Prev: Modules dying when 24V vehicles are jump started
Next: Where is Archimedes Lever??
From: John Larkin on 4 Jul 2010 12:35
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.

John

From: Jim Thompson on 4 Jul 2010 12:50
On Sat, 03 Jul 2010 20:19:47 -0700, John Larkin
<jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote:

>On Sun, 04 Jul 2010 12:46:36 +1000, Grant <omg(a)grrr.id.au> wrote:
>
>>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.
>>
>>Following text is based a screen scraping of a web page I just put
>>up, along with some pictures to describe this issue at:
>>
>> http://grrr.id.au/current-sink/
>>
>>I've not done much with MOSFETs in linear amplifier mode, so some
>>suggestions for stabilising this one Amp current sink circuit are
>>needed.
>>
>>The circuit uses an N-channel MOSFET for the pass element, a 0.5%
>>accurate TL431B reference and a couple dozen 0.1% resistors provide
>>for accurate current sensing with a 2.579 Ohm shunt.
>>
>>The circuit 'works' in the sense that the magic smoke stays where
>>it belongs and no components get overly warm. The circuit needs
>>stabilising as the thing is at present uncompensated and oscillating
>>around 200kHz.
>>
>>The circuit diagram:
>>
>> http://grrr.id.au/current-sink/CurrentSink-circuit-750x736.jpg
>>
>>Circuit description: Fixed 1.000 Amp current sink. Of that one Amp,
>>983mA goes through the current sense resistor, the remaining 17mA
>>current goes via a trimpot scaling circuit to feed the TL431B's
>>Ref pin. A 13V zener limits gate drive voltage during circuit
>>saturation, and a 110 Ohm gate resistor is placed close to the
>>MOSFET.
>>
>>A bank of 24 by 61R9, 250mW, 0.1% resistors to form R9, a 2.579
>>Ohm, six Watt current sense resistor. Calibrating the current
>>sink to 1.000A is done by adjusting the trimpot to so that 2.535V
>>is measured across the current sense resistor, with a 4 1/2 digit
>>Fluke multimeter.
>>
>>Photo of the oscillation waveform:
>>
>> http://grrr.id.au/current-sink/CurrentSink-waveform-R9.jpg
>>
>>The oscillation waveform shows about 2V p-p across the 2.579 Ohm
>>current sense resistor. The MOSFET is happy enough on its large
>>heatsink, it gets very warm to the touch but it's not too hot
>>for continuous operation.
>>
>>Photo of the built circuit:
>>
>> http://grrr.id.au/current-sink/CurrentSink-photo.jpg
>>
>>Photo of the thing shows placement of the MOSFET gate resistor
>>up close to the MOSFET gate where it belongs. The large heatsink
>>allows continuous operation, as the input voltage may go up
>>to 30V, from the nominal 24V.
>>
>>
>>What's needed are suggestions for what stabilising components
>>to use around the TL431B.
>
>
>Cap across it, from high side to ground.
>
>John

(1) Insert moderately high value resistor between pot ("VR1") tap and
"R" terminal of TL431

(2) Then a cap "K" to ""R"

The TL431 is, after all, an OpAmp of sorts :-)

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.

...Jim Thompson
--
| James E.Thompson, CTO | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona 85048 Skype: Contacts Only | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |

Obama: A reincarnation of Nixon, narcissistically posing in
politically-correct black-face, but with fewer scruples.
From: Tim Williams on 4 Jul 2010 13:37
"John Larkin" <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote in message news:3u71369mnbegb23ur93frj6f22ocdl0t6p(a)4ax.com...
> I hate that mosfet symbol. I draw mosfets as
>
>
>
> |
> |-
> --------| |
> |>
> |
>
>
> and
>
>
> |
> |<
> --------| |
> |-
> |
>
>
> which is much more obvious.

Well gee, that's just patently wrong. The arrow is the substrate, so you've drawn a P type substrate follower. Or something. Which has absolutely no physical meaning whatsoever, and is forward biased, which is completely useless.

On the other hand, this symbol is EXACTLY what it is.
http://webpages.charter.net/dawill/tmoranwms/Circuits_2010/Current_Sink.png
Gate: insulated. In order of contact with the gate oxide, the source, substrate and drain all appear on the other side. Source and substrate are tied by wiring. Substrate is a diode junction. The line and arrow mean "diode". That diode is reverse biased. FETs are not minority carrier devices, so that arrow MUST point backwards (except for cases like synchronous rectifiers, when you are using it backwards).

