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From: Grant on 4 Jul 2010 14:21
On Sun, 04 Jul 2010 09:35:35 -0700, John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote:

>On Sun, 04 Jul 2010 23:29:26 +1000, Grant <omg(a)grrr.id.au> wrote:
>
>>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.
>
>You could eliminate D3, R3, C2, C3, and R10. But it would be fairly
>slow. A fast load box probably needs opamps.

Yeah, right -- and let the gate drive go up to 24 to 30V on no load?
No thanks.

Grant.
From: Grant on 4 Jul 2010 14:22
On Mon, 05 Jul 2010 02:17:35 +1000, David Eather <eather(a)tpg.com.au> wrote:

>On 4/07/2010 11:29 PM, Grant wrote:
>> 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.
>
>
>Would a small cap between GND and the junction of R4 and The mosfet gate
>be of any use? SOmetimes a stopper resistor (R4) is not enough on its own.

Might try that once I can do some dynamic load testing, thanks.

Grant.
From: Grant on 4 Jul 2010 14:28
On Sun, 4 Jul 2010 08:08:12 -0700 (PDT), dagmargoodboat(a)yahoo.com wrote:

>On Jul 4, 10:53 am, John Larkin
><jjlar...(a)highNOTlandTHIStechnologyPART.com> wrote:
>> On Sun, 4 Jul 2010 00:36:50 -0500, "Tim Williams"
>>
>> <tmoran...(a)charter.net> wrote:
>> >Your MOSFET is shown as a P type??  33N10 certainly isn't..
>>
>> I hate that mosfet symbol. I draw mosfets as
>>
>>             |
>>           |-
>> --------| |
>>           |>
>>             |
>>
>> and
>>
>>             |
>>           |<
>> --------| |
>>           |-
>>             |
>>
>> which is much more obvious.
>>
>> John
>
>I use the old symbol. I don't like it, but there it is.
>
>Yours looks a little like an IGBT. Might that cause confusion?

I use something like what I see in the data books, at the risk of
being confused about the arrow direction as I was this time ;)

Tim's explanation that it shows the reverse biased junction diode
at least makes sense, rather than confusing with IG whatevers.

Grant.
From: Grant on 4 Jul 2010 14:47
On Sun, 04 Jul 2010 09:35:35 -0700, John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote:

>On Sun, 04 Jul 2010 23:29:26 +1000, Grant <omg(a)grrr.id.au> wrote:
>
>>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.
>
>You could eliminate D3, R3, C2, C3, and R10. But it would be fairly
>slow. A fast load box probably needs opamps.

Twice you've suggested slugging a large cap directly across the TL431,
I'm wary of doing that because the MOSFET is inside the feedback loop.

What am I missing here?

Grant.
From: Jim Thompson on 4 Jul 2010 14:57
On Mon, 05 Jul 2010 04:17:10 +1000, Grant <omg(a)grrr.id.au> wrote:

>On Sun, 04 Jul 2010 09:50:57 -0700, Jim Thompson <To-Email-Use-The-Envelope-Icon(a)On-My-Web-Site.com> wrote:
>
[snip]
>>
>>(1) Insert moderately high value resistor between pot ("VR1") tap and
>>"R" terminal of TL431
>
>I did in new circuit, 200 Ohm, seems low but then there is 17mA going
>past the 'R' terminal.

The "R" terminal itself is relatively hi-Z, just like an OpAmp input

>>
>>(2) Then a cap "K" to ""R"
>>
>>The TL431 is, after all, an OpAmp of sorts :-)
>
>Yes, I tried the opamp classic style controller feedback: small cap
>plus larger cap with series resistor, surprisingly it worked on
>first try values, but might be underdamped. I'll get back to that
>when I make a load delta switcher, just an oscillator driving a
>big MOSFET switching in 5 or 10 Ohms extra load. Need one of them
>anyway for testing responses, last one I made added voltage into
>feedback loop, hardly reusable.
>>
>>Caution! Still a potential for oscillation... the cap from "K" to "R"
>>may need a small series resistor to introduce a "zero"/phase-lead,
>>because of the MOSFET capacitances.
>
>I don't see any oscillation, I've got a noisy mains ground here so
>there's much noise on the CRO even when the thing is powered
>off :( And the thing is powered from batteries isolated from earth.
>
>
>I'm renting, so there's not much I can do about the bad earth --
>did ask for an electrician to check it years ago, he pulled earth
>wire out of the neutral bar, measured resistance between neutral
>and earth and pronounced it okay! Ignoring the fact two other
>units on same earth point were looping earth via their neutral
>blocks.
>
>Hardly a proper earthing test, and we have odd law here that says
>bad earthing that met standards in place when the units were built
>is okay -- even though running an earth connected to water pipe,
>and going through taps before the pipe goes underground breaks
>modern rules.
>
>Grant.

...Jim Thompson
--
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