I am currently trying to measure what I expect to be a VERY large current draw (two large motors drawing power from a LiPo) and don't want to risk blowing out my ammeters, would there be an issue with connecting multiple ammeters in parallel in series with the load?
Ie Vin - (Ammeter in parallel with another ammeter) - (HBridges in parallel) - Gnd
Posted Sep 21, 2016, 9:36 AM UTC
In order to measure the current draw of my electric outboard I've bought a special high-current shunt for the meter. You can also but a volt and ampmeter combination with shunt.
Since an ammeter is a series, low value resistor with a voltage measuring device across it - galvanometer or DVM - the ammeters can be parallelled if they have the same resistance as each other.
If they have different resistances, there will be some calculations to do, to predict the results.
Calculations would be required to predict what will happen, but if you just do it you can simply add the readings on the two meters together.
Would there be an issue with mis-match if the two meters are very out of balance, would one get hot enough to the point that the resistance changes and the value unusable
You might exceed the rating of the lower resistance ammeter.
This. As long as both meters are below their limit they should be as accurate as they were on their own. Obviously a 100A meter in parallel with a 5A meter isn't going to do you much good, but if you have two 50A meters with slightly different shunt resistances and one reads 30 and one reads 40, you can bet that the total of 70 is quite close.
You can use the resistance function to check the other meters Amp reading resistance. They will share the current based on this measured resistance.
If you have a 2 A meter with a 0.1 ohm resistance and a 4 A meter with a 0.1 ohm resistance in parallel, then one meter will show half the current that the other meter shows.
In that case, the two shunts have the same resistance so they would both display the same current... but you wouldn't be able to go beyond 4A total because that would max out the 2A meter.
Easier to get a shunt resistor.
How much current are you expecting?
If both Ammeters have the same shunt resistance, they will share the current.
If not, the lowest resistance meter will take most of the current.
(presumably these are not clip-on ammeters.)
Better to fit a larger shunt, with a suitable meter movement (you can probably remove the shunt to re-use your existing meter).