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From: Tony Williams
Subject: Re: Limiting total wattage for a constant-current dummy load?
Date: Sat, 09 Nov 2002 09:11:25 +0000 (GMT)
NNTP-Posting-Date: Sat, 9 Nov 2002 09:10:50 +0000 (UTC)
User-Agent: Pluto/1.14i (RISC-OS/3.60)
In article <email@example.com>,
John Muchow wrote:
> How can I allow both extremes of current/voltage (10A at up to 5V, or
> 1A(at up to 50V) to flow in this circuit but still limit the overall
> wattage dissipated? I can easily control the current to limit it to
> 10A through the IRF540, but I'm stumped as to how to use the voltage
> to determine the wattage the load would have to dissipate *before* I
> start the darn thing up.
P-Fet = V-Fet * I-Fet. So you do need to do an
analogue multiplication of those two variables.
What sort of analogue multiplier you can use at
this stage depends on what supply rails you have.
The most limited case would be if you just have
a single-rail, probably +12V approx.
The simplest analogue multiplier is a potentiometer
and the simplest mod to do in the single rail case
is to swap your I-control pot for a dual gang pot.
Fet Source +---/\/\---+---/\/\---+---> V-Fet/10
| | ~~~~~~~~
+--|- \ |
V-Fet pickoff | >---+
~~~~~~~~~~~~~ +--|+ /
Fet Drain +---/\/\---+---/\/\---+0v
V-Fet/10>---------+ +-------------/ /<---------->Current control.
Comparator \ \
<----- 0.5V |Dual| 0 Amps
Pick off across the Fet with a differential amp
with a gain of say 1/10. The output of this amp
will then be 5V when V-Fet is 50V.
Put this V-Fet/10 onto the top of the new second
gang of the dual pot. The first gang is exactly
as you had it before.... a Vreference on the top
and the wiper going off to the I-control opamp.
Take the new wiper off to a comparator, whose
other input is set to 0.5V. Make the comparator
give a warning whenever the voltage on the new
wiper tries to exceed 0.5V.
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