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[TCML] Re: Pratical measurement of voltages and current on TCs


Keep it simple always wins.

Use an iron vane ammeter to measure secondary current from the 
transformer. Keep the meter (all parts) isolated off ground and you're 
golden. 0-50ma full scale should be fine. Measuring off one leg is 
sufficient. Unless you burn out one leg, the current will be the same. 
Such meters don't need a rectifier, are rugged and not terribly expensive, 
new or used.

I use a 40kV flyback tester probe for measuring NST-like voltages. I have 
a probe that's AC rated and a suitable meter with 10M input impedance. 
Most meters and DMMs are not 10M in the AC ranges, so you'll read 
dangerously low voltages. you might rig up some sort of divider string, 
but who knows how safe or accurate that will be. I steer towards 
commercial products for measuring voltages like that as was I want to 
really know what I'm dealing with vs. guess with a homemade resistor 
string with a flakey solder joint.

There's no need to overcomplicate stuff with fantasy electrostatic 
voltmeters. I have no idea how anybody even pretends to calibrate such 
things. I have DC power supplies rated to 50kV and have been to facilities 
with 250kV test cages and nobody uses electrotrostatic volt meters, even 
for DC. Trying to measure transient AC RF voltages? Yeah right. Just lie 
like anybody else and says it's a 500,000 or a million volts.

For the secondary all you need need to know is the secondary current to 
ground. Low value shunt resistors between ground and the bottom of the 
secondary is all it takes . Measure the low relative voltages from there. 
It's not going to explode your scope or meter unless you try goofy stuff 
like having multiple grounds for no reason. If you know the current and 
frequency and inductance of the secondary you're all set. You'll want to 
hookup a scope here to look at waveforms and peak currents. The oldest, 
crustiest scope will work fine for this, although the cursors and math on 
digital scopes are helpful.

You can always make things fancier, later if it's really needed.

On Sat, 4 Dec 2021, Joshua Thomas wrote:

> Hello all,
> I'm starting to think ahead about connecting measurement equipment to my
> SGTC. I have experience with measurement of sub-kilovolt equipment, but not
> with the higher voltages a tesla coil produces. I would be interested in
> hearing about the measurement equipment and connections that other coilers
> have used.
> Here are options I have thought of or considered.
> Primary circuit voltage measurement: A potential transformer in series with
> the circuit is likely to be the most accurate, but it is expensive; and I
> expect the inductance of the transformer will change the coil resonance
> frequency. A high-resistance voltage divider (more than 10 Gigaohm?) would
> be simple to implement, should not change the resonance frequency by any
> significant amount, and would draw very little current. However I am unsure
> about what to use as the ground voltage reference of the divider. I use a
> counterpoise system and I would expect it to have transient voltages which
> might be large enough to damage voltmeters or scopes connected to it. My
> other thought is to put in two dividers, one an each output rail, and
> measure the differential voltage between them,
> Primary circuit current measurement: The NST current is in the millamp
> range. The problem isn't the current, it's still the voltage. I could put a
> microamp meter on the voltage divider try to scale it appropriately. The
> impediance of the meter might change the divider, but probally not if the
> divider impediance is very large. The other option that comes to mind is a
> current-sensing transformer, where one of the primary rails runs through a
> toroidal transformer which scales the output to the input. I haven't used
> these before and I'm not sure on the appropriate use.
> Secondary circuit voltage and current measurement: The topload has
> something like 300kV or more, and there's not really anywhere to tap into
> that doesn't effect the circuit directly. My only guess here is remote
> electrostatic sensing. I have absolutely no idea how to implement or
> calibrate this. Definately interested in experiences from coilers.
> Thanks in advance,
> Joshua
> -- 
> Joshua Thomas
> My new email address is: joshuafthomas@xxxxxxxxx
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