From: "Bill Shymanski"
Subject: Re: Limiting total wattage for a constant-current dummy load?
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Date: Sat, 9 Nov 2002 09:02:06 -0600
NNTP-Posting-Date: Sat, 09 Nov 2002 09:01:58 CST
Organization: MTS Internet
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
gets too hot, it could take the circuit out of constant current mode or at
least show an
2. You could probably come up with a comparator and two inputs summed
proportional to current and one to voltage. When the sum of the two
scaled values exceeds
a set point (50 watt eqivalent), it could trip or alarm. This is how
old-time arc furnace regulators
used to work; they *really* wanted to control the quotient of arc voltage
divided by arc current, but
in actuality responded to the difference of scaled voltage and scaled
current. The effect was
practically the same.
"John Muchow" wrote in message
> I've got a 1.2-50V (approx.), 0 -10A constant current dummy load
> circuit (MAX480 op-amp, ICL8069 reference, and IRF540 for the load)
> working beautifully, but I have a problem I can't seem to find a
> solution for.
> I'm limited to about 50 watts for the load right now with the single
> MOSFET device and heat sink I'm using now. But, I can use the load
> with a current "setting" and load voltage that can significantly
> exceed the 50 watt limit if I'm not careful to figure out the wattage
> 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 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.
> 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 arbitrary
> value) with the voltage present at the MOSFET load.
> Can anyone point me in the right direction? I couldn't find any info
> on the IR, National, Zetex, TI, Fairchild, or Linear Tech. web sites.