flyadive(at)gmail.com
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 Posted: Mon Nov 01, 2010 2:28 pm Post subject: AMP METER & SHUNT |
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Hi David;
You do not require two shunts. Only one shunt and a meter capable of reading both Positive and Negative current flow.
If I may, let me describe what a SHUNT is and why it is required.
A SHUNT is exactly what the word says it is... "A way around", "To circumvent" as in a Shun-pike - A way around a pike.
Why are there shunts? It is not practicable to manufacture an AMMETER for ever current range required by different applications. So, there are some basic Ammeters at basic current levels: 100 uA. 1 mA, 100 mA & 1 A.
The SHUNT is placed in PARALLEL with the above ammeter(s) to divert the HIGH current around the Meter, while allowing the small amount through the meter.
For example:-
If you wanted to read 10 Amps you may use a 1 Amp Meter with a SHUNT capable of carrying 9 Amps.
The meter reads 0 to 1 and that would be 0 to 10 Amps with the shunt installed.
How is the value of the shunt determined?
By RESISTANCE.
If the Meter has an internal resistance of say 100 Ohms the Shunt must carry a ratio of (X):
1 Amp is to 100 Ohms
as
9 Amps is to X Ohms
---> WHY 9 AMPS? Because since the Shunt is in Parallel with the 1 Amp meter ...
1 Amp goes through the meter and 9 amps goes through the shunt. A total of a 10 Amp flow.
100 / 9 = 11.11 Ohms for the SHUNT value.
Now I arbitrarily picked 100 Ohms and 1 Amp for the meter.
The real value of the meter would probably be closer to 12 Ohms.
Now, You Can Not just put a DMM across a meter to find the resistance. Because the DMM would load down the meter and give an incorrect reading. This would be indicated by a deflection of the ammeter needle. OF COURSE - If the amperage of the un-shunted meter was high enough there would be less loading of the DMM.
Did this confuse ya? I could go more in depth if anyone is interested 
One more NOW - The direction of the current does not mater, NOT to the shunt... Dave, you do NOT need anymore than ONE SHUNT. The direction ONLY mater to the Meter.
OK, DONE - - - Phew!
--
Barry
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