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Old 28th Dec 2018, 4:05 pm   #1
sunthaiboy
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Default SCR type T31 P800 BFC-3 H11

Seasons Greetings.
I have a large PSU that delivers 120VDC at 30 amps-ish (when it working). It uses a large choke, 3 stud mount SCR's and 3 stud diodes along with a lot of capacitors. I have no circuit diagram. I think at least one of the 3 SCR's is ill (leaky) but cannot identify it. The PSU went bang on run-up as they tend to. Prior to this the output had some unusual ramp waveforms that did not seem quite right.

The SCR's are identified as a AEG (diode symbol) T31 P800 BFC-3 H11.
Can anyone shed light on the technical spec please? I assume it alt least 100 amps (looking at the cathode lead) @ 800 Volts. The PSU is from a R&S SK310 transceiver (part IN 005).

Has anyone had dealings with this sort of PSU design before? It looks very un-conventional and I would like to get working before I build my own replacement.
Thanks
Robert
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Old 28th Dec 2018, 4:38 pm   #2
Sean Williams
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Default Re: SCR type T31 P800 BFC-3 H11

Did it really go Bang?

Reason I ask earlier this year I had to repair an SCR controlled supply, admittedly low volts, but in the end the fault was in the SCR firing circuit, and the heavy (Expensive) devices were all fine....
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Old 28th Dec 2018, 5:01 pm   #3
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Default Re: SCR type T31 P800 BFC-3 H11

The honest answer is I don't know. They test real funny. Anode to cathode is approx 10M Ohms on a DVM both ways. 21 ohms both ways between Gate and Cathode? 0.03V on diode test both ways. The 3rd shows 40 Ohms. Anode to gate O/C both ways. My PIC transistor tester does not recognise any of them. The CCB certainly went bang a type D 20amp. The energising current alone when it worked blew a Type C MCCB. The PSU weighs 70Kg and the transformer part (28VDC section) takes 600W off load. The whole PSU is a very strange design to me.
Robert
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Old 28th Dec 2018, 9:08 pm   #4
Sean Williams
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Default Re: SCR type T31 P800 BFC-3 H11

Im thinking this is a three phase supply, really patronising stupid question, have you checked that it is connected to the correct supply?
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Old 28th Dec 2018, 10:07 pm   #5
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Default Re: SCR type T31 P800 BFC-3 H11

Hi Robert, from distant memory I think AEI "P" types were high speed (for those days) fast turn off types and if 3 phase (looks likely) it could be for a 400Hz supply or even an inverter.

If I get a chance I'll see if I can locate my old AEI data books..

Ed
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Old 30th Dec 2018, 6:53 pm   #6
sunthaiboy
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Default Re: SCR type T31 P800 BFC-3 H11

I can assure you ed, its not a 3 phase PSU. I have a 3 Phase feed to the radio station. I have a HNC in heavy power engineering so am conversant with such arrangements. I worked with 11,000V/33,000V 3 Phase for some of the larger transmitters....

The gate wire on 2 of the 3 SCR's measure 20 Ohms to Cathode.......One measures 10 Ohms. I expected a diode type readings. The anode/cathodes are O/C (several MegOhms) on all.

See Photos of the PSU.

Robert
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Old 30th Dec 2018, 7:01 pm   #7
Sean Williams
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Default Re: SCR type T31 P800 BFC-3 H11

Sorry Robert, I had to ask - The 3 sets of SCR and Rectifier would normally suggest a three phase device.

It does look a serious beast!

Will see if Ed has some info in the devices before we comment any further
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Old 30th Dec 2018, 7:48 pm   #8
sunthaiboy
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Default Re: SCR type T31 P800 BFC-3 H11

An update on the big bang.....it was not caused by the PSU itself, but from a dead short on the output. I found a small off-cut of wire that had dropped into one of the power transistors emitter pin wells (full output voltage). Turning the PSU on its side caused the wire to locate as it did. It may have done damage to the PSU.
When the unit was running on partial load, scoping the DC O/P and ripple showed a waveform that looked like an in-balance in one of the SCR's (a short ramp like waveform exceeding the mean DC level). The 10 Ohm reading on one SCR looks suspicious (pre big-bang)........... either that or all 3 SCR's are faulty in some way. Mind you the gate pulse transformer has a substantial primary winding (read thick solid copper rectangular winding) so maybe 20 Ohms is sensible.
Robert
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Old 30th Dec 2018, 9:30 pm   #9
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Default Re: SCR type T31 P800 BFC-3 H11

Hi Gents, still not located the books, but from memory of the design work done on SCR choppers for DC motor drives we normally used a fast (15uS turn off main (capsule) device) an NO AEI type for the turn off SCR (slow type) and a PO AEI type for the reversing of the commutation cap.

Ed
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Old 31st Dec 2018, 9:59 am   #10
sunthaiboy
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Default Re: SCR type T31 P800 BFC-3 H11

There is no reason why I should not build a simple test rig or use a controlled load based on the premise of 800V and 20 amp minimum load (resistive). Should get an idea of the correct operation and speed of the 3 devices. I guess the use of high speed devices are used to minimise dissipation and quench the back EMF before it exceeds the maximum working voltage especially on light load conditions.

I will check the driver and surrounding components and power it up (when I'm feeling brave), see what happens.
Robert
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Old 2nd Jan 2019, 10:10 pm   #11
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Default Re: SCR type T31 P800 BFC-3 H11

Does the PSU not have a constant current/current limiting function? I have a 0-500v/3A PSU that uses SCR's, mine has a CC mode, but I guess yours is just a straight non variable PSU. Still, you would have thought it would have very good protection what with it's high current capability and should be able to handle a short.

That looks a very well made PSU, the fixing of the SCR's/heatsinks (or are they the diodes) looks odd though, are they just sitting loose inside the chassis?

I have some very big beefy vintage SCR's, I'll dig them out to see if your results are similar to mine.

Andy.
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Old 2nd Jan 2019, 10:18 pm   #12
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Default Re: SCR type T31 P800 BFC-3 H11

I think the SCRs are on machined metal heat transfer bridges- it does look odd though. Even the most highly regarded and competent manufacturers can come up with some seemingly odd ways of doing things, 'work for the sake of it' is a phrase that sometimes comes to mind!
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Old 10th Jan 2019, 7:57 pm   #13
sunthaiboy
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Default Re: SCR type T31 P800 BFC-3 H11

I would be interested to know if you (Andy) dug out out the beefy vintage SCR'S and how they tested Andy? I have just received replacement transistors for the blown one's (during big bang) so would like to re-install everything and power up......after a quick pray.
Robert
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