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Registered Member #54484
Joined: Sun Feb 15 2015, 06:18PM
Location:
Posts: 46
I'm almost done with my coil and a little stuck on choosing the right PFC Cap. The diag I'm following is below. I have a few microwave caps and would rather use one of those instead of buying another component for this. Here's a list of what I have in stock, all test good... 1.00 uF 2300 0.86 uF 2100 0.77 uF 2100 1.05 uF 2100 0.91 uF 2100 My NST is 15000 v 60 ma protected with a Terry Filter. The line filter is RFI EMI 15a 115/250v. Any help is much appreciated especially if you can explain to me how to determine this. Thank you!
Registered Member #33460
Joined: Tue Aug 27 2013, 06:23PM
Location: Seattle
Posts: 46
Do you have an oscilloscope? I think you could look at line current and compare it to line voltage. Pick the capacitor that makes the current most in phase with the voltage.
For current sensing without an inductive probe, try using a very small value resistor in series with the neutral lead. Attach the ground of your oscilloscope probe to the neutral side, and read the voltage on the other side of the resistor. This will be the current, inverted and scaled by the resistor value. For the voltage reading, be sure to attach the ground clip in the same place as the other channel (neutral) and read the voltage on the hot lead.
It will probably be much easier to do this without the Tesla Coil portion of the circuit attached. Load the neon sign transformer by drawing out a large arc.
Disclaimer: I haven't performed the suggested actions, so there's a small chance that the above is nonsense. Power your oscilloscope through an isolation transformer or use isolated differential o-scope probes if you got 'em.
Registered Member #135
Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735
Pulse forming capacitor or Power Factor Correction?
Power factor, most normal power factor on these things is like .86, so that should help with the calc. Not sure why you would worry about power factor on the low side, it will probably calculate out to 47uF to correct for the load, or some large value. It will have to be non-polarized, possibly a motor run cap.
Registered Member #33460
Joined: Tue Aug 27 2013, 06:23PM
Location: Seattle
Posts: 46
I'm quite certain the OP meant power factor correction, given the capacitor labeled "PFC" in the schematic. The theory I present, measuring the phase difference between the line voltage and the current drawn by the NST under load, and adjusting the value of the PFC capacitor to minimize said phase difference, is valid.
What I am unsure about is whether there might be enough voltage difference between the neutral line and the o-scope's internal ground connection to cause a problem when connecting the probe ground clips to neutral.
Registered Member #54484
Joined: Sun Feb 15 2015, 06:18PM
Location:
Posts: 46
Eleccentric wrote ...
Do you have an oscilloscope? I think you could look at line current and compare it to line voltage. Pick the capacitor that makes the current most in phase with the voltage.
For current sensing without an inductive probe, try using a very small value resistor in series with the neutral lead. Attach the ground of your oscilloscope probe to the neutral side, and read the voltage on the other side of the resistor. This will be the current, inverted and scaled by the resistor value. For the voltage reading, be sure to attach the ground clip in the same place as the other channel (neutral) and read the voltage on the hot lead.
It will probably be much easier to do this without the Tesla Coil portion of the circuit attached. Load the neon sign transformer by drawing out a large arc.
Disclaimer: I haven't performed the suggested actions, so there's a small chance that the above is nonsense. Power your oscilloscope through an isolation transformer or use isolated differential o-scope probes if you got 'em.
I do have an oscilloscope but a little hesitant to connect the house wiring directly to it. Also I have very little experience because I have very rarely needed it. What if I just go through a small ac transformer, will I get the same results?
Registered Member #2390
Joined: Sat Sept 26 2009, 02:04PM
Location: Milwaukee Wisconsin
Posts: 381
Eleccentric wrote ...
I'm quite certain the OP meant power factor correction, given the capacitor labeled "PFC" in the schematic. The theory I present, measuring the phase difference between the line voltage and the current drawn by the NST under load, and adjusting the value of the PFC capacitor to minimize said phase difference, is valid.
What I am unsure about is whether there might be enough voltage difference between the neutral line and the o-scope's internal ground connection to cause a problem when connecting the probe ground clips to neutral.
In the U.S. Ground and Neutral are tied to the same point in the service panel. There should be no difference between the two, an easy way to check is simply to probe hot to neutral, then hot to ground. You should see the same reading, if you don't........ You have a bigger issue.
Quick Edit: The ground in the scope will obviously use the mains earth, in this case not using an isolation transformer would be ok. Scope ground, mains earth, and neutral are normally all at the same potential.
Registered Member #1232
Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881
Keep adding PFC capacitance in increments like 47uF across the mains input until the AC supply current drawn from the wall reaches a minimum. Once it starts going up again, take off the last PFC capacitor. You then have found the optimal PFC capacitance for that load. This is the easiest method if all you have at your disposal is an AC ammeter and a box of miscellaneous PFC capacitors.
Obviously take care switching in and out PFC caps, particularly if done with the power on! Watch out for stored charge!
Registered Member #54484
Joined: Sun Feb 15 2015, 06:18PM
Location:
Posts: 46
TeslaMap says 166uF. I'm gonna go with that for now and see what happens. Thank you all for the help. Every time I post a question in this forum I always end up learning something, thank you!
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PFC Capacitor - How do I determine which one I need?
