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From: "Boris Nogoodnik"
Subject: Re: Limiting total wattage for a constant-current dummy load?
Organization: KGB, Ltd.
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Date: Sat, 09 Nov 2002 21:41:13 GMT
NNTP-Posting-Date: Sat, 09 Nov 2002 16:41:13 EST
"Bill Shymanski" wrote in message
> Hmm. I don't like the analog multiplier idea; too many parts.
> 1. Try putting some kind of temperature sensor on the heat
sink. If the
> pass transistor
> gets too hot, it could take the circuit out of constant
current mode or at
> least show an
I've been thinking about it too. But if you apply full 50 volts
at 10 A preset limit you will get 500W for some time before it's
get too hot for the thermistor to sense it.
> 2. You could probably come up with a comparator and two inputs
> together, one
> proportional to current and one to voltage. When the sum of
> scaled values exceeds
> a set point (50 watt eqivalent), it could trip or alarm. This
> old-time arc furnace regulators
> used to work; they *really* wanted to control the quotient of
> divided by arc current, but
> in actuality responded to the difference of scaled voltage and
> current. The effect was
> practically the same.
Addition won't really do it. You must multiply to measure
power. On the other hand you can make a variable current limier
based on the input voltage only.
> "John Muchow" wrote in message
> > I've got a 1.2-50V (approx.), 0 -10A constant current dummy
> > circuit (MAX480 op-amp, ICL8069 reference, and IRF540 for
> > working beautifully, but I have a problem I can't seem to
> > solution for.
> > I'm limited to about 50 watts for the load right now with
> > MOSFET device and heat sink I'm using now. But, I can use
> > with a current "setting" and load voltage that can
> > exceed the 50 watt limit if I'm not careful to figure out
> > in advance.
> > 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
> > 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
> > to determine the wattage the load would have to dissipate
> > start the darn thing up.
> > I'd love have an LED come on that told me "power is too
high" if I
> > selected a current level that would exceed 50 watts (or any
> > value) with the voltage present at the MOSFET load.
> > Can anyone point me in the right direction? I couldn't find
> > on the IR, National, Zetex, TI, Fairchild, or Linear Tech.
> > Thanks!
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