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Vintage diode identification

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Hi there!

I have the old welding machine from 80ies I believe. It has one control circuit and it is supplied by 48AC. Suddenly it lost some functions and I inspected this circuit visually and found bad looking polarized 220mf capacitors and one completely dead diode and bunch of diodes with broken ceramics body. I searched through some welding forums and there was suggestion to change these old diodes to UF4007. I did and that does not helped.

Now I came to conclusion that the problem is absence of correct replacement diode and UF4007 is not the way to go. Circuit contains bunch of these diodes looking to be identical. So I put the rectifier instead of diode bridge to get spare diodes. When I changed UF4007 to original ones circuit starts to work as it should. The problem is that I need more of those diodes to completely change the bad looking ones.

UF4007's behave strange. In some point of circuit they seems preventing forward current. In some other point of circuit it leads to increased current and overheating cap and resistor. In the diagram D16 was one that overheated R1 and C2. D6 was blocking current to relay U5. When I replaced back to original ones both issues disappeared.

Multimeter diode test shows 0.545V. Power supply bench at constant current 0.001A shows that it opens at 0.67V.

Please help me with correct replacement choice!

I am attaching picture of these diodes and the circuit diagram.

Thanks in advance!


Identifying the color coded diodes is somewhat tricky and not always possible / unique.

Judging from the case size, the diodes may withstand slightly more than 1 A. So the UF4007 may be too weak. The faster UF400x comes with more leakage current than the original 1N400x - this may be a problem in some cases.  With just mains frequency rectification for the electronics / relay supply there is no need for fast diode.

There seem to be still a few of the diodes that are looking good. To see if the diode is a fast one (like the UF4xxx) or a normal slow one, one could do a quick test with square wave generator and scope.


they look like TFK (Telefunken) Diodes.
Assuming the color code is grey+green it should be a BYW85.
Replacement according to data sheet would be a 1N5627.

depending on wire diameter (0.85 mm) it could also be a BYX85.
The BYW85 should have 1.35 mm.

with best regards


What sort of welding machine is it - MIG, TIG or an arc inverter?

What brand  and model is it? There may well be a service manual on the WWW which would identify exactly what those diodes are and help you find NOS or current production equivalents.

The only diodes in my collection which look anything like that are Zeners, but yours can't be that.

Thanks guys,

The machine is Dalex-Werke make (German company that still doing good), VDE 0542, Type CGL 160. It is MIG welder. Well, I have searched for the diagram or some other information but without luck. It is vintage machine nowadays so no surprise that there is no info online. I even wrote an email to Dalex company but no reply. So far I narrowed the problem to minimum and I doubt that I could get some info on these diodes on the particular make. My guess that these are quite common diodes of that era.

Unfortunately I do not have square generator and I am not sure that I would be able to measure with certainty even if I had it. Looking at the machine I believe that diode must be slow :)

Yes, I also came across to  these Telefunken BYW85 but was not sure at all on that. Are avalanche diodes used in diode bridges? Initially I thought it should be very common diode. I found out that my local supplier could deliver me Vishay 1N5627-TAP. Does this "TAP" extension matters? Probably it is worth trying to use these 1N5627 and see what happens next.

Well, the wire diameter on original diodes is only 0.3 mm (not even 0.85). Actually 1.35mm is a really beefy one. Yeah, and the body of 1N5627 is 4mm while mine is about 2.5mm. Hmm, 1N5627 seems to be something other than original ones.

Nope, I rechecked with ordinary caliper and the wire diameter is 0.6 no 0.3 as I stated before. The body is somewhat 4.5mm and diameter 2.7mm. I used digital one before and realized that it can not be 0.3. :)


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