Re: shunt question

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "davep by way of Terry Fritz <twftesla-at-qwest-dot-net>" <davep-at-quik-dot-com>

Tesla list wrote:

> Original poster: "by way of Terry Fritz <twftesla-at-qwest-dot-net>"
<RQBauzon-at-aol-dot-com>
 
> ok here it is, but don't laugh, okay:
 
> what does a shunt look like?

	What would you like it to look like?

	I'll Assume this is in reference ot a metering shunt, to
	allow a relatively low current meter to indicate the value
	of a relatively high current.  Such a shunt also allows
	ht meter to be 'here' and relatively light wire to go
	to 'there', where the current is.  Also can allow high
	current wires to be shorter, reducing losses.
	In AC work, it is common to use a current transformer,
	the shunts can be used on AC.

	A shunt is just a precision (more or less), low value
	resistor.  The accuracy of the indication depends on the
	accuracy of the resistor.  (on my motorcycle, i have
	had, for years, a 'shunt' consisting of 12" of #14 wire,
	which allows a 50 mV/1 mA (?) meter to indicate alternator
	output, remotely.  runs fine.)  Care taken in preparing
	the shunt will, of course, improve accuracy.  For rough
	work, simple lengths of wire will do fine.

	If, forinstance, browsing a flea market, a typical shunt
	will have a plastic base.  On this will be mounted a bit
	of resistance alloy, varying according to scale.  say 1/2"
	wide by a few inches long.  Typically shows signs of grinding,
	which is how they are calibrated.  There will be two
	connections at each end, one at each end is high current (big
	screws, lugs, whatever).  This is for the current to be
	measured.  The other two connections, again, one at each end,
	will by 'smaller' as they may carry only a few mA.  With luck,
	the base will be marked with something like:
		10A/50mV
	Which would indicate that if 10A were flowing in the shunt, it
	would have a drop of 50mV, so a 50 mV external meter would
	indicate full scale.  (While not STANDARD, 50 mV is COMMON
	for shunts...)

	A bit of ohms law allows any handy meter to be scaled to
	any current, within limits...
	
	best
	dwp