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Registered Member #59110
Joined: Mon Apr 11 2016, 04:35PM
Location: Camarillo, California
Posts: 74
I've finished preliminary work on a small half bridge coil. It uses the same circuitry as Loneoceans RSSTC3 coil ie a staccato interrupter driving an IGBJT bridge.
In order to get larger sparks I am using a 110/220 step up transformer. It is a rather large unit with a VA rating much higher than the average power that the coil uses. I'd like to buy a smaller one but am unsure how to size a transformer when it is used in a pulse application like this.
Registered Member #30656
Joined: Tue Jul 30 2013, 02:40AM
Location: UK
Posts: 208
A standard voltage double won't work for this (he's not running a fixed DC bus, but is instead using the AC line to give half-sine shaped voltage impulses), though the Villard circuit (as used in microwaves) might be worth investigating/simulating, as it level shifts the AC waveform rather than giving a DC output.
Registered Member #59110
Joined: Mon Apr 11 2016, 04:35PM
Location: Camarillo, California
Posts: 74
Dexter,
Yes I'd like to avoid the transformer. But 220 outlets are rare where I live. I only have one that I put in years ago for a table saw.
I have another puzzling data point. Last night I put in a smaller transformer ~200VA instead of the 500VA unit I am using. Foolishly I turned it on at full voltage (I'm using a variac during the testing stages) and the half bridge became a no bridge.
I'm curious why this would happen. The output voltage (no load) for both transformers is the same, about 250VAC.
I have another puzzling data point. Last night I put in a smaller transformer ~200VA instead of the 500VA unit I am using. Foolishly I turned it on at full voltage (I'm using a variac during the testing stages) and the half bridge became a no bridge.
Transformers can cause a significant overshoot on the secondary if turned on at the wrong time, i.e. at the peak of the primary sine wave. Peak secondary voltage can be up to twice its normal value.
Registered Member #2099
Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716
>> Transformers can cause a significant overshoot on the secondary if turned on at the wrong time, i.e. at the peak of the primary sine wave. Peak secondary voltage can be up to twice its normal value.
Are you sure about that? What makes it happen? For core saturation and high inrush current, zero voltage crossing is the worst time to switch on a transformer. (One slope is worse than the other, depending on the direction of remanent flux from the last time transformer was switched off.) Core flux settles down to its normal, symmetric-around-zero swing soonest if the primary is switched on at a voltage peak.
You're right, klugesmith. I made a gross error in my calculations. I think, correct is, that when you turn on at 0 crossing, there can be a tiny bit of overshoot of about 5%, due to primary resistance.
I haven't thought about saturation effects, but it looks to me, that the effects on the secondary side, as alan sailer worries about, might be less dramatic.
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Step Up Transformer
