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From: Grant on 4 Jul 2010 03:20
On Sun, 4 Jul 2010 00:36:50 -0500, "Tim Williams" <tmoranwms(a)charter.net> wrote:

>Your MOSFET is shown as a P type?? 33N10 certainly isn't..

Oops, I do mean N type, the part number is correct, last time I was
drawing MOSFETs, they were high side switching with P-channels, didn't
notice I drew the arrow wrong way round here.
>
>Compensation across the '431 is the way to go. Besides 10nF, you may
> need series resistance to its input, to set the RC time constant. I
> don't know what R5 and R6 are;

Your eyes probably didn't believe the numbers 20, 12, 12 on the diagram
for trimpot, R5 and R6, the values are in Ohms. I did mention 17mA
going down that part. Yes, it low, I wanted fairly high current
through there to swamp the 20 Ohm trimpot's lower spec tolerance
so the adjustment range worked as expected (it did :)

> R7 is small enough at 150 ohms to
> be tough to drive.

Not from the ~2.5 Ohm source, it isn't ;)

> It's not like the input takes much current, 10k
> ohms is plenty.

Well, I put 200 Ohms in there between R9 and TL431.R, to get
something to work against, then added 1.0nF from R to K, plus
15nF & 3k0 from R to K.

Just come back to the PC now to see if anyone replied to my post,
after adding some caps and stuff to the circuit. Funny thing is
that after writing up the problem, I had to problem guesstimating
a set of values that stabilised the thing first try, when I went
back to the soldering iron.

I think it was a confidence issue, because last time I tried to
stabilise a linear MOSFET power amp (8 n-channels in parallel) the
thing simply didn't work out for me. That was 18 months or two
years ago, and I think I was given some logic threshold devices
to work with.

This current sink is now over-compensated (I think), but I can't
tell until I make a little load switcher to give the thing a step
change so I can watch the response and fine tune it.

Drifts only about 1mV when it warms up, another mV variation from
short circuit to 20 Ohms load. So I think it will be okay for
what I want.

I'm expecting to trim compound power resistors to about one percent
accuracy with this current source, so it is good enough for that.

The compound resistors I'll be trimming only need to be accurate
enough for monotonic operation over a 6 bit range, so aiming for
1% should be okay. Unless, of course, I discover it's not okay.

Grant.
From: Grant on 4 Jul 2010 03:23
On Sat, 3 Jul 2010 21:06:16 -0700, "Michael Robinson" <kellrobinson(a)yahoo.com> wrote:

>
>"Grant" <omg(a)grrr.id.au> wrote in message
>news:furv26hrvj3q0hfve2bkv6crkmh001ruso(a)4ax.com...
>> Hi there
>>
>> I've built a one amp current sink and at present it is uncompensated
>> and it is performing quite nicely as a 200kHz power oscillator, with
>> about 2V p-p across the 2.579 Ohm compound current sense resistor.
>>
....
>I recommend a cap from K to R on the TL431. 10 nF is probably more than
>enough.

Thanks, see my reply to Tim, I put 1.0 nF in there, after adding 200 Ohm
to R9 for the cap to act against.

Grant.
From: Tim Williams on 4 Jul 2010 03:39
"Grant" <omg(a)grrr.id.au> wrote in message news:utb03650jqn6ambteks0c0fgpkv0pqu85m(a)4ax.com...
> The compound resistors I'll be trimming only need to be accurate
> enough for monotonic operation over a 6 bit range, so aiming for
> 1% should be okay. Unless, of course, I discover it's not okay.

And if it's not, you can always go cascode.

You could even make a "corrected cascode", so an op-amp drives the cascoding transistor so as to keep its source at a constant voltage. Something you're already doing with this transistor, so you just do it twice. That should essentially eliminate early effect from consideration. As is, TL431's gain should be more than high enough to not care about this yet.

I'm sure you've taken stock of such things as tempco? A fan might not be a bad idea; turbulence is supposed to make things much noisier, but that should still be less error than the error due to tempco, if the resistors are getting very warm. The resistors down inside that monster stack will run slightly warmer without forced air. Maybe not by much.

The TL431 itself is only about 1% (2.475-2.525V), so you'd need a much better reference to do anything more accurate anyway. At this point you start getting into precision *everything*, calibration procedures and, ehh maybe not as far as tracable references...

