Replacing an old SCR

Loot at the gate to see what is causing it to turn on. The regulator circuit may be oscillating from bad bypass capacitors, or high ESR electrolytics.

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You can\'t have a sense of humor, if you have no sense!

lator

The gate is pulled low through a small resistor, and high through a smaller resistor through a transistor to 15V. The pulses turning it on is just a 3% duty cycle 5KHz square wave.

Is it clean? There may be another, narrower pulse you aren't seeing.

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You can\'t have a sense of humor, if you have no sense!

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It seems to be from a simple 555 based timing circuit, so that's all that there is.

** SCRs have typical turn on times of less than 2 uS - so 6 uS is actually a long time. ** Normal SCRs will not turn off by the gate.

There has to be an interruption in the "on current " to less than 10 mA to guarantee turn off of a 2N6509 - but you also gotta watch out for the

50V/uS dv/dt as well.

..... Phil

The part should latch on in 2uSec, with 80mA of gate current. The pulse train is normally only provided intentionally to ensure latching on slow-rising load currents.

3% duty of a 5KHz drive meets the firing time duration requirement (6uSec).

Cathode hold current for the part is also 80mA.

Perhaps the drive transistor and series resistor is not producing the required drive level. This can be checked without main power applied. It could be damaged or affected by bad supply decoupling or loading elsewhere, by other damaged or aging components.

The part is not a GTO device. The small gate shunt resistor is only there to ensure no self-firing at higher temperatures. The jedec-registered part will not have changed dramatically in the last

20 years.

External snubbers and quenching circuits should be examined for integrity - as they are intended to assist in the switching function by providing temporary hold current, reducing turn-off dv/dt, and forcing commutation when turn-off is required.

RL

"legg"

** The spec says 50mA. ** The spec says 18mA typ.

Individual devices may hold at much lower current - this could be the OP's problem.

Ie - the SCRs in his PSUs were selected.

..... Phil

OP's

So, can anyone recommend a device that would be slow to latch in, but has similar power capabilities to a 2n6504?

Firing and hold current levels are related to gate shunt resistor values.

It wasn't bvious to me that the OP considered latching on to be the problem - that a 'working' circuit was not latching.

I would advise re-examination of operation of the working device relocated into the known-good unit. It may be that latching operation is indeed required, but that quenching circuitry in the defective unit is worn out.

If there's a model number or schematic available, likely culprits might be identifiable.

RL

he OP's

Putting my scope on it, this is what I see:

(Pulse widths exaggerated, obviously)

That's measuring across the SCR (Anode to cathode). With either replacement SCR that I bought, I'm seeing the latching waveform shown on the bottom, in both the good and bad power supplies. I know the 'basic idea' of how an SCR works, and that's just how I'd expect it to.

With the ancient SCR taken from the good power supply, I'm seeing the top waveform, where the SCR briefly conducts and doesn't latch, in both the good and power supplies. Both work perfectly fine like this.

Out of curiosity, I tried putting a series resistor with the SCR (200 ohms), and that made the new SCR not latch up, but the output of the power supply became unreliable. So I'm guessing that the old SCRs were probably hand picked ones that had above average holding currents?

he OP's

There is a 27 ohm resistor pulling the gate to 15V through a transistor (Which is pulsed by some timing circuit), and a 100 ohm pulling it low.

he OP's

The gate is connected to a 100 ohm resistor to ground, and a 27 ohm resistor to a transistor that pulls it to 15VDC for firing. Should I try increasing/decreasing some of those values?

that looks like an issue with the holding current being lower in the new one over the old. Have you tried placing a load on the working supply and then take those readings?

Even though a possible higher holding (I) may exist in the older units, I still think you have an issue with that circuit.

You show a AC source how ever, the gate is being triggered at a free run state? that does not make sense to me. The AC tells me it should be a phase angle firing supply how ever, they normally only fire at the peak and on the down side of the peak, not all the time.

So the next scenario leads me to thinking that maybe you have an old thyristor inductive switching supply with bad caps on the DC bus..

back in the hay days, it was an interesting concept to use thyistors on a DC bus in a inductor/capactor discharge circuit. Still used in many applications today.

Maybe, showing us a schematic of the supply could offer some help here

"

The waveforms show control circuitry firing the scrs (good oor bad)at a very early phase angle, suggesting that NOTHING is actually working as intended. The control circuit might as well be open loop.

I would expect, under light-load test conditions, to see a relatively late phase angle firing in a functional circuit that simply controls the gross line frequency amplitude, downstream.

In a PWM circuit with two scrs (the second might be un-noticeably small) intended to convert at a frequency higher than the line frequency, you would see alternating gate inpulses to the two devices.

How are you determining that everything is 'normal'? It's possible that the 'good' scr might meet an early death, if loaded under current circumstances.

The 'good' waveform might result from bad or open connections in the output power train.

If the 'bad' latching thyristor results in output overvoltage, check the feedback path to the control circuit for open contacts etc etc.

RL

On a whim, I decided to wind a very small toroid (Just 10 turns around a half-dollar sized magnet) and put it in series with the cathode. It seems to have solved my problems. The power supply works with the new SCR, even under load. Nothing is getting hot, and output is stable, even after a while under load. So at least for now it should keep chugging along.

Thanks, all.

For the love of Mike.....a MODEL NUMBER please.

RL