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  • OT-SCR's for power control

    I was going to try building a current control circuit for my welder, and I was going to base my SCR spec on the maximum circuit current which is 50 amps. So I looked for a 50 amp SCR, and came across the BTW69-1200. I think I'm missing some critical info, because I can't get my hear around how this TOP3 cased SCR can handle 50 amps. The leads are 1.20mm x 0.055mm. Seems to me it would be more of a fast acting fuse. Anyone experienced with designing SCR circuits?

  • #2
    Nah its simple, Your wire acts as a heat sink for the lead to keep it from melting.

    At least, thats the only way I can explain my 120A rated TO-220 mosfets (Even smaller then TOP3)

    Some day I wanna try it and see if it works!
    Play Brutal Nature, Black Moons free to play highly realistic voxel sandbox game.

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    • #3
      They DO mention the average current at 32A.... I'm not sure how they square those two.....

      The biggest problem is not the leads, but the solder connections..... at that current many parts use multiple leads, or a different sort of connection.

      it is a 'gray area" of the specs..... not all the limit specs apply at once.... and the "real" limits may be actually outside the part..... getting the current away from the lead without too much resistance and heating

      However, the innards WILL handle the specified currents.... assuming you can hold the case to the temps they quote right with the current.... for the TO-3P case (the one you complain about) you must hold the CASE temp to 75C or less.

      Dissipation is basically current*voltage drop when on, and there can be relatively small drops in these parts, resulting in lower power dissipation.

      Of all semiconductors, SCRs and rectifiers are among the most rugged and hard to kill.
      1601

      Keep eye on ball.
      Hashim Khan

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      • #4
        Your not going to chop the power before the transformer, are you? Thats going to be a mess with the inductive load. I would switch the output of the transformer. Replace two of the 4 diodes in the bridge with scrs. Then you will need a gate drive circuit. There are commercially available ones out there. This is pretty much how it is done in commercial machines,

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        • #5
          Switching that kind of load at power line frequency is going to produce an enormous amount of radio noise. It will wipe out everything on the radio and TV bands for a long distance from your shop. There is a name for a similar circuit that was the first transmitter to send a signal across the Atlantic. It's called a spark gap transmitter.
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          • #6
            Originally posted by Black_Moons
            Nah its simple, Your wire acts as a heat sink for the lead to keep it from melting.

            At least, thats the only way I can explain my 120A rated TO-220 mosfets (Even smaller then TOP3)
            Check the data sheet again. There will probably be an asterix against the current rating and a foot note stating that it exceeds the capability of the package. You still get the advantage of the low Rds-on and lower power dissipation.

            It's quite common to use the same die in several different packages.
            Paul Compton
            www.morini-mania.co.uk
            http://www.youtube.com/user/EVguru

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            • #7
              Originally posted by macona
              Your not going to chop the power before the transformer, are you? Thats going to be a mess with the inductive load. I would switch the output of the transformer. Replace two of the 4 diodes in the bridge with scrs. Then you will need a gate drive circuit. There are commercially available ones out there. This is pretty much how it is done in commercial machines,
              I was planning to do just that to build a TIG. I downloaded the Miller Syncrowave patent which gives you square wave current and the ability to vary the clean to weld current ratio. I bought a Chinese TIG (Jasic WSE200P) before getting very far and still have a 250 amp full bridge and current sensor all mounted on a heatsink sitting on the bench unused.
              Paul Compton
              www.morini-mania.co.uk
              http://www.youtube.com/user/EVguru

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              • #8
                Before, after, all same-same, really, so long as you are not using the SCR as a rectifier. (Transformers need AC)

                The Monarch 10EE "Wiad" version has what may as well be SCRs (thyratrons) directly off a transformer.... for DC motor speed control.

                Both SCRs and thyratrons stay on until the current drops to a low level (hold current). That means they are not quite as nasty with inductive loads as a device which can turn off while current is still flowing. SCRs are still used in some DC motor controls.....