Tim

--
Deep Friar: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms
From: Grant on 4 Jul 2010 07:22
On Sun, 4 Jul 2010 02:39:42 -0500, "Tim Williams" <tmoranwms(a)charter.net> wrote:

>"Grant" <omg(a)grrr.id.au> wrote in message news:utb03650jqn6ambteks0c0fgpkv0pqu85m(a)4ax.com...
>> The compound resistors I'll be trimming only need to be accurate
>> enough for monotonic operation over a 6 bit range, so aiming for
>> 1% should be okay. Unless, of course, I discover it's not okay.
>
>And if it's not, you can always go cascode.
>
>You could even make a "corrected cascode", so an op-amp drives the
> cascoding transistor so as to keep its source at a constant voltage.
> Something you're already doing with this transistor, so you just do
> it twice. That should essentially eliminate early effect from
> consideration. As is, TL431's gain should be more than high enough
> to not care about this yet.

MOSFET has Early effect? Or, do you mean the TL431?

I was concerned the TL431 wouldn't have enough gain, but it seems
good enough for this task.

>
>I'm sure you've taken stock of such things as tempco?

No, such things I measure or get a feel for once built ;)

>A fan might not be a bad idea; turbulence is supposed to make
> things much noisier, but that should still be less error than
> the error due to tempco, if the resistors are getting very
> warm. The resistors down inside that monster stack will run
> slightly warmer without forced air. Maybe not by much.

The resistors sharing 2.5 Watts, 104mW each. Yes, they feel a
little hotter than the heatsink, maybe a fan _is_ good idea,
particularly if I'm using the thing in summer, it gets to 40'C in
here. At the moment, my heater is struggling to get the place
up to 20'C ;)

The controller is bolted really close to the heatsink, there
is a noticeable drift on warmup, but only a mV or so. And, I
measure shunt voltage at the sensing end, not the where the
current is being carried -- four wire measurement.
>
>The TL431 itself is only about 1% (2.475-2.525V), so you'd
> need a much better reference to do anything more accurate
> anyway. At this point you start getting into precision
> *everything*, calibration procedures and, ehh maybe not as
> far as tracable references...

Texas TL431B I'm using here is 2.495V +/- 12mV, 0.5% initial
accuracy, (TL431A is 1%, standard part is 2% -- TI datasheet)
plus I trim the current via voltage across the 0.1% sense
resistor, so circuit accuracy depends on the 0.1% shunt resistor
and tolerance of Fluke 4 1/2 digit multimeter (it _is_ quite
an old meter though, Fluke Model 8050A).

The circuit drifts about 1mV across the shunt for warmup, another
mV for load varying between near zero to 20 Ohms, and another mV
for line regulation (24.8 to 29V on charge, but it's not normal
to charge the batteries whilst using them for this sort of thing,
also not good for the batteries either as the charger not smart
enough to compensate for a load).

Since current is measured at about 2.5 mV per mA through the
shunt, the circuit is accurate to something better than 2mA
plus 0.1% shunt and <mumble> tolerance for the meter used to
adjust the shunt voltage to match the calculated value (2.535V).


I don't know how to tell LTSpice how to model the TL431 and
trust I'd get an accurate simulation. So it's the old soldering
iron method for me. I think it doesn't do trimpots either, but
it's been a while since I use LTSpice. Might have a go at it
later.

Grant.
From: Grant on 4 Jul 2010 09:29
On Sun, 04 Jul 2010 12:46:36 +1000, Grant <omg(a)grrr.id.au> wrote:

>I've built a one amp current sink and ...
....
It's now compensated and on overnight test :) New circuit is up too.

http://grrr.id.au/current-sink/CurrentSink-circuit-2-750x567.jpg

or, see http://grrr.id.au/current-sink/ for the revised web page.

Restoring C1 across the 13V zener had no effect, adding supply bypass
and the usual feedback compensation components worked first try. The
circuit may be under-damped, I need to make a load step switcher so I
can see how the circuit responds for dynamic response, another day.

Makes small sparks when shorting the output with a nickel plated
alligator clip to stainless steel resistor lug. Would you expect
that from a one Amp constant current sink running from 24V?

Power supply is fairly stiff, a pair of 12V, 100AH SLA batteries
through a 35A auto spade style fuse and a couple or three metres
of cable.

Grant.
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