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Dummy making a dummy load box

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bdivi:
Another take on the topic :-)

https://www.eevblog.com/forum/projects/have-you-built-a-resistive-load-bank-ideas/msg2148061/#msg2148061

Doctorandus_P:
Now this is a resistor:

https://www.aliexpress.com/item/Ceramic-2500W-Tube-Wirewound-Resistance-used-as-dump-load-resistors-braking-resistance/32831870690.html

In some of these tubular resistor there is a 3rd terminal in the middle with a screw and these can be adjusted...
Found an example on:
https://www.aliexpress.com/item/WaveTopSign-High-Power-Porcelain-Tube-Adjustable-Resistance-RX20-Ceramic-Resistor-Power-300-1500W-Resistance-Value-0/32966105005.html
And apparently these go upto 10kW, and they have a short time overload capability of 100x...

H713:
Not really so much of a box, but it works pretty well. It's spaced off the back wall of the bench by about 1/2 inches to allow air ventilation (also I was too lazy to mill out pockets for the nuts on the back). Those are 8 ohm "100W non-inductive" resistors from China. I haven't manage to blow them up yet. If I want a 4-ohm dummy load, I connect the second resistor in parallel with clip leads. If I don't need to test two channels at once, I can go up to 32 ohm or down to 2 ohm.

Nobody ever said a dummy load had to be smart. When I get around to re-doing my bench, I'm going to make a rackmount dummy load that will have more options, including 8 ohms at 1500W for load testing big amps, and some higher values for load testing transformers and power supplies. And It's going to have a fan so I never have to stop and wait for it to cool down like I do now.

Another type I've seen for lower values in physics labs is a big spring-like coil of nichrome wire submerged in a tube of water. Supposedly they can take a tremendous amount of wattage due to the high specific heat of water. Just be sure to top it off every now and then.

schmitt trigger:
A "What if" type of question to the cognoscenti in this board:

If you were to do a binary-weighted resistor load (1, 2, 4, 8 ohms and so forth) to be wired in series to obtain discrete arbitrary values.... how would you allocate the resistor power rating for each?

I would assign the highest power rating to the lowest valued resistor.
Let's say you want a load with a maximum 20 volt input voltage. Then the 1 ohm device would be 400 watts, the 2 ohm 200 watts, 4 ohm would be 100 watt and so forth.

Would this be a sensible approach?

SiliconWizard:

--- Quote from: schmitt trigger on January 30, 2019, 08:19:11 pm ---If you were to do a binary-weighted resistor load (1, 2, 4, 8 ohms and so forth) to be wired in series to obtain discrete arbitrary values.... how would you allocate the resistor power rating for each?

I would assign the highest power rating to the lowest valued resistor.
Let's say you want a load with a maximum 20 volt input voltage. Then the 1 ohm device would be 400 watts, the 2 ohm 200 watts, 4 ohm would be 100 watt and so forth.

Would this be a sensible approach?

--- End quote ---

That sounds about right.

If you have a chain of resistors in series, the same current will go through all of them. For each of the resistors Rk, the worst case will be when it's the only one that is used, thus Pmax = Umax^2 / Rk.

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