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PM-1029B meter to variable computer power supply

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wolf9545

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I wanted to make this: How to Convert a Computer ATX Power Supply to a Lab Power Supply - wikiHow so I could have a power supply when I work on electronics.

I then saw this: How to Add Variable Voltage to Your ATX Based Bench Power Supply - wikiHow so I bought all the necessary components. Currently I have everything working. If I turn the pot and put my meter across the output of the variable voltage (the positive probe on my multimeter) and the -12V (for the neg prob) I get a range from 1.2V to 21.5V. So far I am happy.

I thought why not put a meter in my box (I created a wooden box to give me more room) so I don't have to get my multimeter out each time I change the voltage. I found this: https://www.electro-tech-online.com/threads/digital-voltmeter-power-supply.85574/ and it sounded like the exact same thing I was doing.

My problem is that I bought this meter PM-1029B: Circuit Specialists Inc. - LED 5V Common Ground Panel Meter (PM-1029B) but I am having a problem connecting it to the variable circuit. The 5V and ground power the meter fine (it all lights up). You can see where they should connect by looking at this photo: https://circuitspecialists.com/images/PM-1029Bback.jpg. It is the V+ (5V) and the V- (ground) at the bottom left of the image.
Above that is the VIN and the GND connections for the meter. I connected the VIN (output of the variable section) but when I connect the GND to the -12V the whole power supply and everything shuts off. If I wait about a minute I can turn it all back on but the second I re-connect the GND part of the meter it shuts off again.

Any ideas on what might be causing the problems? I am able to submit any photos if needed?
 
The answer lies in the panel meter description "LED 5V Common Ground Panel Meter". It means the measurement input shares a common ground with the meter's power supply. In short (pun intended), V- is connected to GND on the meter board. So by connecting your power supply's -12V line to V- and it's ground line to GND, you have shorted the -12V section of the power supply, and it shuts down, hopefully due to overcurrent protection and not from loss of magic smoke.

You will need a power supply for the meter that is 5 volts above the -12V. You could use an LM7805 voltage regulator with your power supply's ground (0V) as the regulator's input, and the -12V as the ground.
 
The power supply has +12V, +5V, GND, -5V, -12V as fixed outputs.

**broken link removed**

If I am understanding you correctly I would take the ground wire and connect it to pin 1. I would then take the -12V and connect it to pin 2. Then the output (pin 3) would connect to the V- of the meter while +5V would still be connected to the V+ of the meter? Why can't I just take the -5V line and connect it to V-?

Just a reminder the V+ and the V- is just to supply power to the meter. The VIN is the + input for the voltage to be measured and the GND is the ground of the input to be measured.
 
Maybe this will help.
 

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Thank you that does help. Some times a picture just makes trying to explain electronics so much easier. Could you please explain how the LM7805 works? I mean since there is ground and -12V connected to it does it still output the required 5V to power the meter? It has been a while since I worked with electronics and my mind is a bit fuzzy on the "using a positive and negative power to get a voltage.

I think I just did a virtual face-palm in my head. The LM7805 is a 5 volt fixed output right? So the -12V acts as ground and the output of the 7805 will take the Ground and the -12V and convert it into a +5V? Mind still a bit fuzzy. Will I need a heatsink for this or will the metal tab be fine?
 
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Remember that voltage is relative. The original supply had -12V, -5V, 0V, +5V, and +12V. When you re-purpose the supply to get 24 volts from the -12V and +12V lines, essentially you re-assign your "ground" or 0V to the -12V line. So now you have 0V, +7V, +12V, +17V, and +24V. Nothing is really different, you just changed your point of reference, ie. "ground." So the input of the 7805 will be +12V above your new ground and the output of the 7805 will be +5V above your new ground.

I don't think the heatsink will be necessary, the panel meter probably doesn't draw that much current.
 
I understand the idea of the caps (filter out any noise on the line) but why did you change the first one to a polarized cap? Does it matter?
 
I usually put a small electrolytic (~10uF) on the input if the regulator is located any distance away from its source, and a 0.1uF on the output. I don't think it's too critical, use whatever you have available.
 
You're welcome. Good luck with the mods. Remember to check things with your multimeter before hooking up the panel meter to avoid damaging it if something is wrong.
 
I just want to make sure of one thing. Your image:

panel-meter-png.48873


Should the 7805 left pin be pin 1, the bottom pin be pin 3, and the right pin be pin 2?

I know that the actual chip is like this:
**broken link removed**
from left to right pin 1, pin 3, pin 2. I ask because I connected like the pin numbers in the image I originally used but following your schematic and the chip got really hot but it did output the 5V.
 
Sorry I hacked that diagram really quick and didn't put pin numbers, but for the 78xx series of regulators, the pins are in order 1-2-3 (not 1-3-2), which corresponds to left-bottom-right on the diagram, or input-ground-output. Pin 2, the middle pin, is also electrically connected to the metal tab, so don't let it touch other parts of the circuit.

10697-stmicroelectronics-l7805acv.jpeg

Edit: I've attached a better diagram. The voltages in green are what you should be seeing.
 

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If this should be a separate post let me know and I will make one.

I know it has been a while. Everything is working for a while. The 2.5K pot that I have in there has been working fine, able to control the voltage for the variable circuit. Now I do not keep the unit plugged in for a long time (average time is around 10 minutes) while I am using it but somehow my pot died on me. If I take my multimeter and connect it to one of the outside contacts and the other prob to the middle I get either 0 ohms or ~2.5K ohms on my meter, even if I move the knob. Any idea on why the pot would just go? I didn't move any wires around to cause a short and was just using the meter yesterday.
 
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