Old oil filled caps.I installed a ciruit in my welder to control the output current via a foot pedal.
**broken link removed** (the only difference is the potentiometer is a 500 K, not 250..he comments on this in the write up)
I completed the installation last night and tried it out today. It worked great for about 30 seconds, then went to full power. I found one of the scrs in the module shorted out on me. The SCR module is this one http://www.digikey.ca/product-detail/en/ixys/MCC56-16IO1B/MCC56-16IO1B-ND/1652739
So, a couple of questions.
The fella who did the website suggested a capacitor be installed into the welder to protect the welder when the hi frequency box is running. Could the HF have taken out the SCR? I cannot find "bathtub" capacitors on Digikey. They sound like very old technology. What would be a suitable capacitor to protect the welder circuitry from the HF? Or a better way of doing it? Any .05 uf 1000 volt capacitor? The HF box spec out at 3500 kv so I would think that is what I should be looking to protect from or am I missing something?
The circuit breaker feeding the receptacle is a 50 A. The welder (250 AmpAC/DC) is probably rated for less but I am running the welder down around a 100 amps output (max set by front panel control) so input current should have been much lower, guessing 15-20 amps (ish?) no where near the current rating of the SCR.
I am concerned about ordering a replacement SCR and have the same thing happen. Any input for me? I have never done much AC power control so not too familiar with the circuit and general rules (ie rules of thumbs)
Thanks
Spec, the scrs are on the primary side of the transformer so I figure very little current, relative to welding current. I had the max welding current set to 100 of a possible 250 so guessing the scrs were only rectifying up to 20 amps max... .? They are rated for rms of 100 (if I am reading specs correctly) . I mounted the block to the metal chassis of the welder but did not note how hot it was right after it failed. I will need to look at the heat sinking again though, for the replacement.
Oh dear- expensive.yes spec, that is the one (I was using, for 30 seconds...)
Thanks
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tcmtech and spec. Thanks again for your continued interest and support. Much appreciated. You both have me interested in redoing the circuit for a better control but at this point, I have installed it all in the welder and want to move on. I tried to quit being lazy and went through the circuit. I think I see what you mean spec with regards to the scr's being backwards. With my limited recall of old theory, it seems to make sense. Could I use the circuit components as shown, but rearranging the scrs as shown in the image attached to this post? I know the diacs break down at a given value, I think 24-30 volts. Rather than ask you(s) to type in the circuit workings, I will walk through my reasoning and maybe you can just correct me a little if I get off track...
As the power starts going positive, (line going direct to transformer), it cannot complete the path due to SCR1 being off. A charge cycle starts through D3, R5, R4 and C2. As C2 charges to the breakover voltage of D1, at which time it triggers SCR1, completing the path... and the cycle repeats on the other side. Is that a proper analogy or did I miss?
Because I am basically impatient,.......I ordered the snubber components as per the SP100 post above and a new SCR so hopefully, it all can be used. If not, another lesson learned....
And spec, if I have to order more parts, I will try the ones you suggest. Certainly much cheaper. Not available through digikey and I have not used Mouser in Canada yet but could try them. Or I could spec something similar out with Digikey.
Thanks.
Shawno
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