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  • #16
    Originally posted by Aeronca41 View Post
    Actually, they're not that expensive--I was surprised. You would have to buy some really good heat sinks, which would cost more than the diodes, and perhaps an extra fan to blow on them, but not an insurmountable problem. This was the first one that popped up....might be able to find stud mount ones also.

    Click image for larger version Name:	275A DIODE.jpg Views:	0 Size:	89.1 KB ID:	614695
    Thanks for that Aeronca41. I guess those rectifiers match up to the important specs? (What specs must one match? Just voltage and amperage?) And I guess we would need to replace two rectifiers for each of the two legs of 120V? Or is it three total rectifiers? (Rereading the thread, it sounds like the rectifier you linked to is the "main power" rectifier...?)

    What kind of cooler/heat sink did you have in mind? Would a big aluminum thing with lots of fins (like in a computer, only bigger) work? Looking at the data sheet, it looks like max "operating junction and storage" temp is 302°F ... I wonder what happens if you exceed that. Would it shut down...or start to get damaged / burn up?

    Sorry for all the questions! Wish I knew more about electronics.
    Last edited by Helios; 04-24-2021, 02:38 PM.

    Comment


    • #17
      Originally posted by Helios View Post

      Thanks for that Aeronca41. I guess those rectifiers match up to the important specs? (What specs must one match? Just voltage and amperage?) And I guess we would need to replace two rectifiers for each of the two legs of 120V? Or is it three total rectifiers? (Rereading the thread, it sounds like the rectifier you linked to is the "main power" rectifier...?)

      What kind of cooler/heat sink did you have in mind? Would a big aluminum thing with lots of fins (like in a computer, only bigger) work? Looking at the data sheet, it looks like max "operating junction and storage" temp is 302°F ... I wonder what happens if you exceed that. Would it shut down...or start to get damaged / burn up?

      Sorry for all the questions! Wish I knew more about electronics.
      ...And I'm sorry for the confusion--with two different things going in this thread. I hope this will sort it out.

      You have it right. The spec sheet you included is the one for the welding diodes--the big ones.

      If you are talking about the "control voltage" rectifiers, they will be much smaller. These, that lovetofix posted a bit ago, are the control voltage rectifiers.
      ​ Those can be replaced by these:

      ​ These could be mounted to any convenient large piece of sheet metal in the welder, such as where the original seleniums are. They need to be mounting with a good thermal connection to the sheet metal--sand off the paint, apply silicon heat sink compound (generally a white or clear thermally conductive grease made for the purpose), and get a good firm connection with a screw and nut through the middle mounting hole. Should not really be a big deal since they are rated far above requirements, but I'm kind of a "belt and suspenders" guy with electronic stuff. Obviously, you have to get the right wires hooked to the right places. This is a pretty simple installation compared to changing out the main welding rectifiers.

      ------------------------------------------------------------------------------------------------------------------------


      If we are talking about the big main welding rectifiers,

      you would use something like these, or maybe find something with the same specs on a stud mount (but generally, I think this form factor would be cheaper):
      ​ Your are correct on the specs--just have to match the voltage and current requirements, and going higher is just extra headroom for long term reliability.

      Yes, the heat sinks would be the thick aluminum things with lots of fins on them, again with the diodes mounted with silicon thermal grease.

      PLEASE, DON'T ANYONE JUST GO BUY THESE AND PUT THEM IN WITHOUT SOME KNOWLEDGE OF WHAT YOU ARE DOING. I probably wasn't as clear on that as I should have been. This is just a suggestion based on what popped up in a google search. Need to study the data sheets, calculate how much heat will be generated at the diodes by the welding current, and calculate the thermal transfer needed by the heat sinks.

      If someone gets serious about doing this, I may be able to help figure that stuff out. Not something I'm an expert at, but something that can be figured out with a few hours and some google searches--or maybe someone here does thermal calculations every day and knows it off the top of their head.

      Bear in mind that you would never operate any semiconductor at its maximum temperature, and that you have to be careful when reading spec sheets to consider whether they are stating Case Temperature (Tc) or Junction Temperature (Tj)---the semiconductor junction inside the component will be a lot hotter than the outside case. And, case temp is not the same as room ambient. If your welder is sitting at normal room temp, Tc is going to be hotter that the ambient -- as evidenced by the fact that operating electrical "things" are generally warm or hot to the touch. Have to design for cooling accordingly. In general, heat is the enemy of most things electronic.

