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Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
The current pulses of a non-controlled rectifier and a cap are limited by the dv/dt of the mains voltage, which is finite. The current pulses are defined even with no line impedance, no capacitor ESR/ESL and ideal diodes. But if you apply voltage pulses, they have infinite dv/dt. I believe you know the equation for instantaneous current through a capacitor: I = C * dv/dt. I think it should be obvious now and I'll stop here as I don't want to get into an argument...
Registered Member #2292
Joined: Fri Aug 14 2009, 05:33PM
Location: The Wild West AKA Arizona
Posts: 795
We will have to agree to disagree then, I also don't want to get into an argument. I was simply trying to share what I learned from my experience with building SCR supplies for coils.
Destroyer of mosfets, yes that is exactly how it would be setup. If you plan to use it to limit current draw though I would warn against it. Go with a variac or an interrupter controlled method PWM or mains sysncs such as Dr. Dark current was talking about.
I used SCR supply in past to precharge my coils only, I never used them for any long period of time at < 95%
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
The current pulses are worse with SCRs. The reason is that an uncontrolled rectifier does its charging at the peaks of the line voltage, which are quite flat. This tends to flatten and broaden the charging pulses. But a SCR can be fired at any time.
The charging current into a capacitor depends on dv/dt, but in the steady state, the SCR fires as soon as the line voltage gets above the capacitor voltage. So there is no massive pulse, it is just the dv/dt of the line voltage. That is a minimum at the peaks, where regular rectifiers operate, and maximum around the zero crossings. Indeed this is where the controlled rectifier performs worst.
I ran into one problem with my controllers (I have two designs) When trying to charge a big capacitor bank, the system could become unstable at low voltage. The current pulses took bites out of the line voltage big enough to disturb the zero crossing detector.
On the plus side, SCRs are very rugged and don't care about high peak currents. I've blown lots of fuses and breakers with my controllers, but no SCRs. It's always good practice to include a series inductor though, to make sure that the critical di/dt of the SCR isn't exceeded. The inductance needed for this is much smaller than what would be needed to improve the power factor. If you're running a big SCR brick off household wiring, the impedance of the mains wiring may be enough to take care of it.
Destroyer of mosfets, yes that is exactly how it would be setup. If you plan to use it to limit current draw though I would warn against it. Go with a variac or an interrupter controlled method PWM or mains sysncs such as Dr. Dark current was talking about.
Which is right? A or B or am I right that the scr's replace the Bridge
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
I've used B, for a coil where I wanted to run off either SCRs or a variac.
I've also done it with two SCRs replacing two of the four bridge diodes, and a full-wave doubler with both diodes replaced by SCRs. They all worked fine.
I drove all the SCR gates from the same source, using pulse transformers for isolation. You're not really supposed to do that, but it doesn't seem to cause any ill effect. One of the SCRs will just "ignore" the pulses and the other will fire.
Registered Member #2292
Joined: Fri Aug 14 2009, 05:33PM
Location: The Wild West AKA Arizona
Posts: 795
teravolt wrote ...
why not just add 1 or 2 or ohms 200w resistor between the cap and diode or a inductor in series withe the AC to cushion the switching periods?
An inductor is exactly what you would add to smooth out the current pulses in series with the AC line. What I was saying before lines up with what every one seems to be saying but maybe I'm just bad at getting my info across.
Depending on where the SCR fires in relation to the line voltage the pulse can be very short and large. IF it is fired right at the start however it will act like a regular rectifier.
In the past I have used the SCR in the doubler configuration with 2 cap. It seem to work well.
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