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Old 09-28-2016, 08:01 PM
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wa2ise wa2ise is offline
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Using those freebee Harbor Freight DVMs as panel meters

Have an old homebrewed linear adjustable power supply with a meter that was hard to figure out. Volts and amps output, switchable. I figured I could use one of those freebee Harbor Freight DVM to replace the old meter, to become this power supply's panel meter. The power supply can provide up to 27VDC at around an amp. I picked two ranges I'm going to use, 200VDC and 10A. Then I had to figure out how to do the switching the big knob usually does, so I could have a two position switch. I found I could do this with a 4 pole double throw slide switch. Two of the poles would have wires connected to all 3 connections, and the other two poles just two wires each. Another normally switched connection was always connected, so a solder jumper took care of that. Below are some pictures detailing the meter wiring and switching.



The power supply connections to the meter goes as such: The positive output goes to the volts input jack. This positive also goes to the external output jack of the power supply cabinet. The negative output goes to the common jack of the meter. And the negative output of the power supply cabinet goes to the current jack on the meter. There's a current shunt inside the meter that measures voltage developed across this shunt. Thus I don't need to plug or unplug anything when I want to switch from volts to amps on the meter.

There's a trimpot on the DVM's circuit board that I used to adjust the voltage reading. And as the current reading was a little high compared to other DVMs I have, I was able to get it close by adding a 0.1ohm resistor in parallel with the shunt.

I needed to provide a separate 9V power supply to the DVM. Used the guts of a wall wart, with a voltage regulator added to it to get a steady 9V source.

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File Type: jpg metermodeswitch.jpg (49.9 KB, 79 views)
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Old 09-28-2016, 10:23 PM
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Electronic M Electronic M is offline
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I've been meaning to construct a metered isolated variac...This would make a nice cheap meter setup.
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Old 09-29-2016, 02:56 PM
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wa2ise wa2ise is offline
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Quote:
Originally Posted by Electronic M View Post
I've been meaning to construct a metered isolated variac....
Those meters don't have AC amps. Just DC.
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Old 09-30-2016, 10:54 AM
dieseljeep dieseljeep is offline
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Quote:
Originally Posted by wa2ise View Post
Have an old homebrewed linear adjustable power supply with a meter that was hard to figure out. Volts and amps output, switchable. I figured I could use one of those freebee Harbor Freight DVM to replace the old meter, to become this power supply's panel meter. The power supply can provide up to 27VDC at around an amp. I picked two ranges I'm going to use, 200VDC and 10A. Then I had to figure out how to do the switching the big knob usually does, so I could have a two position switch. I found I could do this with a 4 pole double throw slide switch. Two of the poles would have wires connected to all 3 connections, and the other two poles just two wires each. Another normally switched connection was always connected, so a solder jumper took care of that. Below are some pictures detailing the meter wiring and switching.



The power supply connections to the meter goes as such: The positive output goes to the volts input jack. This positive also goes to the external output jack of the power supply cabinet. The negative output goes to the common jack of the meter. And the negative output of the power supply cabinet goes to the current jack on the meter. There's a current shunt inside the meter that measures voltage developed across this shunt. Thus I don't need to plug or unplug anything when I want to switch from volts to amps on the meter.

There's a trimpot on the DVM's circuit board that I used to adjust the voltage reading. And as the current reading was a little high compared to other DVMs I have, I was able to get it close by adding a 0.1ohm resistor in parallel with the shunt.

I needed to provide a separate 9V power supply to the DVM. Used the guts of a wall wart, with a voltage regulator added to it to get a steady 9V source.

I was thinking about doing, that very same thing, ages ago!
I'm going to mount the meter to the front panel of my variable power supply and drill holes in the back of the plastic case to run the meter and power supply leads and leave the selector switch in place.
There's several ways of doing it and you inspired us to practice our ingenuity
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Old 09-30-2016, 01:15 PM
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Quote:
Originally Posted by wa2ise View Post
Those meters don't have AC amps. Just DC.
I've seen AC amps cheapies elsewhere....Hell I may have one that had it's ohms range cooked (from measuring voltage in ohms mode) that I could use....Trouble is I have so much crap (mine and others') to fix that I don't have time to build/modify stuff and won't for a while....I'm thinking of doing a second pick at that TV place in In., and declaring TV purchasing hiatus for a few months till I can widdle stocks/backlog down to a manageable level.
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