SCRs as Diodes question?

SS2017

Joined May 23, 2017
7
Hello all,
I need to connect 4 SCRs to convert a single phase AC source to DC. The SCRs that I have rated for 225AMP in hokey puck package. They will be connect as standard 4 diodes bridge. I am not planning to control them. I just want to use them as diodes. I have controlled SCR in the past and I am aware of how to trigger them via a pulse transformer ...etc. however, is there any way to keep the gate high so they act as diode? In simple, very minimum hardware and uses them as they are diodes?
The AC source will vary from about 20 VAC to 80 VAC coming from a welder transformer. I read several posts and online reading about SCR, however nothing so far meet my requirements.

crutschow

Joined Mar 14, 2008
31,091
SCRs have a higher forward drop than standard diodes, so I don't see why you would want to do that.

But if you want to, just connect the SCR gates to their respective anodes through an appropriate sized resistor (to give the needed trigger gate current at an anode voltage of about 2V).

SS2017

Joined May 23, 2017
7
crutschow,
You are right, the system that I salvaged the SCRs from uses 2v saw wave pulses to trigger the gates. I also noticed about 150mAMP peak gate current. No data sheet for these SCRs. I am only want to use them because I have them.
In the past, I used a calculated resister and diode to get the gate rated current to trigger the SCR on a fixed source. However, as you notice the source will vary. I am thinking the gate current will vary also if I just used the resistor approach. My knowledge about SCR is the gate current effect the drop?
Thank you

crutschow

Joined Mar 14, 2008
31,091
I am thinking the gate current will vary also if I just used the resistor approach.
It will increase until the SCR fires.
Then the gate current will drop to essentially zero.

Below is the LTspice simulation of an example circuit:
The selected SCR turns on when the gate current reaches ≈34mA at an AC voltage of ≈2.57V.
At the point the gate current drops to zero.

Last edited:

JUNELER

Joined Jul 13, 2015
183
Hi,

Can you give the part number if any indicated on that SCR 225A.

SS2017

Joined May 23, 2017
7
crutschow,
Why you used R3? I don't think its required as I don't need to turn OFF the SCR? Other question if my voltage vary from 20 VAC to 80 VAC, do you suggest to do the R2 (as per your drawing) based on 80 volt? I don't think this will work when the source is at 20 volt? I thought of installing a small 240 to 12 volt transformer on the main source and connect the 12VAC to 4 pulse transformers 12 to 3 with full bridge + cap at the 3 to output DC for each SCR, this way they all be triggered all the time. I just think there should be a simpler way for my scenario? I guess I have to try the resister approach and see?

JUNELER,
The part number is 5P50-0225 I could not find any information online, However the system they came from shows to be rated for 300V/225A.

Thank you,

tcmtech

Joined Nov 4, 2013
2,867
I've used SCR's for diodes in low voltage high current power supplies many times. All I ever did was put a few tens of ohm resistor between the gate and anode to get them to fire themselves.

crutschow

Joined Mar 14, 2008
31,091
I've used SCR's for diodes in low voltage high current power supplies many times. All I ever did was put a few tens of ohm resistor between the gate and anode to get them to fire themselves.
Without a gate diode, I would think the gate would zap when the AC goes negative.
Most SCR gates have a low maximum reverse voltage rating.

crutschow

Joined Mar 14, 2008
31,091
Why you used R3? I don't think its required as I don't need to turn OFF the SCR?.........
It's to drain off any leakage current, but I suppose in this application it's not needed.

ian field

Joined Oct 27, 2012
6,536
Without a gate diode, I would think the gate would zap when the AC goes negative.
Most SCR gates have a low maximum reverse voltage rating.
I think you're right - but it gets more complicated than that.

It probably ends up with the same difficulty as you get with MOSFET gates; The anode would probably clamp down the voltage needed for the gate. It should trigger and stay latched before the anode voltage falls, but noisy electrics can cause erratic operation on a rising sine wave.

Normal practice is a pulse transformer and pulse generator. Dual secondaries are common enough. There probably are quad secondary transformers, but series or parallel primaries on a couple of dual secondary units will do the job.

tcmtech

Joined Nov 4, 2013
2,867
Without a gate diode, I would think the gate would zap when the AC goes negative.
Most SCR gates have a low maximum reverse voltage rating.
Never been a problem so far and I have built many over the years using power SCR's of various ages and designs. No need to over complicate something that works.

crutschow

Joined Mar 14, 2008
31,091
Never been a problem so far and I have built many over the years using power SCR's of various ages and designs. No need to over complicate something that works.
That's what they said before the last Challenger launch.

It's not good policy to operate a device outside it's rated limits, even if it seems to work.
It likely is causing the gate-cathode junction to zener and conduct reverse current limited by the resistor, which would be stressing the device.

tcmtech

Joined Nov 4, 2013
2,867
That's what they said before the last Challenger launch.

It's not good policy to operate a device outside it's rated limits, even if it seems to work.
It likely is causing the gate-cathode junction to zener and conduct reverse current limited by the resistor, which would be stressing the device.
Well unlike the challenger launch more than once I have ran them well below freezing temperatures without failure so....?

Sure I agree at a higher voltage it's an issue but at lower voltages it's not so much of one. Especially so with the larger high amperage units. Those by design are very hard to kill at low voltages.