Current Measuring Shunt

The good news this morning is the shunt works better than expected. Calibration is a bit of a challenge, since it’s hard to find 1% precision power resistors in the 1-4 ohm range. Nevertheless, I think it’s close, within 5%. I used a 6 amp/12V power supply and two 4 ohm/50W resistors and two meters to get the best readings I could.

The shunt is pretty basic. If you failed to retain the fuse block from your old Univolt, which had a nice copper plate shunt on it, you can make one for $2.08. If you use #10 wire, the wire tables say you need 12.01 inches of wire for 0.001 ohm (roughly speaking). I wound up using 10.25 inches. If my calibration is off, the shorter wire means the meter will be showing less current than is actually
flowing.

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A scrap piece of plastic can be the backing piece. A dry piece of wood would work fine, since the voltage across the terminals/lugs will always be less than 100 millivolts.

The electrical installation is a bit counter-intuitive for me–I’d have thought these things go in the positive lead of the battery. However, putting it in the negative (common) lead ensures that the wires from the voltmeter are essentially at ground potential. This floats the battery above ground just slightly.

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The fuses in the voltmeter wires are, in my opinion,¬†overkill. The fuses are only needed if the battery common wire becomes open on the ground side of the shunt.¬†It’s ultra-safe, but it would take a short to ground in the wire on the high side of the shunt, close to the shunt, like 12″ or less, in order to drive enough current through the small wire to be dangerous. The voltmeter wires have approximately 8 times the resistance of the #10 wire in the shunt, so the preponderance of battery current would contiue through the large wires. The original fuses are 8″ down the wire. This 8″ is the most “dangerous” part of the length, so the fuse on the other wire protects this short length.

The following refers to the new digital control panel documented on a separate page:
How well does the “amp meter” (the combination of the 200 mv voltmeter and shunt) work? Great! I was concerned that the long wires from the back of the Overlander to the front dome would be noisy. The meter is 3-1/2 digits, so it measures down to 0.1 mv, or 100 milliamps of current. With just a few minutes of observation, the meter jumps around a bit in the tenths digit (1 count only). It shows the water pump pulling 6.5 amps in open flow, ramping up to 7.4 amps as the pressure builds up to shutoff. It also shows turning the porch light on and off (less than 100 ma), and the radio (400 ma). When I get a long enough arm, I’ll check what it shows for the hitch jack.

I just can’t believe it’s this stable and precise. The old analog meters were essentially useless.