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Hello all, i need help with measuring voltage and current on primary coil, how that can be done? I know about current transformers and that i can use them while one primary wire going through it, but that would measure only amps, not volts, am i right? I can use oscilloscope, but input voltage is only 400V, and i think it is too low - danger of destroying channel with overvoltage.
I am currently fiddling with Finn Hammers predikter circuit and want to see zero switching crossing happening on primary. (and that can be done only on drsstc). Would that be just as easy as putting probe to common D-S point in one side of bridge?
Registered Member #7267
Joined: Tue Oct 16 2012, 12:16AM
Location: Detroit, Michigan
Posts: 407
Those are great questions. I think you can just make a simple voltage divider and then just calculate what the actual voltage is. I think that's just a basic way to measure it though, there might be more accurate ways. High voltage probe might work, I'm not familiar with how much they cost.
use a 10x probe, or lower your bus voltage with a variac or dimmer. do you have a digital scope? if so you could charge the bus caps up, then disconnect the power, fire the coil once the voltage drops to a comfortable level and get a one shot capture.
Yes, i have DSO, but i was not aware of how high the voltage could be (DSO i have is new and it would be pity to destroy it so soon). I know this question looks a bit strange in forum with such experienced people, i am still learning a lot.
i can confortably regulate voltage from 70-250V, but voltage divider is also great idea, i will use both to make sure i dont blow things up.
Registered Member #2919
Joined: Fri Jun 11 2010, 06:30PM
Location: Cambridge, MA
Posts: 652
dingo27 wrote ...
Hello all, i need help with measuring voltage and current on primary coil, how that can be done? I know about current transformers and that i can use them while one primary wire going through it, but that would measure only amps, not volts, am i right? I can use oscilloscope, but input voltage is only 400V, and i think it is too low - danger of destroying channel with overvoltage.
I am currently fiddling with Finn Hammers predikter circuit and want to see zero switching crossing happening on primary. (and that can be done only on drsstc). Would that be just as easy as putting probe to common D-S point in one side of bridge?
Any helpful info would be highly appreciated!
You can just variac the bridge down to some more reasonable voltage and then take a differential measurement across the output. (Make sure the measurement is differential!)
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
Not sure why are other reply-ees failing to address this clearly, but connecting your oscilloscope probe to your mains powered bridge output will certainly blow your bridge, and likely the scope as well! You have either to use a mains isolation transformer, so you can ground your bridge - rail, or use differential measurement with two scope probes which I wouldn't trust at all at TC frequencies.
So unless you have an isolation transformer, the only way to go would be to use a ferrite voltage transformer to step down and isolate your bridge output! You could wind your own 10:1 or whatever ratio transformer on a toroidal core, or use existing one like a transformer from PC power supply (you'd need to measure the turns ratio first).
I would also load down the transformer output somehow to minimize allsorts of reflections and noise that may appear. Ideally you could use scope input that is terminated with 50 ohms instead of high impedance!
Registered Member #3908
Joined: Tue May 24 2011, 09:40PM
Location: Gilbert, Arizona USA
Posts: 68
Marko already mentioned the *critical* requirement for mains isolation. +1 Sadly, in my zeal to check the GDT output, I'd forgotten about mains isolation and cremated every semiconductor on the driver board. Fortunately the scope survived the assault.
Registered Member #7267
Joined: Tue Oct 16 2012, 12:16AM
Location: Detroit, Michigan
Posts: 407
Dingo, if you haven't seen this (How NOT to blow up your oscilloscope) I highly recommend you take the time to check this out as it re-affirms what Marko and others are saying. And it very well might save you lots of time, trouble, blown circuits, and money!
EDIT: Pay particular attention to scenario #2 and #3. And at the end he talks about the differential probe for high voltage circuits.
I watched the video and learned a lot, but i might be calm because i actually do use an isolation transformer power supply, which is in no way referenced to ground. Still, i only put ground pin to neutral pole on power supply. Also i use only boost circuit to power tesla coil, which is again referenced to neutral pole on power supply.
I will still hate working with mains without some sort of isolation, and use extreme care. These are high power devices that can kill, and i always have that in my mind.
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Measuring fullbridge voltage output?