      Spec sheets usually have the "maximum" stuff right up front, and then the "operating" info a bit further down. The maximum stuff is nice to know, but that is only for making sure your design won't blow up when you get outside of normal operating conditions for some reason. I tend to do things like this with large amounts of wiggle room--big heat sinks, and an extra fan blowing across them. If you exceed the specs (voltage, current, or temp), you generally cook the component and have to replace it. I might add, on the topic of specs, that these diodes are rated at far higher voltages than you would expect for welding--that is because they have to deal with short spikes of voltage that will occur in any high-current machine with a lot of inductors (transformers, magamps, etc.) as well as any momentary power line spikes that may make it through the transformer.


      You also have to remember that the heat sinks will have to be insulated from the welder case--one of the terminals of the diode (like the bolt in a stud mount, or the rear plate of a through-hole device) is either the cathode or anode of the diode, and must be insulated from other things and connected to the circuit accordingly. The heat sinks would have to be mounted on nylon standoffs or some other insulating material so they are not grounded to the chassis of the welder. None of this is rocket science by any means, but you can't just bolt things down .

      In short, replacing the small control voltage seleniums is a no-brainer; doing the big ones will take some effort.
      Last edited by Aeronca41; 04-24-2021, 04:36 PM.

      Comment


      • #18

        Originally posted by Helios View Post

        Thanks for that Aeronca41. I guess those rectifiers match up to the important specs? (What specs must one match? Just voltage and amperage?) And I guess we would need to replace two rectifiers for each of the two legs of 120V? Or is it three total rectifiers? (Rereading the thread, it sounds like the rectifier you linked to is the "main power" rectifier...?)

        What kind of cooler/heat sink did you have in mind? Would a big aluminum thing with lots of fins (like in a computer, only bigger) work? Looking at the data sheet, it looks like max "operating junction and storage" temp is 302°F ... I wonder what happens if you exceed that. Would it shut down...or start to get damaged / burn up?

        Sorry for all the questions! Wish I knew more about electronics.



        ...And I'm sorry for the confusion--with two different things going in this thread. I hope this will sort it out.

        You have it right. The spec sheet you included is the one for the welding diodes--the big ones.

        If you are talking about the "control voltage" rectifiers, they will be much smaller. These, that lovetofix posted a bit ago, are the control voltage rectifiers.
        ​ Those can be replaced by these:

        ​ These could be mounted to any convenient large piece of sheet metal in the welder, such as where the original seleniums are. They need to be mounting with a good thermal connection to the sheet metal--sand off the paint, apply silicon heat sink compound (generally a white or clear thermally conductive grease made for the purpose), and get a good firm connection with a screw and nut through the middle mounting hole. Should not really be a big deal since they are rated far above requirements, but I'm kind of a "belt and suspenders" guy with electronic stuff. Obviously, you have to get the right wires hooked to the right places. This is a pretty simple installation compared to changing out the main welding rectifiers.

        ------------------------------------------------------------------------------------------------------------------------


        If we are talking about the big main welding rectifiers,

        you would use something like these, or maybe find something with the same specs on a stud mount (but generally, I think this form factor would be cheaper):
        ​ Your are correct on the specs--just have to match the voltage and current requirements, and going higher is just extra headroom for long term reliability.

        Yes, the heat sinks would be the thick aluminum things with lots of fins on them, again with the diodes mounted with silicon thermal grease.

        PLEASE, DON'T ANYONE JUST GO BUY THESE AND PUT THEM IN WITHOUT SOME KNOWLEDGE OF WHAT YOU ARE DOING. I probably wasn't as clear on that as I should have been. This is just a suggestion based on what popped up in a google search. Need to study the data sheets, calculate how much heat will be generated at the diodes by the welding current, and calculate the thermal transfer needed by the heat sinks.

        If someone gets serious about doing this, I may be able to help figure that stuff out. Not something I'm an expert at, but something that can be figured out with a few hours and some google searches--or maybe someone here does thermal calculations every day and knows it off the top of their head.

