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Registered Member #267
Joined: Mon Feb 27 2006, 09:44PM
Location:
Posts: 46
Those are some great links Anders M, thanks.
Steve-
I was referring to the maximum average power from the wall (I shouldn't have said peak). I've been thinking about how I want to soft start the coil. I will probably use a variac until I get the coil working, and then add a soft start circuit. Would a 120V, 10A variac be fine for a coil like the mini brute (1200 W) or should I get something with a higher current rating? I don't want to overload my variac, or be blowing fuses all the time.
Registered Member #33
Joined: Sat Feb 04 2006, 01:31PM
Location: Norway
Posts: 971
You should be able to precisely control how much current the coil draws from the power line by varying the BPS/on-time, so you can always keep it under what the variac is rated for.
I've also heard that variacs can be pushed to twice the rated current for some minutes, so you have some headroom there.
I don't know if you have to do any derating of the variac due to the non-unity power factor of the DRSSTC leading to increased I^2R losses in the variac. Any opinions on this?
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Anders M. wrote ...
I don't know if you have to do any derating of the variac due to the non-unity power factor of the DRSSTC leading to increased I^2R losses in the variac. Any opinions on this?
Yes, the power factor can be terrible, like 0.5, because of the large DC bus capacitance. It actually gets better at variac ratios away from 1:1 because the leakage inductance of the variac smooths out the current pulses.
In practice, though, nobody seems to run coils continuously at full power for hours, so it works out OK. But with DRSSTCs you'll find yourself compelled to turn it up until something blows, so might as well make it a cheap part like a fuse.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Oscilloscope: The cheapest flea market special you can find, because you'll probably damage it sooner or later.
Probes: Two x10 probes at a minimum. I also like to have a x100 high voltage probe, because if you want to scope the output voltage from your inverter, it's over the rated maximum for most x10 probes.
Some ferrite-cored current transformers are also handy. I have two, one with 33 turns and 3.3 ohm burden (1 volt per 10 amps) and another with 33 turns and 0.33 ohm burden to give 1 volt per 100 amps. I fitted BNC sockets to them, so I can just plug them straight into the scope with a BNC cable.
Registered Member #267
Joined: Mon Feb 27 2006, 09:44PM
Location:
Posts: 46
Steve McConner wrote ...
Oscilloscope: The cheapest flea market special you can find, because you'll probably damage it sooner or later.
Probes: Two x10 probes at a minimum. I also like to have a x100 high voltage probe, because if you want to scope the output voltage from your inverter, it's over the rated maximum for most x10 probes.
Some ferrite-cored current transformers are also handy. I have two, one with 33 turns and 3.3 ohm burden (1 volt per 10 amps) and another with 33 turns and 0.33 ohm burden to give 1 volt per 100 amps. I fitted BNC sockets to them, so I can just plug them straight into the scope with a BNC cable.
Registered Member #1535
Joined: Wed Jun 11 2008, 11:37PM
Location: Northeastern Pennsylvania - USA
Posts: 117
20mhz is a good start. You'll be able to see if your tank is ringing up properly. You'll be able to measure how much current is flowing in the tank circuit when using a CT to feed your probe. You'll be able to verify proper interrupter operation. You'll be able to zoom in and see how accurate your zero cross is on a low frequency coil.
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Designing a DRSSTC from scratch
