Meters, supplies and (Re: Reliable Moderate HV testing)

["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>

'tis the Ham Flea Market Time of year.  Copious panel and other
meters available.  Also other Good Stuff Cheap, tho it helps
to know what one is looking for.

I THINK we already missed Dayton, O?

For those in NE, TODAY and TOMORROW are 'hosstraders', which is
huge.  (Date snuck up on me and I have another commitment, both
days....  Hopkinton, NH)

For NE folk, mostly:

	http://web.mit.edu/w1gsl/Public/ne-fleas

is one list.

AND I understand RS has s sale on one of their folding DVMs?
might be worth a check.

As to current measurements generally:
=================================================================

><<Varies as to exactly what's inside the meter.
>     Is it desired to measure AC or DC leakage?
>     What leakage currents are of interest?
>     milliamps?
>     microamps? >>


> Well I would like to measure amps.


	OK, that's different from 'leakage' where this
	started.

>Like how much current is being drawn from the wall by various

>things such as unmarked transformers.


	Some digging may turn up a new or used AC Amps capable
	multimeter/DVM.  Or a small value resistor in series
	and measure the drop across it (as AC Volt ranges are
	more common than AC current ranges.  Or a dedicated AC
	Ammeter either cased, or loose and stuck in a box, say,
	something from a flea market (cf above), or a clamp on
	current probe or ....

	IMPORTANT:
	This can get people killed.  Work Safely.

>I also would like to measure short circuit current from them also.

	
	Cf the caution above.  More important here, as voltages
	are higher.  Any of the above, if in range and suitably
	insulated.

	As to the original 'leakage' inquiry, depending on
	what is desired a couple of approaches are possible.  As
	noted, the 'voltmeter' is (in fact) a milliameter
	(mechanically), with or without an internal 'multiplier'
	(series resistor).  Typical panel meters are of a
	variety of ranges, need to measure that one to be sure.
	Assume it's a 1 mA movement.  (it might not be):
	Leakage could be measured AS IS with a 150VDC (adjustable)
	source: 150 VDC indicated indicates 1 mA leakage.
	Wrap the meter in a rectifier bridge and the same can
	be used to do AC measurements (minor inaccuracies due
	to bridge drop, wave shape).  This is assuming _leakage_
	as low current.  (Be aware that measuring leakage this way
	limits the current, based on the meter resistance.
	meters 'always' have some resistance, milliameters
	can easily have more than expected.  Thus the leakage
	encountered with the meter replaced by a 'real short
	circuit' may be higher.  a LOT higher.)

	For larger currents (as above) better to use a proper
	meter, evading the dissipation problems in the 'shunt'
	(Current sense) resistor.

	Any of this can lead to risks, unless handled with care.

	best
  dwp