Re: Gauge Number from eBay ---- Wiring question, about 24 volt and "bridges" -- update for today


That's good to know. Thanks, John. That means for about 16 bucks Cal could get a battery monitor that seems to work alright, from the reviews, and wouldn't have to buy anything extra:

I keep learning... I thought that the voltage drop rating -- mV -- was for a set current determined by some standard. Say 100 amps, so a 75 mV shunt would have a voltage drop of 75 mV at 100 amp current, whether the shunt was rated to handle 100 amps or 500 amps. No. That'd make life too easy. <g> If your gauge says to use a 75 mV, 100 amp shunt, then you'd better use a shunt just like that:

"Shunts are rated by the maximum current and voltage drop at that current. For example, a 500 A, 75 mV shunt would have a resistance of 150 microohm, a maximum allowable current of 500 amps and at that current the voltage drop would be 75 millivolts. By convention, most shunts are designed to drop 50 mV, 75 mV or 100 mV when operating at their full rated current and most ammeters consist of a shunt and a voltmeter with full-scale deflections of 50, 75, or 100 mV. All shunts have a derating factor for continuous (more than 2 minutes) use, 66% being the most common, so the example shunt should not be operated above 330 A (and 50 mV drop) longer than that."

I'm gonna visit Cal Wednesday to see what's going on with Surprise's wiring.

On 8/31/2020 6:02 PM, john acord wrote:
If you are running 80A, a 100A shunt is fine.  If you run a bit more than 100A it will just heat a little and that's OK for short periods.
I like to keep a shunt and meter around for testing and setting things up.  A meter made for panel mount is generally not so practical, generally needs voltage too, so I just use a shunt and my digital voltmeter.  The photo attached is a 50A 50mv shunt:  each mv read across the shunt is an amp.  I just wire it into whatever circuit I want to measure.
I don't know why 75mv shunts seem prevalent.  ?? Makes the reading an odd factor of amps, ie 75mv = 100A.  So how many amps is 32mv.........??
Remember, a shunt is just a precision resistor.  If you want to measure small currents just go buy a 1% resistor of the appropriate value from the electronic supply house and use the same way as in the photo.
John <>
Words ought to be a little wild, for they are the assaults of thoughts on the unthinking. (John Maynard Keynes)
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