BOC ADR300

[Kawasaki James]

Hi. I've blown a small selenium rectifier on my TIG. I don't know what else (if anything) it's taken out but it would be dead handy if anyone's had any experience of this, or even better a wiring schematic. I'm guessing it feeds a DC coil on one of the relays but the wiring is faded, filthy and a pain to trace.

 

[eddie49]

Assuming it is a bridge rectifier, there would be 5 tags on it. The two outermost ones are joined together and are the negative DC output. The centre tag is the positive DC output. The remaining two tags are for the AC input.
Something like this "KBPC5010" for £2.35 would replace just about anything:

Diode Bridge Rectifier KBPC Metal Case Single Phase 10/15/20/25/30/50A 1000V | eBay

Series: KBPC. KBPC5010 (50A 1000V). KBPC1010 (10A 1000V). KBPC1510 (15A 1000V). KBPC2010 (20A 1000V). KBPC2510 (25A 1000V). KBPC3510 (35A 1000V). Number of Phases: Single Phase.

www.ebay.co.uk

 

[Kawasaki James]

Assuming it is a bridge rectifier, there would be 5 tags on it. The two outermost ones are joined together and are the negative DC output. The centre tag is the positive DC output. The remaining two tags are for the AC input.
Something like this "KBPC5010" for £2.35 would replace just about anything:

Diode Bridge Rectifier KBPC Metal Case Single Phase 10/15/20/25/30/50A 1000V | eBay

Series: KBPC. KBPC5010 (50A 1000V). KBPC1010 (10A 1000V). KBPC1510 (15A 1000V). KBPC2010 (20A 1000V). KBPC2510 (25A 1000V). KBPC3510 (35A 1000V). Number of Phases: Single Phase.

www.ebay.co.uk

Thanks Eddie, it is indeed a bridge rectifier wired as you describe. I'd got my eye on something similar to the ones you've shown. As I don't yet know what the current the rectifier is handling, do you know if there is there any disadvantage to having a higher current rectifier? (say a 50A rectifier where only 10A are required). I was having a fiddle with it today but I also seem to have lost the AC supply to the rectifier so it looks like it took something else out when it shorted. Thank you for your assistance.

 

[eddie49]

To run a relay coil probably only needs 1/4 of an Amp or less, so any silicon rectifier will do, e.g. this 8 Amp one:

Pardon our interruption...

The current rating specified refers to the maximum that the device can handle safely without overheating or burning out, so using a higher rating is no problem.

 

[Kawasaki James]

A fuse might have been a good idea. It really dumped some power when it shorted. Little spheres of molten metal all over, I hope it didn't damage that big cap.

Anyway, in the absence of a wiring diagram I've begun to trace the wiring. Had to clean it off with thinners to see the tracer colours but they're very faded. Can't say I'm impressed with the original wiring routes, could be wired much more efficiently. Also there're some non original wires in there which I'll be investigating. Other than the jumble of filthy spaghetti it's extremely simple, it'll just take a while to make sense of it.

 

[eddie49]

The rectifier is perhaps 60 years old, so not a bad lifespan!
I suppose that all the relays will have the same operating coil voltage, so if you can find a label on one, you will know roughly what AC voltage to look for as the input to the failed rectifier. From the size of the plates, I would guess that the selenium rectifier is rated at about 3 Amps.
Another internal photo:

boc adr 300

I HAVE BEEN CHASING A FAULT ON THE BIG TIG. THE FAN MOTOR WIRING INSULATION FAILED WITH OLD AGE. AND HAS SHORTED DOWN THE CONTROL CIRCUIT BLOWING THE COIL ON ONE OF THE RELAYS. MANY THANKS TO DAVE CACKETT OF WOODFIELD WELDING FOR ALL THE EMAILS TRACING THE FAULT. This machine would not look out...

www.mig-welding.co.uk

 

[Kawasaki James]

Interesting. The wiring looks significantly tidier on that machine, more how I'd expect. Someone's clearly been messing with mine.
The 3 relay coils are 110V AC 110V AC and 24V DC.
I've found a KBPC3504 rectifier in my electrical stocks so I'll use that and see how it performs. I've read that selenium rectifiers have quite a voltage drop compared with silicon ones but I guess I can just measure it to make sure it's not too high for it's purpose and adjust with a suitable resistor if necessary.

 

[Kawasaki James]

My mistake, coils say 110V AC, 110V DC and 24V AC

 

[eddie49]

The voltage capability of a selenium rectifier is determined by the number of plates in series, and I think the rating is about 20 volts RMS per plate. Your rectifier has 3 plates per segment, i.e. 60v, and for a bridge rectifier the current flows through two "diodes" in series, so it could handle 120 volts.

 

[Kawasaki James]

Getting there slowly!

 

[Kawasaki James]

Well she'd finally back up and running. I certainly know a lot more about these old machines than I did a couple of months ago! A bewildering collection of components suddenly begin to make sense after a while. I got about 80% of the wiring mapped out the long way but it was a painful process. A call to Westermans came up trumps for the last 20% as Craig very kindly sent me a manual FOC. Now that's good service.

I replaced the rectifier with a modern equivalent but that was only part of the battle. As I disturbed the wiring loom, 4 or 5 connections came adrift. They are rather fatigued after 50 years or so. Also, a section of wiring had burnt out when the rectifier popped. When everything was replaced the set still wouldn't fire up on TIG. I traced the fault to the main process selector can switch as the 110V AC control circuit was not energising. Also, one of the terminals was permanently closed circuit in every switch position which was a bit alarming.

First off, the switch exploded all over the workshop when I dismantled it. I found everything but one spring. Also noted that one of the 430V terminal screws was stripped and there was some burning at the loose connection. Anyway, as the argon spot weld setting never gets used (it requires a few terminal changes inside the set) I pinched some of the good components from that terminal to renovate the rest of the switch. Fortunately the terminal that was permanently closed was a redundant one. I note that these cam switches are not necessarily 'fail-safe'. What had happened is that the plastic cam follower had disintegrated so the cam was unable to open the contacts and the spring held them closed.

Also noted that the auxiliary transformer (110V, 24V secondary) was linked through the the 380V primary instead of the 440V one. I'm receiving 430V so the control circuits have been running at around 130V and 32V which may have been causing a bit of unnecessary stress.

Long story long, I fired her up this afternoon and she's welding very nicely once again. On TIG I can still hear a relay or contactor clicking every few seconds but until I have someone else present, I won't be able to see which one it is. Can't remember whether it's always done this or if there's still a minor fault or whether this is normal. At least I'm happy now that I understand the whole machine.

I suppose the bottom line is an explanation of what the rectifier does on the set for anyone else in the future with the same problem. It's all about the argon delay circuit. Most of the rest of the controls run on AC but the argon delay relay is energised through a large capacitor which slowly discharges through various resistors depending on the time setting, hence DC is required. It rectifies 110V AC . A long winded explanation but there you go.