Potential divider wattage calculations

[netdudeuk]

Hi

I'm looking at using a split rail supply (+-12V or +-9V) with a potential divider using a 1kΩ trimmer resistor (https://uk.rs-online.com/web/p/products/5220883/) to produce a voltage output within that range.  This would be to create a variable offset bias for an op amp.

Here are my calculations to work out the wattages -

+-12V supply

I=V/R
I=24/1000
I=0.024A

P=VI
P=24*0.024
P=0.576W


+-9V supply

I=V/R
I=18/1000
I=0.018A

P=VI
P=18*0.018
P=0.324W

This makes me think that a 1/2W trimmer would be ok for the +-9V supply but not for the +-12V supply.  Do I have this right ?

I guess that going for a 10kΩ trimmer instead of the 1kΩ would allow me to use this trimmer resistor with either voltage setup ...

[LateLesley]

yes, your calculations look right. Is there any particular reason to go with such a low resistance trimmer though? if it's feeding the bias if an op-amp, a 10K trimmer would be good enough, and you may even get away with 100K. The benefit of this is it reduces power consumption, and the power dissipated in the trimmer. I'd try and pick as high as a value that you can get away with. Op-amp inputs tend to be in the megΩ range, so you shouldn't need too low a value trimmer. I'd try 100K, if it's just for a voltage divider to an input.

[netdudeuk]

yes, your calculations look right. Is there any particular reason to go with such a low resistance trimmer though? if it's feeding the bias if an op-amp, a 10K trimmer would be good enough, and you may even get away with 100K. The benefit of this is it reduces power consumption, and the power dissipated in the trimmer. I'd try and pick as high as a value that you can get away with. Op-amp inputs tend to be in the megΩ range, so you shouldn't need too low a value trimmer. I'd try 100K, if it's just for a voltage divider to an input.

Thanks for the prompt reply.  I've ordered a couple of 100k and a couple of 10k to experiment with.

[rs20]

It can also be a good idea to have a capacitor between the wiper of the trimmer and ground. If the value of the capacitor is appropriately chosen (there's a wide range of valid values, just apply T=RC with the value of the trimmer as R as a rough approximation), this will have virtually no effect on the responsiveness of the adjustment (at least as far as a slow human observer can tell), but can calm down any scratchiness coming from the trimmer, and also kills any RF noise on that highish impedance node.

[netdudeuk]

It can also be a good idea to have a capacitor between the wiper of the trimmer and ground. If the value of the capacitor is appropriately chosen (there's a wide range of valid values, just apply T=RC with the value of the trimmer as R as a rough approximation), this will have virtually no effect on the responsiveness of the adjustment (at least as far as a slow human observer can tell), but can calm down any scratchiness coming from the trimmer, and also kills any RF noise on that highish impedance node.

Thanks for the tip.  I'll do that.