I don't suppose you use the symbols that show the extra zener as well? I've always been tempted to rebel against that symbol by drawing the substrate 'dash' with zener wings.

And you could draw a MESFET with a schottky "long S" gate, same as schottky BJTs. It would even be acceptable to use one line instead of a gate and three dashes, since they're pretty much all the same hunk of material, no insulators.

Tim

--
Deep Friar: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms
From: Grant on 4 Jul 2010 14:17
On Sun, 04 Jul 2010 09:50:57 -0700, Jim Thompson <To-Email-Use-The-Envelope-Icon(a)On-My-Web-Site.com> wrote:

>On Sat, 03 Jul 2010 20:19:47 -0700, John Larkin
><jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote:
>
>>On Sun, 04 Jul 2010 12:46:36 +1000, Grant <omg(a)grrr.id.au> wrote:
>>
>>>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.
>>>
>>>Following text is based a screen scraping of a web page I just put
>>>up, along with some pictures to describe this issue at:
>>>
>>> http://grrr.id.au/current-sink/
>>>
>>>I've not done much with MOSFETs in linear amplifier mode, so some
>>>suggestions for stabilising this one Amp current sink circuit are
>>>needed.
>>>
>>>The circuit uses an N-channel MOSFET for the pass element, a 0.5%
>>>accurate TL431B reference and a couple dozen 0.1% resistors provide
>>>for accurate current sensing with a 2.579 Ohm shunt.
>>>
>>>The circuit 'works' in the sense that the magic smoke stays where
>>>it belongs and no components get overly warm. The circuit needs
>>>stabilising as the thing is at present uncompensated and oscillating
>>>around 200kHz.
>>>
>>>The circuit diagram:
>>>
>>> http://grrr.id.au/current-sink/CurrentSink-circuit-750x736.jpg
>>>
>>>Circuit description: Fixed 1.000 Amp current sink. Of that one Amp,
>>>983mA goes through the current sense resistor, the remaining 17mA
>>>current goes via a trimpot scaling circuit to feed the TL431B's
>>>Ref pin. A 13V zener limits gate drive voltage during circuit
>>>saturation, and a 110 Ohm gate resistor is placed close to the
>>>MOSFET.
>>>
>>>A bank of 24 by 61R9, 250mW, 0.1% resistors to form R9, a 2.579
>>>Ohm, six Watt current sense resistor. Calibrating the current
>>>sink to 1.000A is done by adjusting the trimpot to so that 2.535V
>>>is measured across the current sense resistor, with a 4 1/2 digit
>>>Fluke multimeter.
>>>
>>>Photo of the oscillation waveform:
>>>
>>> http://grrr.id.au/current-sink/CurrentSink-waveform-R9.jpg
>>>
>>>The oscillation waveform shows about 2V p-p across the 2.579 Ohm
>>>current sense resistor. The MOSFET is happy enough on its large
>>>heatsink, it gets very warm to the touch but it's not too hot
>>>for continuous operation.
>>>
>>>Photo of the built circuit:
>>>
>>> http://grrr.id.au/current-sink/CurrentSink-photo.jpg
>>>
>>>Photo of the thing shows placement of the MOSFET gate resistor
>>>up close to the MOSFET gate where it belongs. The large heatsink
>>>allows continuous operation, as the input voltage may go up
>>>to 30V, from the nominal 24V.
>>>
>>>
>>>What's needed are suggestions for what stabilising components
>>>to use around the TL431B.
>>
>>
>>Cap across it, from high side to ground.
>>
>>John
>
>(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.
>
>(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.
From: Grant on 4 Jul 2010 14:17
On Sun, 04 Jul 2010 10:15:09 -0500, John KD5YI <mycalldotjocjodotjocjo(a)xoxy.net> wrote:

>On 7/4/2010 6:22 AM, Grant wrote:
>
>(snip)
>
>> 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.
>
>
>Use the LT1431 which comes with LTSpice. Look in the Op-Amps folder.

Thanks.
>
>Cheers,
>John
First  |  Prev  |  Next  |  Last
Pages: 1 2 3 4 5
Prev: Modules dying when 24V vehicles are jump started
Next: Where is Archimedes Lever??