3
\$\begingroup\$

I'm an EE, not a physicist or chemist, so please forgive if this question is dumb.

I need a resistor that can dissipate many kW for 10 us, followed by > 10 ms off time.

Since the duty cycle is ~0.1%, I could use a wirewound rated for > 100 W, but the large inductance those things have makes them a bad option. While the bulk is not a problem, the hundreds of dollars they cost is.

I figure if I mix clay from a pottery store with aluminum powder or some other conductor finely ground up the resulting slop could be made to have almost arbitrary resistance. The desired resistance is 50 kohms although it does not need to be very precise. Anything within a factor of 2 would be nice.

I'm also willing to dissolve salt in water and allow the water to vaporize as a cost of doing business. It's not ideal but it's not out of the question either.

Is this in the realm of feasibility or is this all ridiculous? I don't know a lot of lattice structure crystaline chemistry stuff, and sometimes you've go to care about that sort of thing when it comes to conductors.

\$\endgroup\$
14
  • 2
    \$\begingroup\$ Last time I did something similar, we ended up with screen printing conductive paste on ceramic substrates and water cooling them. Controlling the thickness of the printing was the hardest part. If it’s not mass production, there are some regular 1/2 W THT resistors with spectacular high peak power rating, in the order of 100 kW for 1 us. At 1 kW average, you would need a couple of thousands of them though. Give Kanthal or Isabellenhutte a call? \$\endgroup\$
    – winny
    Commented Mar 20, 2023 at 17:47
  • 11
    \$\begingroup\$ 1 MW and a 50 kohm resistor means a voltage of 224 kV (no matter how briefly it is applied). Are you sure you know what you are doing? \$\endgroup\$
    – Andy aka
    Commented Mar 20, 2023 at 17:50
  • 5
    \$\begingroup\$ What are you actually doing? \$\endgroup\$
    – Tim Williams
    Commented Mar 20, 2023 at 18:04
  • 2
    \$\begingroup\$ @JamesStrieter more basic than that: application. What value inductor (L, DCR)? What waveforms? What diode? What switch? Can you even do what you are proposing to do (are there other component limits of which you aren't aware)? Not being an EE or physicist, asking for a specific resistor (especially of such ratings) sounds either very ambitious -- or you might be missing something. \$\endgroup\$
    – Tim Williams
    Commented Mar 20, 2023 at 19:34
  • 2
    \$\begingroup\$ I would look into liquid resistors. Much easier to adjust the resistance by changing electrolyte concentration and/or changing the submerged area of the electrodes. 10kW for 10us is 100mJ which is negligible in the context of e.g. a 5-gallon bucket of electrolyte. \$\endgroup\$
    – vir
    Commented Mar 20, 2023 at 20:02

2 Answers 2

Reset to default
2
\$\begingroup\$

I figure if I mix clay from a pottery store with aluminum powder or some other conductor finely ground up the resulting slop could be made to have almost arbitrary resistance.

This is actually pretty much what ceramic composition resistors are, which are commercially available. They have a very high pulsed power rating, comparable to the old fashioned carbon composition resistors (which for some reason aren't made anymore--I'm not sure why); they can typically handle many hundreds or even thousands of times their continuous power rating in pulsed applications. (They may be rated in pulse energy rather than power, as that's what the fundamental limitation is.)

If it's also a high voltage application (which I assume it must be for several kW into several kΩ), you may need to spread the voltage out over multiple resistors. You could make your desired 50 kΩ by putting fifty 1 kΩ resistors in series, for instance, and provided the PCB layout is appropriate, you could get fifty times the working voltage compared to a single resistor.

\$\endgroup\$
2
\$\begingroup\$

There's a type of resistor that's made exactly for applications like yours (pulsed, low inductance, high power): Metal film resistors in transistor packages.

Here's an example of a 20W type that can take extreme pulses such as the one you expect to dissipate in the resistors: https://www.vishay.com/docs/50005/rto20.pdf

Depending on the actual voltage you expect to encounter, you could put 10 to 20 of them in series, giving you a total voltage rating of 5kV to 10kV and a power handling capability of 200W to 400W. The momentary overload capability of this resistor string would approach 100kW for 10µs, according to the datasheet. (Each resistor can handle 20W continuously, >5kW in 10µs pulses, and 500 Volts)

Keep in mind that you have to cool the resistors appropriately. Additionally, if the voltage spikes exceed 2kV, you have to put insulating pads between the resistors and the heatsinks to avoid dielectric breakdown of the resistor's package. Alternatively, you can tie the heatsinks to one leg of each resistor (or group of resistors) to lower the voltage between the resistor and heatsink. Note that this puts the heatsinks at high voltage.

\$\endgroup\$
1
  • \$\begingroup\$ Try reading the question again maybe. Then read my comment under the question. I might have misread what the op said but, so might you. \$\endgroup\$
    – Andy aka
    Commented Mar 20, 2023 at 18:03

Not the answer you're looking for? Browse other questions tagged or ask your own question.