Are Mega Testers Accurate for Testing Puck Thyristors?

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Mega testers can effectively determine if puck-type thyristors are functioning by applying 250V and checking for zero impedance, indicating they are operational. However, the accuracy of this testing method may be influenced by the need for proper clamping, as specified in the thyristor datasheets, which often require significant force for optimal performance. While testing without clamping may yield positive results, it is essential to recognize that older devices might not perform reliably without the necessary pressure. Proper clamping ensures effective thermal and electrical conduction, particularly for aging components. Ultimately, while a successful test at low current suggests functionality at higher ratings, clamping remains crucial for long-term reliability and performance.
roro36
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I just started working on a furnace that uses the puck type thyristors for the main power switching. Every now and then, one of them blows and we replace it. We ususally use a mega tester to test wheteher they have blown or not i.e. put 250V across and short the gate to the anode. A zero impedance means its working fine. I am however not too sure how accurate this is for testing? In the data sheets of these thyristors it says they need to be clamped with 500kg of force in some cases. Now by running 24Vdc through these thyristors is this test being performed adequately? Does the device need to be clamped and does the amount of current running through it matter. If its rated for 1300A and I run 1A through it and t works, doees this mean it ill work for high amps or not?

Thanks
 
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roro36,

Yes. SCRs and TRIACs work or don't work, no middle ground; so if it works @ 1A, it should work @ rated current. Make sure when replacing one to clean the heat sinks & use a good quality heat sink paste. Use a torque wrench to get the clamping pressure as close as possible.

Fish
 
How important is the clamping? If testing without is fine, what is the importance of the clamping in the machine?
 
roro36 said:
How important is the clamping? If testing without is fine, what is the importance of the clamping in the machine?

Maintaining proper thermal and electrical conduction.

Fish
 
Sorry, stupid previous question, meant for testing? Is it not necessary to have the SCR or Diode clamped for testing?
 
roro36,

AFAIK, the only reason for clamping during testing is to ensure proper electrical connection. If it works "unclamped", you should be good to go, if it does not, you may want to clamp it and re-test it. These devices have a large contact area between the metal ends and the silicon that may not perform properly unless clamped. In most cases a "new device" will work just fine w/o clamping, but mechanical and thermal stresses may prevent older devices from operating properly w/o pressure.Fish
 
Thanks.
 

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