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From: Tony Williams
Subject: Re: An interesting little prob.
Date: Thu, 31 Oct 2002 08:32:20 +0000 (GMT)
References: <firstname.lastname@example.org> <email@example.com>
NNTP-Posting-Date: Thu, 31 Oct 2002 08:32:23 +0000 (UTC)
User-Agent: Pluto/1.14i (RISC-OS/3.60)
In article ,
Winfield Hill wrote:
> . program --, current sources
> . | /
> . raw _|_/ 0-2A -> ===== L1 current
> . Vin --x-|_|_|-----------/////--------x----(O) supply out
> . | _|_ 0-50mA -> |
> . x-|_|_|-------, D1 D2 |
> . '----/\/\-----x--|>]--x---[<|--x
> . | | |
> . ,--[150k]--' x---, |
> . | | | |
> . | | \ V | |
> . VDAC--- | --|+ \ |/ | |
> . 0-5V | | ---------| | |
> . x---|- / |\ | |
> . | | / Q1 \ | | | \
> . | PNP | | '--|+ \
> . [49k9] 0V | | --- high for
> . | '-------|- / CV mode
> . 0V | /
> The Q1 PNP shunt transistor gives me pause. It must be able
> to handle 10W (for Tony's 500mA 20V rating) to say 40W, and
> is no doubt a darlington type.
Darlington (or similar) yes. But why 40W?
> Consider the change in Vbe when going from conducting the
> low standby bias current to conducting the full clamp current:
> this can be considerable, say 200mV or more. That's not very
The V-clamp is primarily a safety clamp in my application.
As noted earlier, less-than-perfect is an acceptable
price for having a nice quick low-R guillotine.
We can even operate in an acceptable CV-mode if we need to.
This is a big machine, with (literally) yards of cabling
between the stimulus and the load. It is routine to have
kelvin connections, read actual load voltages with IEEE DVMs,
and let the computer calculate the (Ax+B) control characteristic
of any stimulus.... all done online in 100mS or so.
(IEEE488 is magic stuff.)
> I'm also not very happy with the imprecise CV mode detection.
> Maybe Tony will come up with solutions, as he so often does.
Connect the -ve of the comparator to the top of D1?
In fact, I have to bring actual Vout back down for
an ADC to report back to the remote controller. So
it can be tapped off before the ADC and compared with
the Vout setpoint that the DAC originally commanded.
Ok. Thanks to all for the suggestions and the points
of focus. This job will probably surface as something
working (or not of course) in a few weeks. I'll report
on what resulted.
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