Hi Don, the center tap of these transformers are connected to the case and
therefore would be grounded. If you design your tester so that it operates
with the output voltage referenced to ground, then your transformer will
work just fine.
I have used microwave oven transformers like that with good results. They
have one side of the hv winding connected to the core which is grounded.
In your case, either a half wave circuit using only one leg of the
secondary or a full wave circuit using both legs can be used. You can also have
separate rectifiers for positive and negative test voltages and that could be
useful. Of course you can also test components between the two output
polarities, you just have to be careful that the component has no path to
ground
while testing.
The other needed part will be a way to vary the voltage and a small Variac
would work well for that.
Current limiting is essential for many tests as some insulators are damaged
by even a very small current flow. I don't want to discuss how I know
that!
An old fashioned VOM with a 5KV or higher dc scale makes a great current
limiter due to the high resistance of the meter. A 20,000 ohms per volt
instrument has a 50 microampere movement and at full scale, 5000vdc, 50
microamperes of current flows through an internal resistance of 100 megohms.
For test purposes, you would never allow full scale current flow, but just
enough to see the meter move above zero.
73,
Gerald K5GW
In a message dated 11/25/2010 8:49:43 P.M. Central Standard Time,
donroden@hiwaay.net writes:
Has anyone used neon transformers to build an insulation tester ( Megger
).
I need something to go to around 10KV for testing capacitors ( type 291 )
under a high voltage /low current condition.
Don W4DNR
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