More shunt rigging [RE-wrenches]

[Ray Walters]

Maybe this is a stupid question, but why don't they use a 1000A/ 100 mV
shunt from applied power with a Trimetric. It reads just the same as a 500
A 50mV shunt.

Ray
 At 09:26 AM 12/18/00 -0800, you wrote:
>>Slight difference between 
>>two shunts will cause unequal current distribution, and the meter 
>>will read the higher of the two ... I think!
>
>	Windy;
>
>	Remember, two shunts in parallel, connected with sufficiently heavy
>conductors, act as one resistance, therefore there will be only one
>voltage, the sum of the voltage drop across them both.
>	Look at it this way, if you parallel connect two batteries with large
>interconnects, then read the voltage of each battery, it will be the same
>on both right? At least it will be close enough for you to assume that they
>are both at the same voltage for practical purposes.
>	If you were really stressed about accuracy, I'd put the meter across the
>high side of one shunt and the low side of the other
>
>>The best way is to put a resistance in series with the meter.
>
>	I agree that one shunt is the way to go. I use a 50 mv/100 A shunt on my
>PV input, driving a remote 1mA meter movement in series with a 200 ohm,
>15-turn pot. I calibrated it by reading the output of the shunt with a DVM
>and transposing the reading to the meter, adjusting the pot to match. Works
>great. A 50 mv meter movement wouldn't work because the resistance in the
>wiring back to the metering position caused the meter to read low.
>	This brings up the case for custom shunts and meter movements. I have
>constructed many shunts using brass threaded rod in a variety of sizes, and
>used all manner of salvaged meter movements to provide metering for odd
>values or to make use of 'vintage' panel meters for custom installations.
>Try finding a 10, 15 or 30A shunt. Try loacting a shunt to match a 50uA
>meter movement.
>	One of my better projects was engineering and constructing a -30/0/+10
>shunt and meter that used off-the-shelf components and a standard 0-100uA
>meter movement that self-center-zeros when the current isn't going either
>direction.
>
>>Write a big " Multiply X 2 " on the meter.
>
>	Many meters have removable faces, I used a Brother Labelmaker to print new
>values for the graduations on the meter face, so it reads 0-100A instead of
>0-50.
>
>>The shunt will be fine as long as it doesn't carry 1000 amps long 
>>enough to get hot.
>
>	Heating in the shunt isn't going to be much of a problem. At 1000A, a 50
>mv shunt is dissipating 1 watt, hardly enough to make it even warm. The
>larger factor would be the voltage drop in the circuit, and at .001 volt,
>even that seems insignificant.
>	Yours in theory,
>
>	-S
>
>
>
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 michael.welch at homepower.com

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