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Thread: Selenium rectifier replacement

  1. #1
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    Selenium rectifier replacement

    I have a older miller scp200c that I recently acquired It appears the rectifier is on its way out as my no load voltage is at about 1/2 if what it should be throughout range. I have checked the straps and they are on the appropriate terminals although the wiring diagram and the input wiring strap location diagram are different. The wiring diagram shows it is wired for 208. 230. And 475. The strap diagram shows 230 460 and 575. I will have to try to ring these out with my meter to see which one is correct. It is serial number M282321 Should it be the selenium rectifier restistance increasing therefore lowering my output voltage is there a way I can put in some newer silicon diodes or bridge rectifier to replace my current rectifier. I am sure someone has done this before as these are tough old welders great for a shop with no worries of fancy board failures. I will try to attach both pictures of the wiring diagram here also but I am a better welder than computer guy

  2. #2
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    Re: Selenium rectifier replacement

    I canít seem to be see how to attach pictures is there a way to do this

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    Re: Selenium rectifier replacement

    I will need to edit my first reply. My no load voltage is bang on with the dial it is when an arc is struck it drops to about 50% of no load voltage. I compared this to my 180 miller mig and it drops about 25% Is this normal with the scp200c with the selenium rectifier I know they have greater losses across them but I would like some info from someone with a scp200c to compare with

  4. #4
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    Re: Selenium rectifier replacement

    Selenium rectifier! Now there's a collector's item. It's amazing how small (physically) a diode can replace one of those antiques.

    Your SN is too old for it's manual to be available on the Miller site but I would start by checking the brushes for the main transformer. I have seen them get rusted into their holder preventing contact with the transformer winding.
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    Re: Selenium rectifier replacement

    I have done that they are all clean and the surface of the windings are clean that they ride on I have tested all the caps all test at 5800 to 600o uf. I compared the voltages to my miller 180 open circuit and while welding. My newer miller 180 settles out at 21 to 22 volts when welding a 1/4 plate. The old scp200c holds about 22 volts max when loaded but doesnít seem to have the same crispness or evenness when welding it is like it is at too low of a voltage setting I would like to know if it is the nature of the old beast to weld like that and the selenium rectifier is limiting my voltage. I would like to talk to someone with one with either the old style rectifier or newer diodes to compare to

  6. #6
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    Re: Selenium rectifier replacement

    usually Miller welders are 208-230-460 or 230-460-575. The manuals do not reflect this all the time. The welder data plate is the only way to tell what the primary voltage of a welder IS. Placement of the jumpers is different for both. For your out put voltage. Measure the AC output voltage of the transformer, multiply by 1.414.
    and compare to stud voltage. should be close. 30 vac times 1.414 = 42.42 vdc. with bad capacitors this voltage will be lower.
    You also can make this measurement under load. if the ac voltage is higher than the DC voltage you have a problem with the diodes.
    The voltage drop on a silicon diode is .5 to .7. selenium is 1. approximately. So the 1.414 might be a little off.
    The selenium diode assembly can be replaced with 4 silicon diodes on two aluminum plates. two forward and two reverse diodes. Check the newer parts list for diode specs.

  7. #7
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    Re: Selenium rectifier replacement

    I will try to get the AC voltages in the next day or so. I did check them but only for comparison I think I got 27AC volts on both but I donít remember if that was maxed out or not. If I look at the wiring diagram on the welder it says it is set up for 208. 230 and 475. But the tag at the input says 230 460 and 575 for the jumper straps I have gone through it and feel pretty confident the input chart for the jumper straps is correct it is just weird why the wiring diagram would state differently. My plan is to replace the old selenium rectifier but I just want to be sure that is my problem. Thanks for the help I will test the voltages and post my results

  8. #8
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    Re: Selenium rectifier replacement

    The selenium rectifier is old tec. Today we use silicon rectifier there far less leaking over the old low cost selenium rectifier.

    Go to Amazon and find a rectifiers

    Dave

    FYI: I have replace a lot of selenium with silicon. The only you may is increase voltage and less heat.

    Quote Originally Posted by wrench9 View Post
    I have a older miller scp200c that I recently acquired It appears the rectifier is on its way out as my no load voltage is at about 1/2 if what it should be throughout range. I have checked the straps and they are on the appropriate terminals although the wiring diagram and the input wiring strap location diagram are different. The wiring diagram shows it is wired for 208. 230. And 475. The strap diagram shows 230 460 and 575. I will have to try to ring these out with my meter to see which one is correct. It is serial number M282321 Should it be the selenium rectifier restistance increasing therefore lowering my output voltage is there a way I can put in some newer silicon diodes or bridge rectifier to replace my current rectifier. I am sure someone has done this before as these are tough old welders great for a shop with no worries of fancy board failures. I will try to attach both pictures of the wiring diagram here also but I am a better welder than computer guy
    Last edited by smithdoor; 12-17-2019 at 10:40 AM.

