how to make 100W non inductive resistor

Not a "specification that can be met" NUMBERS????

You can bifilar wind a wirewound resistor and hook the windings together at one end so the coupled inductances cancel...sort-of. NUMBERS???

, to the point where a 6 foot long resistor would be a-okay

NH-series:

Also comes in non-inductive:

On a sunny day (Wed, 10 Feb 2010 16:11:38 -0800 (PST)) it happened " snipped-for-privacy@wwc.com" wrote in :

In the early days we made a light dimmer for stage lights by cutting of the end of a fluorescent tube, filling it with water and salt, and hanging a piece of iron in it from a wire, making it a variable resistor.

Joerg gave you an elegant solution, but those look a bit pricey. If you want a less elegant but cheaper DIY alternative, you could take nichrome heater coils and wind them non-inductively on a ceramic former.

Well, you can't have more than 100pF across your 1500 ohm resistor, nor more than 6.4uH in series with your 40 ohm resistor, otherwise it looks more reactive than resistive. If you need reactive current under 10%, cut these by 10 and things look more impressive: a 250W vitreous resistor will easily run 10pF to ambient, more if shielded; and it has to use less than 32" of wire (assuming 20nH/inch). A noninductive winding will fair somewhat better.

If your signal is 1.0MHz sine, you can cancel any annoying inductance or capacitance with some more of the other type. OTOH, if it has harmonics, you're nearly SOL because they will be affected the most by parasitics.

Tim

Packed carbon granules in a non conductive tubing.

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Lets say the solution ends up being some kind of linear element (wire, nichrome, carbon granules in tube etc...) but its a few feet long. Can I reduce the inductance by sliding the whole assembly inside a close fitting tube made of mu metal?

No, not really. It'll greatly increase the capacitance though (I'm guessing very roughly 100pF?). Mu metal isn't what you want anyway, it doesn't work above 10kHz, let alone 1MHz.

You can, however, reduce the inductance by making it physically small. They make thick film resistors that are expensive as hell but would do nicely at high frequency. Downside is you need $500 of them to make the series you want...

What frequencies and waveforms are you using? What accuracy? Can you cancel reactance with tuning?

Tim

Odd requirement. You can chain a lot of resistors (say, 100 15 ohm

Inductance can be low, OR you can choose to model the individual taps' voltage drops as I*R + L* dI/dt, and note that a bit of added mutual inductance, in series with the tap wiring, can cancel the L-containing term.

Because power is dissipated in the resistors, there WILL be a B-field at the resistor surface, and that means there IS always some inductance. Zero inductance means no B field, no B field means Poynting's vector is zero, and Poynting's vector surface integral around a resistor is the power dissipation (which we know is NOT zero).