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The only way to be sure that you are testing only the SCR is to take the module you included data on out of the circuit. Set up a simple bench test circuit to check for performance. Since you will be working with out a scope, use a DC supply and just gate the SCR only and see if it fires. You can tell if there is voltage drop across the load resistor you use, or if no voltage there it will all drop across the SCR until it is fired. If the load has voltage across it all the time and you do not have to gate the SCR then the likelihood is that the SCR is shorted. Be sure to place the SCR in the test circuit, not the diode that is in the same package. By the way, you can get some really decent scopes for little money. Especially on Ebay.
mordred said:
testing the biasing is certainly the best method however another quick check is to take a resistive check with your DMM + to anode -to cathode resistance should be a low number. on the reverse bias - to anode + to cathode resistance should be high a shorted junction will show a low resistance in both for/rev bias and high resistance an open junction in both states. it is similar to a diode resistive check in that case
I'm sorry but I must disagree. You can do a DMM check on a Thyristor from gate to cathode and get a low resistance if the device is undamaged, but standard forward and reverse resistance readings (diode test) will be high in both directions because it hasn't been gated. A DMM
WILL tell you if the SCR PN junction is
shorted because it will read low in both directions, it
won't tell you if the device is
opened.
Hi,
Not sure just how familiar you are with thyristors, but what Randy suggests is correct.
But to complete his suggestion:
1. Use a load in the range of 100W (like a plain light bulb) in series with the thyristor anode-cathode and feed with any standard AC mains supply.
2. Use a DC supply (5 to 10 Volts should do the trick) in series with a small resistor like 100Ohms (so you don't damage the thyristor gate) and feed it between the gate and the cathode (don't bother about polarity).
The load current should only flow as long as the thyristor gate current flows.
The diode in the thyristor module can be checked with any diode tester (or DMM).
Lets start with the easiest check of all, one you can do with the thyristor in place and with the load (heating elements) connected. No disassembly required.
Place your temperature control loop in manual and set the output at 0%. You should read near 0 V at the thyristor output. Ramp up the output of your control loop. As the output increases you should read a voltage that is close to an equivalent percentage of your line voltage. IE, if line voltage is 480V, then at 10% you should read about 48V, at 50% you should read about 240V, and at 100% you should read about 480V. If you get a fairly linear reading at the output then the semiconductors and firing circuit are just fine. If you see something like 50% voltage at 0% command, then one of the SCRs is shorted out and you need to replace the module.
Another thing that can cause havoc in a resistive heating system is if one of the heating elements has developed a short circuit to ground somewhere along the element path - something that you may not pick up on right away because it won't trip any breakers for blow a fuse, especially if there is a transformer in the heating circuit as well.
Maybe I'm asking an obvious question, but has anything else changed in the system at all? The problem you describe is not one that would send me looking at the SCRs or triacs right away.
If this is something you do often? If so I recommend that you bite the bullet and purchase an oscilloscope. Its a tool. If you saw the necessity of the DMM and purchased one, then the same thought and justification process should apply to the scope, despite the additonal cost.
Alaric said:
Another thing that can cause havoc in a resistive heating system is if one of the heating elements has developed a short circuit to ground somewhere along the element path - something that you may not pick up on right away because it won't trip any breakers for blow a fuse, especially if there is a transformer in the heating circuit as well.
To expand on this just a bit:
Sometime a problem with a heating element will look like a power control problem because it only shows up when the elements are hot and/or you are at a high power output from the power controller. Elements move around as they heat up. These can be a real bear to track down.