  9. #9
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    Re: Selenium rectifier replacement

    Did you buy individual diodes and fabricate your own rectifier or I also see full bridge rectifiers available in the 1000v 275 amp range are these OK to use as it makes things a lot easier.. FYI I have replaced the diodes on a couple old thunder bolts and realize it is not a big job to fabricate a couple heat sinks and make a rectifier for this one should not be too hard but if I can use the ready made full bridge rectifier it would make it real easy

  10. #10
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    Re: Selenium rectifier replacement

    Quote Originally Posted by wrench9 View Post
    Did you buy individual diodes and fabricate your own rectifier or I also see full bridge rectifiers available in the 1000v 275 amp range are these OK to use as it makes things a lot easier.. FYI I have replaced the diodes on a couple old thunder bolts and realize it is not a big job to fabricate a couple heat sinks and make a rectifier for this one should not be too hard but if I can use the ready made full bridge rectifier it would make it real easy
    I like using bridge/full-wave but if can not find a bridge the size I need then build one.

    I would use the 1000v 275 amp bridge.

    Dave

  11. #11
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    Re: Selenium rectifier replacement

    Thanks it sure makes it a lot easier if i purchase the full bridge rectifiers. What do you use for the heat sink. Bolt it to a piece of aluminum with some heat sink paste. I didn’t know if the ready made full bridge rectifiers were as good as using seperate diodes and fabricating one

  12. #12
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    Re: Selenium rectifier replacement

    Three types of diode (rectifiers) . 1. single diodes of varying shapes and sizes. stud or axial lead. 2. Diode blocks from two to six in different sizes. must be mounted on a AL ,plate heat sink. 3. Then there are diode heat sink assemblies of different sizes. Plates with pressed in diodes and assembled in two to six diode configurations.
    No extra heat sinks needed. your choice on if you build your own or buy one assembled. Just use diodes with a PIV rating of 250 volts or more and the complete assemble is rated for 200 amp or more. 250 to 300 amps is a good buffer range.
    ON your line jumpers, yes the diagram pasted inside the welder might show the wrong primary connections. Miller sometimes used a second small sticker with the correct primary connection next to it. They fall off on these older machines. I have glued a lot of them back on. Your data plate and the label on the primary terminals should match and that is what you use to connect the welder.
    If you can't tell, connect 230 volts and then measure the voltage at the 115 volt feeder plug. Should be between 125 and 115 volts if connected right.
    Voltage will depend on your line voltage. could be 230 on the low side and 250 on the high side anywhere between is ok. same thing with 460. it ranges between 460 and 490 volts. 575 is found in Canada and border areas of the northern USA. 208 varies between 205 and 212

  13. #13
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    Re: Selenium rectifier replacement

    I my plan is to buy 2 complete bridge rectifiers and mount them on an aluminum plate in place if the old selenium rectifier. It is the most cost effective way and probably the easiest. I will probably buy a couple on line. . I know they may not be the best quality but it will let me see if it solves my low output voltage on the welder when under load. From what i see it should be able to hold about 37 volts max under load where mine is in the 22. 23 volt range Thanks for the reply

  14. #14
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    Re: Selenium rectifier replacement

    Sound great

    Dave

    Quote Originally Posted by wrench9 View Post
    I my plan is to buy 2 complete bridge rectifiers and mount them on an aluminum plate in place if the old selenium rectifier. It is the most cost effective way and probably the easiest. I will probably buy a couple on line. . I know they may not be the best quality but it will let me see if it solves my low output voltage on the welder when under load. From what i see it should be able to hold about 37 volts max under load where mine is in the 22. 23 volt range Thanks for the reply
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    Re: Selenium rectifier replacement

    OK... I know ziltch about welders. I have a comment though pertaining to your rectifiers. IF the transformer has a CENTER TAPPED secondary, most likely the rectifiers are connected for a full wave rectification with two diodes. There could even be four diodes with split into pairs of two in parallel to easy the current load on the old selenium stuff. For such arrangement, a bridge circuit will gain nothing. Also if you do rewire the new diodes as a bridge across the entire secondary, you will have TWICE the rectified full wave voltage over the previous arrangement of the selenium with a center tapped transformer connection.

  16. #16
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    Re: Selenium rectifier replacement

    I do replace Selenium rectifier with silicon diodes or bridge rectifier.
    But the DC voltage will be higher using silicon rectifier. In some cases you may to adjust for higher voltage.

    Dave

    Quote Originally Posted by wrench9 View Post
    I have a older miller scp200c that I recently acquired It appears the rectifier is on its way out as my no load voltage is at about 1/2 if what it should be throughout range. I have checked the straps and they are on the appropriate terminals although the wiring diagram and the input wiring strap location diagram are different. The wiring diagram shows it is wired for 208. 230. And 475. The strap diagram shows 230 460 and 575. I will have to try to ring these out with my meter to see which one is correct. It is serial number M282321 Should it be the selenium rectifier restistance increasing therefore lowering my output voltage is there a way I can put in some newer silicon diodes or bridge rectifier to replace my current rectifier. I am sure someone has done this before as these are tough old welders great for a shop with no worries of fancy board failures. I will try to attach both pictures of the wiring diagram here also but I am a better welder than computer guy

  17. #17
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    Re: Selenium rectifier replacement

    I am waiting for my diodes now and will build my bridge with them . That is the exact problem I am having now is my DC output voltage is too low .Even with the welder cranked al the way up I can only get 21V dc output. All the outputs from the transformers before the selenium rectifier check out in spec. I am hoping this solves my problem

    Brad

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