Karl's Power Supply SCR question?

RogerN · 2010-10-02T02:48:50+00:00

I looked up a bit of SCR info to see how to wire Karls SCR's to act as rectifiers. I'm posting this to confirm or correct me if I'm wrong. Many diagrams I saw show an AC supply, a load, and the SCR to vary the power to the load. They used a variable resistor to control power to the gate to fire the SCR sooner or later in the rising + portion of the sin wave. If Karl wired his gate to the anode with a diode (1N4004 maybe?)it should allow forward voltage to the SCR gate and prevent reverse power if there is an internal resistor present (Per SCR tutorial provided in the "Power Supply" posts). This should turn on the SCR a soon a voltage was high enough on the gate, Vf of the 1N4004 + SCR gate turn on voltage (~4V).So, if I'm thinking about this correctly, if Karl connects the Anode of a 1N4004 diode to the anode of his SCR and connects the gate to the cathode of the 1N4004 diode, the SCR should work like a rectifier. Correct?I would recommend wiring it up this way and then powering it from a lower voltage transformer through a current limiting resistor (or light bulb) to verify everything is working as hoped. Maybe power it from a wal wart with AC output, then maybe 120V line cord with fuse, then the 230VAC mains. Just be careful, 230VAC rectified to DC with capacitors charging to peak is nothing you want to get zapped by! It's nothing to be terrified of, I work with 480VAC mains rectified and filtered almost daily, but I don't take a chance on touching it.RogerN

Oct 02, 2010 27 Replies

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DoN. Nichols 15 years ago

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Hmm ... thyristors, not SCRs. And it looks as though the one illustrated has the cathode connected to the heavy-gauge wire, not the stud. Now -- the remaining question is still "are *all* of them the same?"

Part number does not seem to match. Just because it looks the same -- are you sure that it *is* the same?

O.K. According to Wikipedia, "Thryristor" is an alternative term for SRC -- but it also can refer to a bunch of other devices.

Make sure exactly what you have -- by matching the part number, not just by finding something which looks similar. Most of the data sheets for 71RC80 devices appear to be on Chinese sites now, and those are blocked from here.

Good Luck, DoN.

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Martin H. Eastburn 15 years ago

They both had it - a pot can adjust the phase trigger voltage. The R-C time constant of the pot/cap combination sets a time delay which equates to a phase angle and at the trigger voltage there is a massive rush of current through the main terminals.

It is all how one looks at it. RC is used with AC power - the load is DC partial wave.

Four terminal Thyristors are used for gate-turn-on and gate-turn-off operation. Two gating - an anode gate to turn it off and the cathode gate which is used to turn it on.

There are many version of Thyristors.

Mart>

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Martin H. Eastburn 15 years ago

Don't stack the SCR's!!!!!

The are likely high voltage already.

Stacking means putting anode and cathodes end to end.

If you do that - the gates will be floating up at higher and higher voltages. The one you showed us was 400V DC. Likely a 220v gated rectifier.

No manual and no cable set that might lead you to believe the use ?

Mart> >

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Richard the Dreaded Libertaria 15 years ago

It's trivial. Just connect the gate directly to the anode, and voila! Diode!

Seems kind of a shame that he didn't keep any of the control electronics, however.

Cheers! Rich

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Don Foreman 15 years ago

That violates reverse gate voltage spec. Circuit behavior may not be as expected or hoped for.

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Rich Grise 15 years ago

Yes, I concur - it should have a diode from gate to cathode to clamp the reverse gate voltage, and of course, a resistor to limit dissipation.

What I had in mind was the time I made a center-tapped full wave controlled power supply, and drove the mongo SCRs with 4Nsomething optos, and just connected them straight from anode to gate, because when the gate fires the SCR, its forward voltage drops to a diode drop.

But there was no direct connection for the negative half-cycle, so based on my experience, I did, in fact, neglect to include the problem of the negative gate voltage.

Thanks, Rich

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