Trimpot in Parallel to a Resistor

[Brushthrower]

Hi,

Please excuse me if this is an obvious question, but I couldn't find information on this elsewhere. Could some kind soul explain to me how to solder a trimpot in parallel to an already existing resistor. Having trouble figuring out what to do with the third leg. A diagram would be extremely helpful (being the visually orientated person that I am).

Thanks!

[John Lyons]

I depends what you are trying to do with the trim pot.
Are you trying to change the resistance or?
Let us know and we can help you out.

john

[earthtonesaudio]

To make a fixed resistor smaller using a trimpot, connect the wiper of the trimpot (usually lug 2) to one end of the fixed resistor, and one of the remaining lugs (1 or 3) to the other end of the fixed resistor.  This will let you change the value of the fixed resistor from the new value (the fixed and variable resistors added in parallel), all the way down to zero.

Also it doesn't hurt to connect together lugs 2 and 3 (or lugs 1 and 2) of the trimpot, which can help reliability in case the trimpot ever has some malfunction.

[frank_p]

Take a look at this topic also, it might help:
http://www.diystompboxes.com/smfforum/index.php?topic=70732.0

[Brushthrower]

Hey,

Thanks for the responses everyone. I'll give that method a shot, Alex.

To answer your question John: I've done a "Creamy Dreamer" mod to my Russian BMP with good results, but it also had the side-effect of making it noisier when idle. I read a suggestion that placing a trimpot in parallel to the 100k resistor to ground would create a noise gate and thus rectify this problem. Thought I'd give it a shot.

[aron]

Remember that all it is doing is lowering the value of the total resistance to lower than the trim pot value.

(from the purchasing parts page)
You can put resistors in parallel to get other values.
    For 2 values: RT = (R1 X R2 )/ (R1 + R2)
    For 2 or more: RT = 1 / ( 1/R1 + 1/R2 + 1/RN ...)

[Brushthrower]

So, instead of putting resistors in parallel, lowering the value of the resistor should have the same effect?

Hmmm...the trimpot compromised the sound of the pedal as I was worried it would, giving it more of a...well...flatulent sound. Not quite what I wanted. Any suggestions on reducing the amount of ambient noise while still maintaining the original sound?

Thanks,

Richard

[John Lyons]

Post or link a schematic and we can see what you have going.
Otherwise it's hard to speculate. 

john

[aron]

>So, instead of putting resistors in parallel, lowering the value of the resistor should have the same effect?

Yes. Think about what it might be doing. It might be mis-biasing the transistor so it's cutting off at low signal levels. Yes, you might not like that sound. I guess you could try metal film resistors and lower noise transistors.

[Spuzzum]

I know this is an old post, but...

Putting resistors in parallel with a trimpot has other benefits as well.. such as bulking up the power rating of a 1/2w pot by adding 1/2w resistors in parallel.

I'm currently building some constant current LED drivers, that require a fixed resistance calculated by whatever the out current you're trying for. At 1000mA, it requires a 0.5ohm resistor, and burns 1/2w. But to be "safe".. it should be a 1w or higher. Pretty straight forward, but I want an "adjustable" constant current driver. So.. instead of paying $12 for a Bournes 1w trimpot, I've decided to use a 1K 1/2w trimpot, in parallel with (3) 1K 1/2w resistors. This makes a 250ohm 2w trimpot/variable resistor.. at less than $5.