit (2 bjts). I've just simulated it. I
wrote it's better than the first one but it exhibits a "not so small"
temperature dependency. Of course, this time the simulation couldn't be
so accurate.

Thank you anyway because (currently) I solved using a circuit based upon
you first idea.

Marco
 
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