Digital Voltmeter Help - Solve ATX PSU Issue

In summary: Yes, there is conductance between the negative side of the +5V power supply and the negative sense input. So the meter's negative supply is the same node as the meter's negative sense input. Don't drive it with anything and you'll be fine. If you connect only the +12V supply to the positive sense input, the meter should correctly read +12V.
  • #1
madhatter4
4
0
Hello all,

I made a custom power supply using a standard ATX computer PSU. I took the -12V and +12V leads to get 24V then put a LM338 on the positive side for a 1.25V to 22V adjustable ouput. Works great.

I wanted to put a digital voltmeter on the adjustable output so I would know what voltage was being supplied at the banana terminals. I bought an LED digital panel meter from ebay and wired the voltmeter supply to +5 and ground from my moded ATX PSU. When I connect the adjustable +12V and -12V leads from my PSU to the sense leads on the voltmeter my PSU shuts off. I am guessing its because of a short caused by the negative sense lead and the supply ground being connected somewhere in the voltmeter PCB. If I only connect the + lead from the adjustable output to the voltmeter I get a reading of about half what I should as its only measuring from the + side of the adjustable output.

Is there a simple way to either isolate the voltmeter power or trick the meter into thinking it is seeing an additional +12V? Would adjusting the Vref on the voltmeter do the trick?

Thanks for any help you can offer.
 
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  • #2
Try using your voltmeter to measure the voltage on a standard 1.5V AA battery first, to make sure it's functioning properly.

- Warren
 
  • #3
I have. If I use a separate power supply for the volt meter and connect both sense inputs from the adjustable output the Voltmeter reads correct.

My problem is that I want to use the +5 and ground from the same power supply as the -12 and +12 that I want to measure.

Here is a pic of the meter I am using.
ME-PM105-2.jpg


PSU +5 goes to meter 5V+
PSU ground goes to meter 5V-

This is a diagram of my ajustable output that I want to measure. This is from the same PSU that supplies +5 to the meter.
http://jon.simnets.com/wiki/uploads/admin/LM338K_Voltage_Regulator.jpg


PSU +12 goes to meter + IN, actually I am reading the output of the LM338
PSU -12 goes to meter - IN, When connected the PSU shuts off, when not connected the meter reads half of the actual voltage because its reference is from ground instead of -12V.

I am trying to find away around this. Could an opamp be used to combine the +12 and -12 then adjust its output to be double what the output of the LM338 is?

Im trying to explain as best I can. Sorry if I am confusing.
 
Last edited by a moderator:
  • #4
If you're applying +12V and -12V across the meter (24V total) across your meter, it appears you are exceeding it's range. The one you've pictured appears to have a range of only 20V.

Use a handheld ohmmeter to see if there any conductance between the negative side of the +5V power supply and the negative sense input.

- Warren
 
  • #5
The total voltage is outside the meter range but I won't be using above 20V vary much if ever. The meter will display a - if it is over range.

I have not tested to see if there is conductance between the negative side of the +5V power supply and the negative sense input, but from the DIY panel meter circuits I have been looking at they are. I will test mine when I get home.

The chip in my meter is an ICL7107 and I am assuming its wired smiler to fig 15 on page 12 of the data sheet but for a 20V scale. I have tried to figure out how mine is wired but its hard to follow traces that run under the LED segments on my meter.
http://www.intersil.com/data/fn/fn3082.pdf"
 
Last edited by a moderator:
  • #6
I have tested my panel meter and there is conductance between the negative side of the +5V power supply and the negative sense input.
 
  • #7
So the meter's negative supply is the same node as the meter's negative sense input. Don't drive it with anything and you'll be fine. If you connect only the +12V supply to the positive sense input, the meter should correctly read +12V.

- Warren
 

Related to Digital Voltmeter Help - Solve ATX PSU Issue

1. What is a digital voltmeter?

A digital voltmeter is an electronic device used to measure the voltage level of a circuit or component. It typically has a numeric display to show the voltage reading.

2. How can a digital voltmeter help solve an ATX PSU issue?

A digital voltmeter can be used to test the voltage levels of the different components in an ATX PSU, such as the power supply, motherboard, and graphics card. By comparing the readings to the recommended values, it can help identify any faulty components that may be causing the issue.

3. What are some common issues with ATX PSUs that a digital voltmeter can detect?

Some common issues that a digital voltmeter can help detect in an ATX PSU include overvoltage, undervoltage, and voltage fluctuations. It can also identify if there is a problem with the power supply itself, such as a blown fuse or faulty components.

4. How do I use a digital voltmeter to troubleshoot an ATX PSU issue?

To troubleshoot an ATX PSU issue using a digital voltmeter, you will need to first disconnect the power supply from any devices and unplug it from the power source. Then, using the voltmeter, you can test the voltage levels of the different connectors and compare them to the recommended values. This will help identify any abnormalities or faulty components.

5. Can a digital voltmeter be used for other types of power supplies?

Yes, a digital voltmeter can be used to test the voltage levels of various types of power supplies, including ATX, ITX, and SFX. However, it is important to make sure that the voltmeter is set to the correct range and that you are familiar with the specific voltages and connectors for the type of power supply you are testing.

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