Looking for diodes for montgomery ward welder

[patrick_g]

Patrick, you are very close. Welding duty cycle time is based on 10 minutes. Weld 3 min out of 10 and that is 30% at a stated current. Raise the current and the duty cycle will shorten. Lower the current and you may weld longer.
Keep lowering the weld current and at some point the duty cycle will be 100%, once that point is reached you may weld at that current all day long.
Small hobby welders have their 100% point too low to weld anything but thin stock. If you want a welder that can run at the 100% point and do useful work you need $$$$$$.
If you weld past the 100% point and the thermal safety opens, stop welding and keep the machine powered on to keep the cooling fan running. Once the safety resets, the unit must be allowed to cool for the full 10 minutes.

CLOSE?? What did I say that wasn't gospel??? I may have left something unsaid regarding turning current low enough to get to 100% duty cycle but that is a sin of omission. What is my sin of commission? I only intended to explain what I explained that wasn't previously explained. Thank you for adding a little info for completeness that was beyond my intent.

If you think your response is of little interest to the masses then reply by PM.

Thanks,

Pat

 

[SPYDERLK]

Brent, read the section, "Basic Operation". Diode bridge - Wikipedia, the free encyclopedia

First half cycle:
The current flow is from the AC source left to right, through the UPPER RED path to the +.
The return flow is from the right to left, through the LOWER BLUE path to the AC source.

Second half cycle:
The current flow is from the AC source left to right, through the LOWER RED path to the +.
The return flow is from the right to left, through the UPPER BLUE path to the AC source.

There are always two conducting diodes in series, all the forward voltage and all the reverse blocking voltages are added.
The same current flows through the RED & BLUE diodes.




Grasshopper, when you can take the diode from my hand it is time for you to go.

So I was right the first time around. That's what I get for thinking about things when I'm up after midnight.

Thanks for the link! ... It sure looks to me like the back/reverse biased diodes are in //, so reverse blocking capability would not add.

Unlike the situation in a series circuit, where the voltages divide among elements, the voltage in a parallel circuit is seen fully by each leg of the circuit. The forward biased diode in each leg that is in series with each of the reverse biased ones wouldnt help much [~ 0.7V].
The forward conducting diodes are in series as you say. This results in a DC output voltage loss limiting DC potential to the peak AC value less about 1.4V [2x0.7].
larry

 

[BrentD]

Thanks for the link! ... It sure looks to me like the back/reverse biased diodes are in //, so reverse blocking capability would not add.

It's an optical illusion. They appear to be in parallel if you lok at the diode bridge as a load on the transformer, but that's not what it is exactly.. Iif you consider the entire circuit at the moment of one cycle then the diodes are in series. The circuit goes like this: Current flows out of the main transformer through the first diode, through the inductor, down the negative welding lead through the arc, up into the positive lead through the second diode and then to the other leg of the main transformer. So the diodes ARE in series, but not directly. They have the arc and the stabilizer inductor between them. Each of the other two diodes is in parallel with one of the first two, but reverse biased, so they would only conduct when the voltage from teh main transformer swings to the opposite polarity.

 

[Samshine]

Transit i'm sorry i need to make a correct. I coundn't get my big head inside the welder.
Correction: The two heat sinks that i though was contacted to the welding
case as a spacer between them, therefore all three heat sinks
are isolated from the case.
I also noticed another wire coming from a different transformer bolted to heat sink number one. This wire comes from the transformer that also has wires running to the thermanl block to make voltage changes. 230 or 208 voltage.
There are three transformers in this welder. There are two up high with the sliding arm between them that moves up and down to make welding amp. settings.
The tranformer below, has four wires coming from it. 2 wires going to the pos. welding lead. And 2 wires going to heat sink which has number 3 and number 4 diodes bolted to it. There is a jumper plate going to the neg. welding lead. from also heat sink with number 3 and number 4 diodes.
Sorry for not giving you full details.
Thanks for all your help. samshine

Will someone please tell me if this diodle will work in my welder if if buy four of them? I keep calling century but, they keep trying to sell me something else besides a diode, or keeps telling to call back tomorrow.
Thanks samshine
Semikron SKR 130/16 250A 1600V Rectifier Diode DO-8 - eBay (item 350238374298 end time Feb-06-10 04:47:44 PST)

 

[Samshine]

