Is it line ( 60 Hz ) or other/variable frequency?
Current/voltage range ?
How much distortion is acceptable?

The best solution seems to be to use pulse width modulation
at a much higher frequency and then -- if necessary --
filter it out.

Plesae read FAQ240-1032
My WEB: <http://geocities.com/nbucska/>

Iam dealing with audio frequencies and I need the device to act as a resistor and offer a resistive load in the circuit.

Can you tell me more about your application?

Plesae read FAQ240-1032
My WEB: <http://geocities.com/nbucska/>

I am loading audio power amplifiers with a voltage controlled load to confiqure impedence and distribute power in conjunction with a transformer system, but first I need to get the voltage controlled resistor element sorted. An inverse parallel pair with blocking diodes is about the best I have tried but it is not great!

The amplifier usually loaded with matched impedance to
maximize the power. Adding resistor just wastes power.

What do you have, what are you trying to do and what are the data ?

Plesae read FAQ240-1032
My WEB: <http://geocities.com/nbucska/>

Why must you use a FET, can't use a Bipolar for some reason?

Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com

I am maintaining correct loading on the amps but with a dummy load. The useage of the amplifiers is independent of any requirements of effeciency etc. I am primarily concerned with the dummy load and am trying to use mosfets instead of wirewound resistors as a means of dumping unwanted amplifier power. The sound qualities of the amplifiers are monitored and need to be undamaged by the process. The application is a bit weird and esoteric but the principle of mosfets operating as resistors is well used at low power for signal control and at high power for dc control. I need ac at high power! Any thoughts?

Quote:

...am trying to use mosfets instead of wirewound resistors as a means of dumping unwanted amplifier power

Why? It's more expensive and more complex, and in all likelihood less linear too.

What's so wierd about the application? This isn't one of the 'wierd' things that make audio electronics different to all other branches of electronics, is it?

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  I don't suffer from insanity. I enjoy it...

To use conventional resistors I need also heavy duty switching to select the combinations, so 50w resistors and 10A switching, and how do I come up with a constant impedence variable level control? I started with resistors etc. and cannot get what I want that way. A 30A 125W mosfet costs about 50p so I am trying to find a way of using them as variable (or pre setable) power resistors that work with musical signals.
The application is for musicians, how weird do you want?!

nbucska's first idea is a good possibility.

Try to write a spec of what you are trying to build in terms of required voltage / current / power / linearity / THD / frequency range / cost etc. Feeding it out piecemeal doesn't make it easy to see the whole picture.

As for musicians being wierd, absolutely!

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  I don't suffer from insanity. I enjoy it...

particularly when they spell "weird" differently from everyone else

As for the IRF540, http://www.semiconductors.philips.com/acrobat_download/datasheets/IRF540_S_1.pdf shows NOT a zener but the substrate diode.  Also note that unless you keep the part nailed to 25ºC, you'll have to derate its power capability

likewise,
http://www.irf.com/product-info/datasheets/data/irf540n.pdf shows on the bottom of page 2 that International Rectifiers doesn't know the difference between a diode and zener.

It's unlikely that you'll find a power MOS device with a floating substrate.

TTFN

MYSELF WHEN YOUNG DID EAGERLY ... work in a service.
A man came in with a special request: as a musiclovor
with perfect hearing he wanted perfect sound -- he has
read somewhere about gluing a piece of eggshell into the
middle of the dynamic loudspeaker to improve the high
frequencies.

During the argument we connected a loudspeaker to a
generator -- he couldn't hear 12 kHz...
 

Plesae read FAQ240-1032
My WEB: <http://geocities.com/nbucska/>

I think I have found a workable solution, it enables me to get rid of modulation at the gates and schottky rectifiers by-passing the substrate diodes should reduce distortion, I shall check things out and report back.                                                                                                     
 

Hey, I never claimed I was normal... and at least this isn't the Engineering Language & Grammar Skills forum!

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  I don't suffer from insanity. I enjoy it...