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Replacing an old SCR

E

[email protected]

Hello.

I have a pair of old power supplies here, and one of them has a failed
SCR. It's a 2N6509.

However, when I went and replaced it, I found that the new device did
not work. I put a scope on the SCR, and found that it's being turned
on and off very very quickly. It's only on for about 6 microseconds,
then left off for about 200us. So I think on the working one, with the
old SCR, it's not being given enough time to latch in. To verify, I
removed the working old scr and put it in the bad power supply. It
worked just fine.

I tried 2N6509s from two different sources, but they both latch up,
like one would expect an SCR to. My guess is that this power supply
(It is old, like 20 years old) relied on the imperfections in the SCRs
to work properly. The new ones seem to have no gate turn off action.
Any thoughts on what I should do? The triggering voltage is 15V, so I
tried an IGBT in series with a diode, but that didn't work. I just
blew a fuse. Putting the scope on it, I found that my diode wasn't
chopping off all of the negative voltage. It seriously reduced the
negative portion of the sine wave (From -170V to about -50V), but this
is still well over the Vec breakdown for the IGBT. That part confuses
me, too. It was just a standard 1N400x diode. I tried two in series
and it didn't get any better.

I looked for some GTO SCRs, but I couldn't find anything. I couldn't
find any reverse blocking IGBTs on mouser/digikey, either.

Thanks all.
 
E

[email protected]

   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.
Click to expand...

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.
 
J

Jamie

Hello.

I have a pair of old power supplies here, and one of them has a failed
SCR. It's a 2N6509.

However, when I went and replaced it, I found that the new device did
not work. I put a scope on the SCR, and found that it's being turned
on and off very very quickly. It's only on for about 6 microseconds,
then left off for about 200us. So I think on the working one, with the
old SCR, it's not being given enough time to latch in. To verify, I
removed the working old scr and put it in the bad power supply. It
worked just fine.

I tried 2N6509s from two different sources, but they both latch up,
like one would expect an SCR to. My guess is that this power supply
(It is old, like 20 years old) relied on the imperfections in the SCRs
to work properly. The new ones seem to have no gate turn off action.
Any thoughts on what I should do? The triggering voltage is 15V, so I
tried an IGBT in series with a diode, but that didn't work. I just
blew a fuse. Putting the scope on it, I found that my diode wasn't
chopping off all of the negative voltage. It seriously reduced the
negative portion of the sine wave (From -170V to about -50V), but this
is still well over the Vec breakdown for the IGBT. That part confuses
me, too. It was just a standard 1N400x diode. I tried two in series
and it didn't get any better.

I looked for some GTO SCRs, but I couldn't find anything. I couldn't
find any reverse blocking IGBTs on mouser/digikey, either.

Thanks all.
Click to expand...
The last time I checked, a IGBT was a "Isolated Gate Bipolar Transistor".
You may want to rethink you're tinkering.

Or maybe I should not of replied to you? I noticed the "GMAIL" account.
It seems that a few around here have some fetishes with GMAIL people.


http://webpages.charter.net/jamie_5"
 
P

Phil Allison

I have a pair of old power supplies here, and one of them has a failed
SCR. It's a 2N6509.

However, when I went and replaced it, I found that the new device did
not work. I put a scope on the SCR, and found that it's being turned
on and off very very quickly. It's only on for about 6 microseconds,
then left off for about 200us. So I think on the working one, with the
old SCR, it's not being given enough time to latch in.
Click to expand...


** SCRs have typical turn on times of less than 2 uS - so 6 uS is
actually a long time.

I tried 2N6509s from two different sources, but they both latch up,
like one would expect an SCR to. My guess is that this power supply
(It is old, like 20 years old) relied on the imperfections in the SCRs
to work properly. The new ones seem to have no gate turn off action.
Click to expand...

** 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.

http://www.onsemi.com/pub_link/Collateral/2N6504-D.PDF



...... Phil
 
E

[email protected]

The last time I checked, a IGBT was a "Isolated Gate Bipolar Transistor".
   You may want to rethink you're tinkering.

Or maybe I should not of replied to you? I noticed the "GMAIL" account.
  It seems that a few around here have some fetishes with GMAIL people.

http://webpages.charter.net/jamie_5"
Click to expand...

Like I said, when I transplanted the not-blown SCR out of the working
power supply into the other one, it worked perfectly. It successfully
goes back into blocking mode after the gate voltage ends. I've tested
it with a scope. But when I replaced the SCR with a new device, it
failed, latched up like you would expect an SCR to. So my thinking
was, I need some kind of gate turn off thyristor. But I couldn't find
one on mouser or digikey, so my next thought was: IGBT with a series
diode.
 
L

legg

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.
Click to expand...



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
 
E

Eeyore

The triggering voltage is 15V
Click to expand...

Via what resistance ? SCRs trigger on a turn-on current. Also it's
possible the current may be marginal if the device isn't latching on.

Graham
 
E

Eeyore

Like I said, when I transplanted the not-blown SCR out of the working
power supply into the other one, it worked perfectly. It successfully
goes back into blocking mode after the gate voltage ends.
Click to expand...

That's not how an SCR works ! An SCR latches on after being triggered by a
CURRENT, you'll blow it up if you feed 15V to the gate and once latched only
ceases conducting when the load current falls to (about ) zero ( the holding
current ). Look at the ON Characteristics here.
http://www.alldatasheet.com/datasheet-pdf/pdf/11509/ONSEMI/2N6509.html

It seems you're misunderstanding this PSU totally.

Graham
 
E

Eeyore

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.
Click to expand...

I have come across some SCRs where you could turn them off with a negative
gate voltage but it's not standard practice. Maybe the PSU maker was using a
'defect' in some manufacturer's SCTs to make the circuit work and 'proper'
2N6509s from other sources don't do this.

To turn off an SCR the way you seem to be suggesting you need a gate turn-off
thyristor.
http://en.wikipedia.org/wiki/Gate_turn-off_thyristor

Graham
 
B

Baron

Eeyore Inscribed thus:
That's not how an SCR works ! An SCR latches on after being triggered
by a CURRENT, you'll blow it up if you feed 15V to the gate and once
latched only ceases conducting when the load current falls to (about )
zero ( the holding current ). Look at the ON Characteristics here.
http://www.alldatasheet.com/datasheet-pdf/pdf/11509/ONSEMI/2N6509.html

It seems you're misunderstanding this PSU totally.

Graham
Click to expand...

I agree ! Its likely that there is some kind of failure that is
allowing the new SCR to unlatch because the current through it can't be
sustained.
 
P

Phil Allison

"Baron of any kind of a Clue "
I agree ! Its likely that there is some kind of failure that is
allowing the new SCR to unlatch because the current through it can't be
sustained.
Click to expand...


** Thick as a plank ....




.... Phil
 
L

legg

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

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
 
E

[email protected]

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
Click to expand...

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

http://imgur.com/2jrnL.png (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?
 
E

[email protected]

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
Click to expand...

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.
 
E

[email protected]

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
Click to expand...

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?
 
J

Jamie

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

http://imgur.com/2jrnL.png (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?
Click to expand...
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

http://webpages.charter.net/jamie_5"
 
L

legg

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?
Click to expand...

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
 
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