SCR diode testing

I have a Monarch EE1000 that has been updated with a Southcom solid state control. While using it today after about 15 minutes it just stopped. Upon checking, found that it had both tripped the circuit breaker and blew a fuse in the machine disconnect box. Finding the blown fuse about used up my knowledge of the electronics, so called in a industrial electrician friend to examine it. He found out that it appears that one or more of these SCR diodes has shorted.

My question is this. Is there a way to test these diodes? There are six total, and they do the "heavy lifting" converting the 3 phase input into DC that powers the spindle motor so are an important part of the machine. We have narrowed it down to 2 potential diodes, but do not know how to check them, or worst comes to worst where to buy replacements.

Dale Nelson

Yes,
If they are truly shorted you can test this with an ohm meter. A normal SCR would show a very high resistance between the anode and cathode (untriggered state) but if you measure a low resistance vlaue the device is shorted. One should measure the SCR in both directions red lead to cathode and again with the red lead to anode to be sure you have covered all the cases. Just becasue it's not shorted doesn't mean it's functional. Functional testing is a little more difficult. See: The Silicon-Controlled Rectifier (SCR) : Thyristors

Craig

And, to avoid confusion, you should test with the device disconnected.

it is possible for the drive circuit (often another smaller SCR) to be shorted, which causes the driven one to appear shorted in use, but not when tests at low voltage.

Sometimes the "diode" setting on a Fluke meter will help, as it uses a higher voltage than the regular ohms setting, which is low to avoid forward biasing diodes..

However, that setting does not measure ohms, but rather voltage drop.... so it can be confusing.... a good diode should therefore read around 0.4 to 0.6 volts drop.

With SOME SCRs, you can test action on the diode setting by shorting the gate to anode, and applying the positive lead to the two, and negative to cathode (so-called "bar end"). The scr, if it can be so measured, should show open with the two not shorted and a low drop when the gate and anode are connected.

High current SCRs often will not test well that way, so you must fall back on seeing if they are shorted.

SCR, more or less like bearings, are standardized commodities. Newark | US - Electronic Components Distributor | Electronic Parts Distributor Mouser Electronics - Electronic Components Distributor or DigiKey Electronics - Electronic Components Distributor are the equivalent of McMaster for electronic parts. They will have dimensioned diagrams of form factors so you can match more easily, ship fast and are cheap. Be generous on the voltage ratings, 1kv devices not much more than 400V devices. It is not unusual for a device to be
rebadged with an end user designation, so a direct correlation with a Mouser listing may not be possible, but the form factor should be a pretty strong hint as to the amps rating and the voltages output by
the power supply an indication of what the PIV rating should be (at least 2x the volts output of the supply).

A big thank you to all who replied. The way these SCRs are configured, it seems to be clear that it is one or both fed off of the center leg of the 3 phase input, so once they are found to be the for sure culprit, and if the replacements are cheap, that is a no brainer to replace at least these 2, or all 6 if the price is right. This past year has been a real bear for fixing machines, some of them have become a real money pit, so if this is a reasonable fix cost, will make 2013 be off to a good start! I hope

Dale

Check the drive circuitry also....... Most applicable with IGBTs, it still applies to SCRs.

The SCR failing may have killed the drive. or the reverse.