Current Meter Shunt

[DTB Radio]

This little idea was gleaned from a number of websites, so I can't say its original.

If you have a power supply with no current read-out, its relatively simple to make your own. First, determine the current rating of the power supply, and then select a solid copper wire with a high enough guage to VERY freely pass that amount of current. I run a 60-amp supply in my shop, and I use 4-guage solid copper for the main feed.

Here's how to make the meter shunt: Take a length of the large-guage wire (in my case I used about 4 feet), and coil it around a convenient NONCONDUCTIVE form. Second, attach one end of the coil to the supply's positive output, and the otehr end to your distribution point. Next, find a panel meter that has a very high internal resistance, and which is graduated in equal increments up to a number thats either at or somewhat above your supply's current rating. Connect the positive side of the meter to the end of the coil thats closest to the power supply, and the negative of the meter to the other end. You may need to play with exact tap points to get an accurate readout, but once done, you will have a fairly accurate current meter. You can use a fixed, high-power resistor to roughly calibrate your meter, or use a calibrated ammeter to check your accuracy.

 

[Beetle]

I'd suggest that the meter you select should be a DC millivoltmeter. It should be marked somewhere, probably on the face, with an inscription something like "FS 50" or "FS 100", indicating the movement will give a full scale reading when 50 or 100 millivolts is applied.

Now, with a wire table which shows the resistance per 100 feet or 1000 feet of the various wire gauges, select the size of wire needed, as DTB said. Then consult the wire table and use Ohm's Law to determine how much of this wire it will take to produce a voltage drop of 50 or 100 millivolts when the full output current of the power supply is flowing through it. This is the length of wire you need. Connect this length of wire, coiled up in any convenient way, ACROSS the meter terminals (not in series!).

 

[C2]

Or you could just buy one of these?

 

[Beetle]

If that thing that looks like a copper strip really IS a copper strip, then it's the shunt and things should work fairly well. However, I wouldn't trust the accuracy of that meter. The contact resistance will change every time you connect it to the circuitry, and just a few milliohms' change will change the meter reading.

This would be good for a general indication, but don't use it for any real quantitative readings.

 

[C2]

Could be, I'm sure it's +/-10% at FS...

Would you not face the same problems when using the homebrew meter?

Anyway, the only reason I post that is becuase they come for $18...I think accurate movements cost more.

I have one that goes up to 10 amps just like the one pictured. I mounted it to my power supply and it seems to do OK for that purpose.

But this is the homebrew forum, and I think I'll build one since I have a few movements already.

Just need to find a resistance chart...

 

[Beetle]

The nice thing about homebrewing is that you can tweak things to be a little more accurate.

Freecell posted a good wire table in the "Solid State Amps" topic just a few minutes ago. Should work!