                A decent amount of filtering will take care of radio noise..... it may not take care of "line harmonics" of lower order, which are controlled by law in some places (europe, for instance).
                1601

                Keep eye on ball.
                Hashim Khan

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                • #9
                  Transformers need a change in the current. That doesn't necessarily mean AC. Pulsing DC works also, you just don't get the polarity switch. Even though it's rectified, it will reach 0 and shut the SCR off.

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                  • #10
                    For an SCR controller full wave bridge 2 quadrant control, you may be able to recruit a DC motor SCR controller, just replace the bridge with higher power out-board components and feed the board itself from the normal 120vac supply.
                    They use a 5k pot for control.
                    I have used these in the same manner with higher rated bridge components for bridgeport DC table motors.
                    You may want to place a large inductor (choke) in the bridge output.
                    Max.

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                    • #11
                      Originally posted by EVguru
                      Check the data sheet again. There will probably be an asterix against the current rating and a foot note stating that it exceeds the capability of the package. You still get the advantage of the low Rds-on and lower power dissipation.

                      It's quite common to use the same die in several different packages.
                      Not that I saw. There was a operating limits chart, that actualy.. EXCEEDED 120A at I think below 50C case tempature, but that area was shaded out with a note "Exceeds die bonding limits".. hahah. I assume that means the mosfet vaporises from the inside out.
                      Play Brutal Nature, Black Moons free to play highly realistic voxel sandbox game.

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                      • #12
                        SCR & MOSFET Ratings

                        Current ratings are "based", (mis)using Ohm's law. Its calcualted on the resistance through the device when it is in full saturation, ie fully on, and the maximum rated voltage, where Current = Voltage/Resistance.

                        These ratings do not mean the the device will survive very long at these loads without catastrophic failure due to overheating, its just the maximum possible current that it can conduct...unless you run some massive heat sing with cryogenic cooling, LOL.

                        Kind of like a 220 MPH speedometer on a car...just because it goes 220, doesn't mean that you are a good enough driver to keep it under control!
                        Old P&W "C" 12x36 lathe, Covel SG, DoAll T&C Grinder, Rottler SG9M, HP7A, F69ATC

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                        • #13
                          Originally posted by Swamp Donkey
                          These ratings do not mean the the device will survive very long at these loads without catastrophic failure due to overheating, its just the maximum possible current that it can conduct...unless you run some massive heat sing with cryogenic cooling, LOL.

                          Kind of like a 220 MPH speedometer on a car...just because it goes 220, doesn't mean that you are a good enough driver to keep it under control!
                          Oh yea, thats another thing to look out for, Lots of devices current ratings are speced assuming.. 20C case tempature! Ie, basicly watercooling with a water chiller! Or maybe a heatsink in -40c weather.

                          When you consider a realistic case tempature of say 70C you could maintain with a decent sized heatsink and a normal 20~30c ambiant room tempature, with a 125C max junction tempature, you end up with typicaly half the 'absolute max' current rating. (And thats WITH the junction tempature at the 'max')
                          Play Brutal Nature, Black Moons free to play highly realistic voxel sandbox game.

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                          • #14
                            The SCR in teh datasheet shows a 70C CASE temp max...... actually fairly reasonable compared to 25C case temp on other data sheets.... but in practice may not be easy to hold.... it is 45C total rise from case to ambient....

                            Originally posted by TheAndroid
                            Transformers need a change in the current. That doesn't necessarily mean AC. Pulsing DC works also, you just don't get the polarity switch. Even though it's rectified, it will reach 0 and shut the SCR off.
                            You have to take the volt-seconds out of the core somehow. Primary side, or secondary side, once you load up the core you need to take out the energy before the next pulse. Otherwise you effectively have net DC, and it will saturate.

                            A transformer cannot have more than a small net DC in any winding unless specifically designed for it.
                            1601

                            Keep eye on ball.
                            Hashim Khan

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                            • #15
                              Problems with DC offsets causing core saturation is why old sinewave TIG setups had a huge capacitor bank in series with the torch.

                              I used to have one. I nearly threw it away a couple of times, but it was too heavy to put in the car. I left it out in the back lane and even the local scavengers gave up on it after a dozen yards or so. I managed to give it away in the end.
                              Paul Compton
                              www.morini-mania.co.uk
                              http://www.youtube.com/user/EVguru

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