Potentiometers and computer Power Supply

[CRE]

Hi everyone,

First off I love this site! I found it a few days ago and it has been a huge help in creating a circuit I've wanted to build for a while.

Here's the situation: I'm building a voltage meter using a LM348 (quad op amp) as a comparator. I have three reference voltages to set. I was originally going to use resistors to divide the voltage but found that usings pots would be much better since I can adjust the references as needed. Now for the questions, I have 2 100K pots rated for .1W, I know the op amp's power requirements are very low so I don't think the small pots will be a problem.

I understand that if I wire one leg to ground, the other leg to my PSU's +5v line and use the wiper as output I'll get my divider (so that when centered/balanced the output will be ~2.5v). The first question is do I need to worry about this putting a load on my PSU? You know in a way it can't handle? It's capable of ~40A on the 5v rail. Also, would this be too much for the post to handle? As I understand it the low currentdraw from the op amp's inputs shouldn't pose a problem, but if there is a possibility could I perhaps buffer the pots using a resistor on the wiper or one on each of the main pins?

The second part is this, I need three reference points; one high, one mid, and one low. Can I wire the high and low using pots as described above and simply use two equal resistors between the output of both pots to get a consistant mid point? So that if I were to adjust the high vref higher, the midpoint would follow maintaining it's position between the two? If so, does it matter what size resistor? I'm sure it does to a degree but would 1K suffice?

Thank you!

 

[Ldanielrosa1]

I recommend against having one reference dependant upon another. This can cause a lot of frustration when trying to adjust them.

The power supply isn't likely to notice the current required by the references. And using an opamp as a unity gain follower should do a fair job of buffering the references.

 

[CRE]

Thanks for the reply, I did experiment with dividing the output of the two pots for a mid point but I found that I do prefer the extra control attained by using a pot on the mid vref as well. I think from my experiments today that I may need to actually go with 1K pots, the 10K are just too sensitive for this project (serves me right for using dirt cheap single turn pots). Anyway, The drop from 3 10K pots to 3 1K pots between the +5v and ground won't cause an issue will it? Like I said before, I'm a newbie with only enough knowledge to do damage.... albeit in very interesting ways that my parents never would have imagined. ;D

This circuit is rather impractical for what I'm using it for but I set out to do this project using an op amp/comparator do see if I could actually get it running. Well, it works!!!! Now I just need to work the bugs out. And if in the end I end up using something along the lines of a LM3914 well, I'll have a very nice little battery tester! 8)