Improvised power resistor

You can wind nichrome wire around a ceramic core.

I'm curious if anyone will mention how a power mosfet can be turned into an active resistor.. D from BC

This would be more appropriate in sci.electronics.basics

Depends what resistance you want and how much inductance and temperature drift you can tolerate.

100meters of thin insulated wired on a drum typically has a resistance of a few tens of ohms but has signfiicant inductance. You can usually get access to both ends of a wire on a drum to connect drums in series or parallel.

The lead from a graphite pencil is a few ohms end to end.

Adam

Depending on the value, salt water in a ceramic mug. Power capacity is strictly a function of how hot you will let the water get and the thermal transfer characteristics of the ceramic.

Jim

On a sunny day (Mon, 28 May 2007 06:48:35 +1000) it happened Thomas Carlan wrote in :

Sure 20-50W lightbulb.

(So now 'what resistance', also note a lightbulb when cold has a 10 times lower resistance then when warm).

Or you can wind your own resistors using special wire with a low temp coefficient.

Probably the best way is to do a web search for "electronic load". Lots of info and schematics out there.

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I heard of one guy who took one of those hollow tubular resistors and ran Surgical Tubing down the center of it then circulated water through the tubing and back to a large reservoir kept cold.

Don't know how well it'd work.

Robert

Any kind of metal will work as a (generally low-value) resistor. Steel strapping, for example, but the resistance won't be very constant with temperature.

Best regards, Spehro Pefhany

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A long length of hookup wire. A light bulb. A couple of wires in a bucket of water.

^ salt or hard

Jim

Metal ribbon.

It's useful to put some of the story here on s.e.d. First, we'll make the MOSFET's current follow Ohm's law for a resistor, I = V/R, using a current-sense resistor Rs, and a resistive divider, R1/(R1 + R2).

--+--------------+- V | | R2 | | __ ||-' +-----|+ \\__||-, | ,--|-_/ | | |___________| R1 | | Rs | | --+--------------+---

If we select Rs so that R = Rs (1 + R2/R1), and also R1>>Rs, we'll have made a nice active power resistor. It's necessary to add an RC to the opamp output, to prevent oscillation, as we've written about here many times before, but that's the basic idea. If the R1 R2 voltage divider ratio involves a multiplying DAC, it's easy to create a digitally-programmable power resistor.

It doesn't matter much so long as it isn't super pure.

Yes.

Stainless steel safety tie wire works well, if you don't need very high resistance values. Supporting the wire so it can heat air or water without shorting is more trouble than just making resistance. e.g.

What range of resistance are you needing, and energy at what frequency do you need it to absorb?

I've been using a 50W tubular resistor as a test load recently.. I got it dissipating 100Watts..so I keep it in a bucket of water... D from BC

Cool! Looks like a great way to dodge buying power resistors. D from BC

I liked all the suggests posted..wires, ribbons, ionic liquid, bulbs ..etc.. But I gotta add one obvious wacky suggestion. (IFAIK..not posted yet..) Using lots and lots and lots of 1/4W resistors together!!!

D from BC

How do you insulate the electrical connections from the Water?

Robert

Pure water is a poor conductor. All my connections to the power resistor are bare.. I put it in a plastic bucket. The only thing bad that happened was a little rust and there's steam. D from BC