100W Rheostats on ebay, has anyone used them?

[ScottBurr]

I'm working on a project that requires some very high power variable resistors for test purposes, I found these on ebay, the price seems to good to be true, has anyone ever bought / seen one?

Thanks in advance,

Scott

https://www.ebay.co.uk/itm/100W-100-Ohm-Ceramic-Wirewound-Potentiometer-Rotary-Resistor-Rheostat-FK-/232369880225?_trksid=p5731.m3795

[hayatepilot]

These look like old stock that someone found in a warehouse. Just look at the bent mounting lug.
I like that all 3 pictures are the same, just rotated a bit. 

They where probably used in passive loudspeakers to adjust the bass/treble content.

[Brumby]

Old stock for this sort of thing might actually translate into something decent - assuming they haven't been abused...

[MarkL]

I've used them for power supply testing.  They work better in some cases than an electronic load.

The only thing I would point out is that they do have current limits, which you should take into account when selecting the right model.  For example, you can't set a 100W 100 ohm rheostat to 10 ohms, put a continuous 3A through it, and expect it to last even though that's technically less than 100W.  The localized heating might burn out or deform the resistance wire.

[ScottBurr]

@MarkL

Thanks for the response, agree with your point, so you have actually bough some off this seller? The quality is OK?

Thanks,

Scott

[MarkL]

I bought a bunch of 100 ohm 100W years ago, similar to the picture, but it wasn't from that seller.  Sorry if I misunderstood your question and you wanted to know about that specific seller.

The ones I bought were made by Ohmite and were good quality.

[ez24]

I have had good luck with these

https://www.aliexpress.com/item/W-5-OHM-High-Power-Wirewound-Potentiometer-Porcelain-Adjustable-Resistor/32356477023.html

As someone said pay attention to power.  So at 2 1/2 ohms this is rated at 50watts.

[Brumby]

The only thing I would point out is that they do have current limits, which you should take into account when selecting the right model.  For example, you can't set a 100W 100 ohm rheostat to 10 ohms, put a continuous 3A through it, and expect it to last even though that's technically less than 100W.  The localized heating might burn out or deform the resistance wire.

The simplest approach I can suggest is that you run with I²R to get the current and use that as the limiting factor.  100W for a 100 ohm load is 1 amp.  So don't let your current get over 1 amp and you should be fine.

Going over 1 amp will mean you need think about what you are doing a lot more - and that you might be risking damage to the rheostat.

[CopperCone]

What exactly causes spot heating on a rheostat thats made of pretty thick drawn wire?

[Gyro]

The wiper contact.

[MarkL]

Yes, the wiper contact can.  But if you stay under the current spec, it should be able to handle it with minimal heating.

What I was referring to was that if you set the rheostat to some value in between the end points, you're only putting current through part of the resistance winding.  The heat created is then only dissipated by the portion of the ceramic body where the current is running.  (Assuming a two terminal connection to the rheostat.)

Unless specified otherwise, the power rating assumes the heat is dissipated over the whole winding and body.  So in the example I gave above, if you dissipate 100W through 1/10 of the full winding, it's going to get a lot hotter in that 10% of the winding and possibly damage it.

That's all.  I was just saying to take the current into account in rheostat applications as well as the power.  The rating considerations are a little different than a fixed resistor.