More shunt rigging [RE-wrenches]

[R. Sparks Scott]

>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



- - - - - - -
To send a message:
 RE-wrenches at topica.com

The archive of previous messages: 
 http://www.topica.com/lists/RE-wrenches/

To unsubscribe send a message to: 
 RE-wrenches-unsubscribe at topica.com

To check out the other RE-Wrench participants:
 www.mrsharkey.com/wrenches/index.html

Hosted by Home Power magazine: 
 www.homepower.com

For info contact list moderator by email:
 michael.welch at homepower.com

___________________________________________________________
T O P I C A  http://www.topica.com/t/17
Newsletters, Tips and Discussions on Your Favorite Topics