Transit after looking at the drawing. The diode hook up looks good.
The only problem i see is the transformer hook up.
This transformer has 4 wires coming from it.
Two wires hook up to the dc pos. welding lead.
Two wires hooks up to the number C heat sink. Also another hook up hooks from the same number C heat sink and runs to the dc neg. welding lead.
Like i mentioned earlier this welder has three transformers. I was told one was a secondary transformer. These other two tranformers are the ones at the top. The slider bar runs up and down between the transformers to change the amp. There is two wires running from one of these transformers and hooking up to the dc neg welding lead as well. Also there is just one wire running from one of the transformers up top and hooks up to number A heat sink.
Thanks for all your help. samshine

Will someone please tell me if this diodle will work in my welder if if buy four of them? I keep calling century but, they keep trying to sell me something else besides a diode, or keeps telling to call back tomorrow.
Thanks samshine
Semikron SKR 130/16 250A 1600V Rectifier Diode DO-8 - eBay (item 350238374298 end time Feb-06-10 04:47:44 PST)

 

[SPYDERLK]

It's an optical illusion. They appear to be in parallel if you lok at the diode bridge as a load on the transformer, but that's not what it is exactly.. Iif you consider the entire circuit at the moment of one cycle then the diodes are in series. The circuit goes like this: Current flows out of the main transformer through the first diode, through the inductor, down the negative welding lead through the arc, up into the positive lead through the second diode and then to the other leg of the main transformer. So the diodes ARE in series, but not directly. They have the arc and the stabilizer inductor between them. Each of the other two diodes is in parallel with one of the first two, but reverse biased, so they would only conduct when the voltage from teh main transformer swings to the opposite polarity.

... It sure looks to me like the back/reverse biased diodes are in //, so reverse blocking capability would not add.

Unlike the situation in a series circuit, where the voltages divide among elements, the voltage in a parallel circuit is seen fully by each leg of the circuit. The forward biased diode in each leg that is in series with each of the reverse biased ones wouldnt help much [~ 0.7V].
The forward conducting diodes are in series as you say. This results in a DC output voltage loss that limits the DC potential to the peak AC value less about 1.4V [2x0.7].
larry

Youve just made a case for the forward biased diodes being in series. We agree... It is the reverse biased diodes that are at issue. If their PIV is exceeded they break over and become a brief load on the transformer until they are toast. Since they are in parallel the PIVs do not add.
larry

 

[Samshine]

Thanks everyone for helping me. After reading everyones post. I know i should change every diode so there want be a weak link. But before i buy all four. I wanted to buy one the same size, to make sure nothing else was wrong.
If anybody got a manual of this welder, Montgomery Ward powr kraft 250 amp. ac/dc welder? I need to find the voltage of these diodes. I want to thank all that helped me. Thanks samshine

 

[Samshine]

'''GOOD NEWS''' I keep calling Welding cutting tools and asso. (Divison of Lincoln electric). I guess he got tired of me calling, he finally looked the part up. The diode that is in my welder, its not 150 amp. It is 250 amp. 300v. This company can't order the part. But they gave me a part number of the diodes. I looked a lot of places on the web trying to find a part with the same part number. I couldn't find anything, do to the part as been discontinued. They also gave me a number to A&A Hydraulics (Divison of McGivern Enterprises), which has the part in stock. If you looking for parts and can't find them any where, give these people a call. I got the numbers if you need them. I call my welder mechanic and gave him the item number of the diode that is on e-bay. He is going to let me know what he thinks tomorrow.
I think a lot of what made the diode go bad, is when it was midnight, and time to go home, the welder go cut off a few times without letting it cool down enough.
Again i want to think everybody who helped me. I couldn't never done it without you help. Thanks a lot, samshine

 

[BrentD]

If the original diodes are 250 amp 300V diodes, then ANY 250+A diode with a PIV rating of 300V or greater will work.

This diode on Ebay would be a perfect replacement. Now, what we are trying to determine by asking you about schematics is whether you need 4 identical diodes or 2 diodes with anode to case and 2 with cathode to case.

My guess is that if the rep only gave you one part number, then all 4 are probably the same, but if you can help Transit complete the schematic, we can be sure.

 

[BrentD]

Youve just made a case for the forward biased diodes being in series. We agree... It is the reverse biased diodes that are at issue. If their PIV is exceeded they break over and become a brief load on the transformer until they are toast. Since they are in parallel the PIVs do not add.
larry

Ah, but you have to remember that we are dealing with 2 parallel strings each with 2 diodes that are in series with the arc between them. (The two parallel strings just happen to be connected to separate transformer legs.)

If the forward diodes are in series then the reverse-biased diodes are also in series. Take a look at the attached schematic for a better view.

Colored lines and arrows indicate current flow during each transformer phase. (I.e. Blue lines, left side of transformer is - right is +. Red line, Left is + Right is -.)

You can see that at any point in time the diodes in the green box will always be in series and the diodes in the turquoise box will also always be in series.