        Bear in mind that you would never operate any semiconductor at its maximum temperature, and that you have to be careful when reading spec sheets to consider whether they are stating Case Temperature (Tc) or Junction Temperature (Tj)---the semiconductor junction inside the component will be a lot hotter than the outside case. And, case temp is not the same as room ambient. If your welder is sitting at normal room temp, Tc is going to be hotter that the ambient -- as evidenced by the fact that operating electrical "things" are generally warm or hot to the touch. Have to design for cooling accordingly. In general, heat is the enemy of most things electronic.

        Spec sheets usually have the "maximum" stuff right up front, and then the "operating" info a bit further down. The maximum stuff is nice to know, but that is only for making sure your design won't blow up when you get outside of normal operating conditions for some reason. I tend to do things like this with large amounts of wiggle room--big heat sinks, and an extra fan blowing across them. If you exceed the specs (voltage, current, or temp), you generally cook the component and have to replace it. I might add, on the topic of specs, that these diodes are rated at far higher voltages than you would expect for welding--that is because they have to deal with short spikes of voltage that will occur in any high-current machine with a lot of inductors (transformers, magamps, etc.) as well as any momentary power line spikes that may make it through the transformer.


        You also have to remember that the heat sinks will have to be insulated from the welder case--one of the terminals of the diode (like the bolt in a stud mount, or the rear plate of a through-hole device) is either the cathode or anode of the diode, and must be insulated from other things and connected to the circuit accordingly. The heat sinks would have to be mounted on nylon standoffs or some other insulating material so they are not grounded to the chassis of the welder. None of this is rocket science by any means, but you can't just bolt things down .

        In short, replacing the small control voltage seleniums is a no-brainer; doing the big ones will take some effort.

        Comment


        • #19
          Originally posted by Helios View Post

          Thanks for that Aeronca41. I guess those rectifiers match up to the important specs? (What specs must one match? Just voltage and amperage?) And I guess we would need to replace two rectifiers for each of the two legs of 120V? Or is it three total rectifiers? (Rereading the thread, it sounds like the rectifier you linked to is the "main power" rectifier...?)

          What kind of cooler/heat sink did you have in mind? Would a big aluminum thing with lots of fins (like in a computer, only bigger) work? Looking at the data sheet, it looks like max "operating junction and storage" temp is 302°F ... I wonder what happens if you exceed that. Would it shut down...or start to get damaged / burn up?

          Sorry for all the questions! Wish I knew more about electronics.
          Well, tried to post a good bit of additional info, but it booted me out and said I am "unapproved". May get posted by a moderator on Monday, I guess.

          Comment


          • #20
            Click image for larger version  Name:	Sr2.jpg Views:	0 Size:	360.7 KB ID:	614710 Click image for larger version  Name:	Sr3.jpg Views:	0 Size:	277.1 KB ID:	614711 Click image for larger version  Name:	Sr1.jpg Views:	0 Size:	302.9 KB ID:	614712
            These are all the selenium bridge rectifiers I could find. There may be one more, have to take a look at the circuit diagram. The last one is the main power bridge.
            I imagine a silicon bridge would not produce the heat that a selenium bridge does because of the lower forward bias voltage drop. So, the heat sink required may not be as large as that shown for the selenium one.
            Last edited by 07wingnut; 04-24-2021, 04:50 PM.

            Comment


            • #21
              The oldest diagram I have shows a total of 5 including the main one.
              Nice pictures. I have never seen a selenium rectifier in a welder before.

              I agree on the power diodes--forward drop in a silicon is certainly less than the seleniums. Whatever diodes you choose will likely have an "on" resistance specified which will allow calculation of power to choose a heat sink. Those seleniums have a LOT of surface area.

              Comment


              • #22
                Originally posted by Aeronca41 View Post

                Well, tried to post a good bit of additional info, but it booted me out and said I am "unapproved". May get posted by a moderator on Monday, I guess.
                Thank you sir...I for one really appreciate the help and expertise!

                Comment


                • #23
                  Dang it, Wayne! I have other stuff to do!

                  I can’t let my helga die. I will have to address her in issues as soon as feasible.

                  Thanks for the info brother. I’ll certainly hit you up when it’s time!

