Hello,

This might be actually rather foolish idea, but by all means I`ll give it a shot

What I need is something that can compare certain voltage (with some offset). Say I want to test the 12V line from a typical PC PSU. If the voltage is 12V (or anything between 11.5V to 12.5V), the output should be "floating", otherwise should be near ground.

The same thing for 5V and 3.3V lines...

Thank you

 

Have a look at Rob Paisley's comparator page:
http://home.cogeco.ca/~rpaisley4/Comparators.html
About halfway down the page there is a circuit called "Voltage Window Detector Circuit"
It's a window comparator. You can almost use that circuit, except since your supply voltage may vary, you will need a fixed reference voltage at the point labeled "V - REFERENCE 2". A Zener diode from that point to ground would create such a fixed voltage reference.

 

I was actually pretty sure that I need to use a comparator, but this circuit has everything I need.

Thank you

 

...only one question (which might be actually computer related):

As you know, there are several wires coming from the PSU that deliver the same voltage (e.g. 12v). My question is: do these wires take the voltage from the same place, so if one wire delivers 12,3 volts, all wires from the same type would deliver 12,3 volts, right?

Thank you.

 

They basically come from the same place, except for the +5v standby, which is always on, and not the same as the other +5v. However, you could always have a bad connection in the harness....

Also, don't forget that you will need to have a load of a couple of amperes on the main +5v before the voltages begin to regulate properly.

 

In case you want to buy instead of make one; Amazon has one I am looking into buying.

http://www.amazon.com/LCD-Power-Supply-Tester-voltage/dp/B001AYEFHY

I think I got the link right.

I just figured it would cost more than $25 for me to make one.

Note: The one thing I dislike is that it likely is putting only a slight load on the PSU.

Tim S.