Disc type Thyristors
Disc type Thyristors
(OP)
Hi all!
I probably know the answer but have been unable to convince my customer as to why it is necessary to clamp a disc type thyristor when wanting to test it's properties with a multimeter. Without contacting a thyristor manufacturer can you guys tell me me the reasons? Any suggestions as to how much to clamp a device (the force and area) and what sort of readings to expect? I would put them into an insulated vice and tighten and compare measurements with other thyristors to identify 'suspect' devices. Any other tips?
Thanks in advance.
Drivesrock
I probably know the answer but have been unable to convince my customer as to why it is necessary to clamp a disc type thyristor when wanting to test it's properties with a multimeter. Without contacting a thyristor manufacturer can you guys tell me me the reasons? Any suggestions as to how much to clamp a device (the force and area) and what sort of readings to expect? I would put them into an insulated vice and tighten and compare measurements with other thyristors to identify 'suspect' devices. Any other tips?
Thanks in advance.
Drivesrock





RE: Disc type Thyristors
Don't put the thyristor in a vice!! There are purpose-made heatsink / clamp arrangements which exert a uniform and carefully calibrated clamp force on the device. Too little pressure and you get inadequate contact to the die; too much and you distort the package and fracture the die. A fractured die will probably work for a while, then fail without warning. Usually the failure will take out a load of other expensive components (or is it just my world that happens in?
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Sometimes I only open my mouth to swap feet...
RE: Disc type Thyristors
Some (not all) puck thyristors rely on outside pressure for contact between disc and semiconductor. It is soldered (always?) cathode side, but pressure contacted anode side.
I wouldn't use a multimeter. OK, you can do that to test gate-cathode (anything from a few ohms up to 100, polarity independent), but for reverse and blocking voltage, I would use a megger.
Remember, these are sturdy things so a correctly used megger is not a problem. But, connect before starting megger. Some devices have a capacitor inside and if you connect after starting the megger, this capacitor is charged and will cause a current to flow (charge carrier separation) and this might trigger the thyristor "over head" and cause a very intense discharge current with a lot more A/us than the device is built for.
Same thing can happen if you try to trig the thyristor with voltage applied - that's why we have commutation reactors in the working circuit.
I made that mistake once. After havning destroyed two thyristors, I realised that something was wrong. Expensive lesson.
Meggers without that internal capacitor should be OK, though. Most are.
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Disc type Thyristors
Most of the puck thickness is in the copper pads. The die is usually no more than about 1/8" thick so, for example, in a 1" thick puck the copper pads on each side are close to 1/2" thick.
Most of the pucks I've seen start at about 2000lb of clamping pressure. So putting some pressure on them with a vise is not likely going to hurt them unless you are dealing with the smallest of puck sizes. The largest puck I've personally seen required about 15000lb of clamping pressure.
If you really want to see one opened I can probably dig up a picture or 2 and post them.
RE: Disc type Thyristors
I have never heard this "clamp to work" stuff even after years in the semi business. How bizarre! 15klb!!!
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com
RE: Disc type Thyristors
This pic shows the device with the seal at the cathode cut open. You're looking into it from the cathode side. You can see the gate lead connection - the black piece coming in and going to the white part in the middle. The white part actually has a spring loaded pin that hits on the gate of the device. The red is just a rubber insulator around the outside. On the left is the cathode block that was removed.
Here's a different device with the parts separated. The disc on the top right is the actual SCR wafer. You can see the gate which is the small dot in the middle. This device has an auxiliary gate which is the second ring you can see in the middle. I believe you basically trigger a small device that in turn triggers the main device. The disk on the top left is just a plate that just turns over and lays on top of the wafer.
This device looks like it was an over-voltage failure. Also could have been temperature I guess.
As you can see, it's all held together by pressure. You can also see how thick the contact faces are. They're around 1/2" thick copper each.
RE: Disc type Thyristors
Keith Cress
Flamin Systems, Inc.- http://www.flaminsystems.com