measures wrote:
>
>>
>>Hi Anybody ever built a highpot using an auto ignition coil .Have to
>>provide it with Low voltage high frequency supply but should work .Am I
>>missing some thing?
>>Thanks Glenn N3huv
>
>The potential has to be completely adjustable.
Agreed - otherwise you miss some very important information about *when*
breakdown begins.
> 60Hz transformers with
>variacs work best.
I've had very good results with a small laser(?) power supply module
which consists of a one-transistor oscillator with a step-up transformer
and voltage multiplier. (IIRC, Fair Radio was advertising something
similar about a year ago.)
The whole module gives about a 400:1 step-up of the applied DC voltage,
so all it needed was an LM317 variable DC input to give a variable DC
output of about 2-10kV.
The positive output feeds a clip probe via 10M of series resistance
(several high-voltage resistors in series to get >10kV breakdown
capability). Output voltage is not routinely measured, but the pot
setting to the LM317 was calibrated using an electrostatic voltmeter.
I floated the module and its DC supply just above chassis potential in
order to use a large 50uA meter in the ground return. This allows an
ordinary meter to be used without special insulation, and an earthed
metal case for the whole instrument.
Anticipating Rich's next comment, only positive voltage is available
with respect to earth. I couldn't see any future application which
required negative voltage, where the object under test couldn't be
turned around instead of the voltage.
DANGER: even with 10M of series resistance, this unit will give you a
very nasty bite! Even a small capacitance like a 1000pF feedthrough
charged to 10kV is very dangerous. The tester should include a
permanently attached discharging wand containing 10M of high-voltage
resistance.
Thanks to Rich for pointing out how useful a hi-pot tester can be.
Next time it's used, I will add an audio amplifier which can be
connected straight across the meter.
73 from Ian G3SEK Editor, 'The VHF/UHF DX Book'
'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.com/g3sek
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