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Old 11th December 2013, 01:51 PM   #1
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Default Cermet or conductive plastic

I realize this topic has come up before, but could not find the answer in any earlier thread.

For a tone control to adjust an active speaker for room placement, I need to incorporate a potentiometer. In order to minimize noise and feed the next stage with a low source impedance, I want to limit the resistance of the pot to 2.5K. This obviously drives currents up. I simmed the maximum current to pass through the pot to be 10 mA @ 3 V. To dissipate 30mW in a closed pot will heat up things, so tempco obviously is of importance.

If this were a stationary voltage divider, I would no doubt choose fixed resistors with a low tempco and known low excess noise. However, because it has to be variable, I am stuck with using a pot and this is where there seem to be two options:

Cermet, with 150 ppm/C, or conductive plastic, with 1000 ppm/C. Both are lousy, but the figure for conductive plastic almost takes it into thermistor territory.

Given these figures, it would seem to be a no brainer to go for cermet, but I have no clue as to the excess noise generated by the two technologies.

It would be much appreciated if someone could fill me in.
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Old 11th December 2013, 06:48 PM   #2
AndrewT is offline AndrewT  Scotland
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Assuming a maximum input signal from a cdp of around 2.2Vac, then a 2.5k pot will have to dissipate ~1.9mW
The average dissipation is likely to be much less than 500W

The heating effect is so close to zero that ppm tempco is almost irrelevant.
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Old 11th December 2013, 09:20 PM   #3
Johno is offline Johno  Australia
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Embarrassing moment = most of your current is flowing thru the capacitor, not the potentiometer!
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Old 11th December 2013, 09:39 PM   #4
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Quote:
Originally Posted by Johno View Post
Embarrassing moment = most of your current is flowing thru the capacitor, not the potentiometer!
Well, not really. It's not quite the schematic you might think of.
filter.jpg

R80 and R81 are two legs of a pot. Current through R80 is shown in this sim. I can change it for a fixed resistor though, also better for other reasons, so that should take care of that part of the problem.

Still interested to know if measurements exist of excess current in cermet vs conductive plastic, just for the sake of making things better.
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Old 12th December 2013, 10:17 AM   #5
AndrewT is offline AndrewT  Scotland
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2.2Vac into 2k5 is ~ 0.88mA
That is well within the capability of a volume pot.
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