PI trimmer power rating

[sunnydaze]

Hello,

Would appreciate if one of the more knowledgeable members could explain how to determine the minimum power requirement for a potentiometer (trimmer) as used as a phase inverter balancing trimmer.

I know the basic ohm's law and power formula - just not sure how to use them in this case. Not sure if I'm only concerned about the voltage drop across the trimmer or the input voltage. Specifically, I'm wondering about the minimum for a 10K trimmer on the PI plate resistors in a circuit with around 300VDC going into the trimmer. The voltage drop across the pot on the high side is around 10 volts or so, depending on the setting.

Assuming the setting is in the middle on a linear pot, this would be 10VDC / 5,000 = .002 (2 ma). I thought I was supposed to multiply this times the input voltage of 300VDC to calculate the dissipation = .6 watts.

But the part listed in the Dumble 124 BOM, is rated much lower than that at .15W, aside from that, the spec sheet shows its a carbon type. Makes me wonder if I should be calculating differently.

http://nl.mouser.com/ProductDetail/Pihe ... nCesZF4%3d

Seems like a lot of DC voltage on a carbon trimmer

What am I missing?

Thanks
Mike

[Roe]

the pot doesn't see 300v, its more like 10v. So 10x10/5000=.02A (2mA).

Wattage: 2ma x 10v = 20mW

[sunnydaze]

the pot doesn't see 300v, its more like 10v. So 10x10/5000=.02A (2mA).

Wattage: 2ma x 10v = 20mW

Thanks - it's the voltage drop across each side of the pot that is the concern rather than the input voltage. I guess this trimmer is actually in series with the plate resistors (one on each leg) which would allow for more power handling too. I guess another way to look at it is usually 1/2 watt resistors are OK for 12ax7 plate resistors - the trimmer in series needn't be very large.

Any comments on carbon versus cermet in this application?

Thanks

Mike

[TUBEDUDE]

Cermet is considered to be more stable.

[sunnydaze]

Cermet is considered to be more stable.

That's what I thought. Shouldn't have much impact, if any on the sound - the trimmer is before the plate resistors.

Thanks
Mike