                  Comment


                  • #24
                    Great pictures “07wingnut.” After looking as those I found two more on my machine as well. As “Aeronca41” noted earlier, since my machine is a ‘74 I do not have the BIG one.
                    Attached Files

                    Comment


                    • #25
                      I am not good with electrical components and understanding the intricacies of circuitry. If someone figures out the proper replacement for those four rectifiers I would be super happy to pay a finders fee or whatever else to to be able to share part numbers and build specifications for any mods needed.
                      In fact, maybe someone would be interested in doing a group buy of all the components and then split them up into flat rate shipping boxes to make a replacement “kit”. I for one would be happy to pay a markup, handling fee, and shipping to know I am getting the right parts.
                      Anyone interested?
                      Thank you to everyone for all the good info and pointing out the potential failure point on these wonderful old machines.

                      Comment


                      • #26
                        CAVEAT: This post is only about the four smaller selenium rectifiers--the big main weld rectifiers in the oldest machines before silicon diodes are a different topic.

                        I took a look at a later 300AB/P manual--all 4 of the selenium rectifiers (except the one in the main welding current path) were replaced by Miller part number 035914 (which is obviously no longer a valid part number after all these years). The good news is the parts list has the specs--30 amps, 400 volts--quantity 4 used per machine. I found a couple of parts at Mouser that will fit the bill very nicely--single screw mount, rated for 35 amps, 400 volts, with male FastOn connectors, all for around 5 bucks apiece. One has low stock right now, but they have hundreds of the other one. Thus, you could simply cut off the end of the wires that are in your welder now, crimp on female FastOns, plug them in (being sure to get the right wires to the right place), either drill a new hole or use one of the old mounting holes, install with one screw and nut, and you're good to go. It is not good practice to mount things like these that need tight mechanical contact for heat transfer using sheet metal screws--use a machine screw and nut. As I said earlier, I would probably scrape off the paint and add some silicon heat sink compound, but that is probably way overkill for the load that will ever be on these things. I wouldn't sweat that if you don't happen to have any around. DO NOT put regular grease in there--just mount it with the screw. Good contact with the sheet metal should be all you need. It would be interesting if someone with a later machine were to dismount one of these and see if they used thermal grease, and if they scraped off the paint.

                        You can see the info here:

                        https://www.mouser.com/Semiconductors/Discrete-Semiconductors/Diodes-Rectifiers/Bridge-Rectifiers/_/N-ax1mf?P=1z0z810Z1yzxhyxZ1yuo9uaZ1yzxpgd

                        Unless a group buy would r
                        each a quantity of more than a hundred pieces (not very likely), the cost savings per piece is not enough to warrant the extra shipping to divvy up the group buy. Might be some minor shipping cost savings, but I doubt it would be much. Good idea, but doesn't look like it saves enough money to be worth it. And this is all the info you need to order parts.

                        You can find cheaper ones (2-3 bucks) at Digikey, which would probably be fine, but Vishay and ON Semiconductor (Motorola) are well-known top brands I recognize. I have been out of the parts spec business for many years, and I don't recognize any of the brand names at Digikey. That is not to say they are inferior parts--Digikey is a top-notch supplier, and doesn't sell junk--just that I don't recognize the names. Probably any of them would be fine, for less money.


                        https://www.digikey.com/en/products/...SOwCqAK4fMziQA

                        I've posted this before, but here's some info on how to test a diode bridge should anyone be interested.

                        Click image for larger version

Name:	DIODE BRIDGE TESTING.jpg
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                        • #27
                          I have seen mentioned on this board that there are some really high voltages associated with the HF circuits. Does this circuit have a bridge rectifier in it, if so the specs for the bridge would have to be a lot higher for voltage. There is also a diode involved with the remote/standard switch which may the intermediate size selenium rectifier in the first picture.

                          Comment


                          • #28
                            You are correct about high voltages associated with the HF, but the diodes don't help make that. All four of them are replaced with the same part number in the later machines.

                            Comment


                            • #29
                              Holy cow, Aeronca41, what a lot of great info. This is all getting saved on my computer for future reference.
                              Thank you very much for your help.

                              Comment


                              • #30
                                Originally posted by 07wingnut View Post
                                Everything on the machine seems to work fine, except for not being able to test the remote with foot pedal, since it came with no foot pedal. At the price these are selling, I may have to build my own. The major part, a 16 ohm 150 watt slide resistor is not that expensive.
                                I have a Miller RFC-23A pedal I'd sell for less than the typical eBay prices.

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