Looking for diodes for montgomery ward welder

[Treemonkey1000]

You should replace all 4 of them.

 

[Steve Bosshard]

Ditto - replace all four. (Caveat - provided they are the same part # and part of the same circuit).

 

[Samshine]

The tech support man i talked to over the phone, told me the diodes with the part number he gave me, is the same ones that are in the welding machine now.
samshine

 

[Treemonkey1000]

The tech support man i talked to over the phone, told me the diodes with the part number he gave me, is the same ones that are in the welding machine now.
samshine

So there you go. You will have to scrape up the money it sounds like for all 4. You can not be sure which ones are damaged completely or weak. No sense in taking any chances by not replacing them all. Too bad he didn't give you some more spec's to see if they could be cross referenced. As all have said in this long thread. Good heat sinking, use thermal compound when mounting them, good clean connections to the diodes, and make sure the fan is working correctly..

 

[Transit]

150 amps is less than I had thought, that defiantly says that the diodes are cooked. To bad you did not get the voltage ratings. I would not bother replacing the diodes with the same 150 amp rating, I would be looking at a 200 amps minium and perfer 250 + amps.
640-200UF60 200UF60 Ruttonsha Diodes - Standard Recovery 200 Amp 600 Volt Power Diodes
640-200UF10 200UF10 Ruttonsha Diodes - Standard Recovery 200 Amp 100 Volt Power Diodes

640-250UF80 250UF80 Ruttonsha Diodes - Standard Recovery 250 Amp 800 Volt Power Diodes
640-250UF60 250UF60 Ruttonsha Diodes - Standard Recovery 250 Amp 600 Volt Power Diodes

640-300UF80 300UF80 Ruttonsha Diodes - Standard Recovery 300 Amp 800 Volt Power Diodes 800 V 300 A 5000 A 1.35 V
640-300UF100 300UF100 Ruttonsha Diodes - Standard Recovery 300 Amp 1000 Volt Power Diodes 1000 V 300 A 5000 A 1.35 V
640-300UM120 300UM120 Ruttonsha Diodes - Standard Recovery 300 Amp 1200 Volt Power Diodes 1200 V 300 A 5000 A 1.35 V
640-300UF140 300UF140 Ruttonsha Diodes - Standard Recovery 300 Amp 1400 Volt Power Diodes 1400 V 300 A 5000 A 1.35 V
640-300UF10 300UF10 Ruttonsha Diodes - Standard Recovery 300 Amp 100 Volt Power Diodes 100 V 300 A 5000 A 1.35 V
640-300UF20 300UF20 Ruttonsha Diodes - Standard Recovery 300 Amp 200 Volt Power Diodes 200 V 300 A 5000 A 1.35 V
640-300UF60 300UF60 Ruttonsha Diodes - Standard Recovery 300 Amp 600 Volt Power Diodes 600 V 300 A 5000 A 1.35 V


Other things to keep in mind is how wires are connected to the center pin of the diode, soldered or terminal with pigtail.
If the wires are soldered to the center take care not to over heat the diode.
The diode-heat sink must be electrically isolated from the other diode-heat sink and the welder case. This means that there are insulating hardware to keep the heat sinks from shorting to each other.
I would take pictures and make notes, trust nothing to memory.

 

[BrentD]

This Diode on ebay would be a perfect replacement part. Not sure of the stud size of that diode, but if it's smaller or larger than current, you can always a washer or drill new mounting holes in the heatsink.

You would need 4 of them.

If you're going for a serious upgrade you could go with This 400A diode which is the same price as the one above and more than double the capacity of your existing diodes.

(I build tesla coils for a hobby so I'm used to dealing with high-voltage and high current circuitry.)

 

[Transit]

Brent, good find. The 250 amp meets my minium requirement with a 66% gain in current rating. The 400 amp stud is very attractive, however the used condition gives me concern. If the heat sink needs to be drilled to accept the diode so much the better. Drill and tap the hole, the thread will increase the contact area between the diode and heat sink making the heat flow more efficient.

 

[Samshine]

True it might be 150amp. But if i got four (two above and two below).
That means i get 300 amps. out of it right?
samshine

 

[Samshine]

I took a look at he diode Brent sent me off of e-bay. My diode looks just like that one, except were the copper braid wire is connected to the big end. On this one it looks like it has about a 3/4 inch connecting. Were mine only has a 1/4 inch connecting. Looking at the ruler it looks like it has a 5/16 inch stud. That is what mine is. I like transit's concern. I dont want to put used parts in the welder.
My only concern, me not knowing what the voltage is on my diodes. How do i know if i am picking the right diode for my welder?
I know i sure liked the price on the one on e-bay.
But will this diode work, not knowing the voltage rating of the diodes in the welder now.
Thanks for all your help.
samshine

 

[BrentD]

My only concern, me not knowing what the voltage is on my diodes. How do i know if i am picking the right diode for my welder?

Like somebody said before a diode is (basically) to electricity what a check valve is to water or air. It allows voltage to only flow in one direction. Just like a check valve, there is a limit to how much backwards pressure (i.e. voltage) it can block before it breaks down. (Forward voltage doesn't matter unless it is astronomically high.) Where things get a little different with electricity is that you can put two diodes in series and their "peak inverse voltage" adds up. These diodes in question are 600V diodes so putting two of them in series would give you a diode rated roughly 1200V (more or less based on some unpredictable technical factors that are mostly irrelevant here)

The primary transformer in your welder steps voltage down from 220V to a lower voltage, usually no more than 80-90 volts in "standby". When you strike an arc that voltage will drop by as much as half.

So if you consider that, then the 600V diodes are more than enough to handle that voltage.

Someone earlier mentioned the inductor. For those that don't know, and inductor is just a coil of wire wrapped around a ferro-magnetic core. It's used in welders to help smooth the DC arc because one of the rules of electricity says that current traveling through an inductor can't change magnitude or direction rapidly due to the magnetic field induced in the core by the current flowing through the coil of wire. When the current starts flowing, that magnetic field stores up part of the energy from the current up to a certain point and then if the current starts to drop, the magnetic field dumps energy back into the coil momentarily boosting the current. When the current starts to rise, the magnetic field in the core will grow again and absorb some of the current and slow the currents rise up to the point where everything is back in balance.

The downside to this is that when the arc is broken the magnetic field in the inductor collapses rapidly. The speed of the collapse creates a spike of high voltage much higher than the initial voltage in the circuit and the welder has to handle it somehow. (This phenomenon, called "inductive kickback" is true for any inductive coil, be it a motor winding or a relay coil.) Fortunately, that voltage spike will produce current flowing in the same direction as the original circuit.

On DC welders, the inductor is placed after the diode rectifier bank, so the kickback from breaking the arc can't flow back toward the diode bank. It is just dissipated (again through physical laws too involved for this discussion.) The only way you would get serious inductive kickback across the diode bank would be if you killed power to the welder while a rod was stuck.

So what this little physics lesson is saying is that 600V diodes should be more than sufficient for your welder, unless you intentionally abuse it by repeatedly sticking rods and killing the power while they're stuck or deliberately connecting the two welding leads together, cranking the power way up, and flip the switch on and off a bunch of times.