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adding volt and amp meters to power supply

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waterbug

New Member
I am building a Lm388 1.3 - 30v power supply from the internet. I was able to find a nice broken power supply that has a volt and amp meter. I have supplied a .bmp image to show how the meters are wired together. My question is how to wire the meters into the curcuit.
 

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Sceadwian

Banned
Hook them up the same way they were hooked up in the old power supply. I'm guessing you just pop it in series with the + lead.
 

duffy

Well-Known Member
Check and make sure the amp meter has the shunt resistor (it will NOT be that 3.9k, it will be close to zero ohms). This resistor often just looks like a little metal plate or wire screwed across the ammeter terminals, like it was shorting it out. Start it off with a low current (like a milliamp) if you are unsure, because if you bang the needle you can wreck it. The ammeter goes in series with the load.

That 3.9k resistor is probably a voltage dropping resistor for the voltmeter. The voltmeter goes across the load. Try a low voltage first, and see how it responds.
 

Sceadwian

Banned
Make sure you don't hook it up backwards too, cause then the needle will go the wrong way =) You'll notice this right away. Try it with a AA battery with a 1500 ohm resistor in series, that'll give you 1.5 volts and 1ma. If it registers okay with the battery you should be able to just take it right on to your circuit on the output.
 

colin55

Well-Known Member
Firstly you will to find out the reason for the 3k9 resistor. What is the scale on the voltmeter.


What is the scale on the ammeter.
 

waterbug

New Member
REply

The power supply was a McIntosh(not Apple) programable 0-13.5V 0-6A power supply. all of the guts have been stripped out(why it had soooo many resistors and caps plus 2 regulators is a mystery to me) so the way it was hooked up is unknown. The shunt for the Ameter must be in that circuit.(These are matched Modutec meters. could the amp meter have an internal shunt?) The scale for the meters 25v 10a. The V meter will be parallel to the output and the A meter in series? or some other way? Like from the pot?
 
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duffy

Well-Known Member
Some ammeters have internal shunts, check first with a low-current source as suggested. Yes, the volt meter is in parallel and the ammeter is in series.
 

Hero999

Banned

waterbug

New Member
Great catch

I'm glad you caught that error. I mis-drew the curcuit. I attached the R3 resistor to it's correct point now(to the wiper of R1 and pin2 of LM388). It sure shows how it pays to review before applying power.
 
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