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I need to make a pi attenuator which must support voltages of about 100 V and 20A but only on a pulse of 250ns.

But I do not know what resistor to use because it creates a power of about 2kW but only 500uJ.

All the power resistors that I see are sized according to the power and not the energy so I do not know how to choose.

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  • \$\begingroup\$ What's the pulse-pause ratio? \$\endgroup\$
    – Janka
    Aug 21, 2019 at 14:25
  • \$\begingroup\$ Pulse power rated ones? Isabellenhuette? Bourns WS? Vishay CRCW-HP? \$\endgroup\$
    – winny
    Aug 21, 2019 at 14:35
  • \$\begingroup\$ It's a manual test, so about 1 second between pulses \$\endgroup\$
    – Adrien
    Aug 21, 2019 at 14:38
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    \$\begingroup\$ Normally a wirewound, where the energy gets distributed uniformly in a mass of metal is good for pulse power, however your 250nS pulse length may be above the frequency that most wirewounds go to. The problem with many resistors are that they are adjusted to final value by laser cutting a slot, which promotes failure at the end of the slot. High power resistors are adjusted by thinning the film, more expensive.What's the tolerance required, maybe bulk carbon would be adequate? You may be forced to test several resistors. Just buy a few metal film ones, and see if they work, what's to lose? \$\endgroup\$
    – Neil_UK
    Aug 21, 2019 at 14:43
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    \$\begingroup\$ Keep looking. Many, many power resistors are specified in their data sheets on energy as well as power. \$\endgroup\$
    – Andy aka
    Aug 21, 2019 at 15:22

2 Answers 2

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@AndyAka got to me first on this part: this is a common issue, and many power resistors are specified for this.

What you need to do is get on the net and start slogging through resistor data sheets. Start with power resistors, and possibly with ones that are rated for pulsed power. You're looking for a chart in the data sheet that relates either average power to pulse time, or that gives you the energy of the allowable pulse vs. time.

Not every manufacturer will help you out with this, and it's not something that is put front and center in marketing material. So you gotta dig. And absolutely positively don't assume that once you choose a resistor you can just let your purchasing department buy any old resistor with the same size and power rating -- you'll need to specify that resistor.

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A big wirewound resistor won't have any trouble handling your pulse power requirements, but it would be quite inductive, even the "non-inductive ones" and that may interfere with your short pulse duration.

You can also use a carbon composition resistor, these have huge pulse power handling capabilities, but they're not very stable or accurate.

I'll assume you need low inductance, therefore I'll go with SMD chip resistors.

Go to DigiKey, and make sure to check "Pulse withstanding" in the "features" box. Then select value, power, etc like any other resistor.

Here's an example resistor datasheet with the specs you're looking for.

Now go to page 5...

enter image description here

According to the text below the graph, this gives pulse power for a continuous pulse application as long as dissipated power is within spec (this is the case here). Your pulse duration is below 1µs, so one of these 2512 resistors should be enough (barely).

However, the datasheet guarantees a "permissible resistance change" of +/- 4% which may be too much for your application, especially if you're making a precision attenuator.

Solution:

1) Use higher precision resistors (I selected the cheapest).

2) Use a number of them in parallel or in series to spread voltage and power. They're cheap, if you can use 10 resistors instead of one, they'll get 10x less peak power each, so they'll have an easier time and drift less. Also, lowering the pulse power requirements opens more choices of more accurate resistors.

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