Darren
What would you call an extended period of time stuck in the ON position, it was working fine and then BOOM
What i have always said is that the FET's fail cold.
I use an analogue Meter to monitor the current through the coil, and it always reads zero while running normally, the digital psu reads 1 or 2 mA under 10kHz depending on the fundamental frequency being driven at, if i go higher the more on time the higher the current.
My analogue meter has 3 taps 50mA, 500mA and 5 amp ranges, like a fool in the last test i did, i pulled the plug from the ammeter 5 amp range and plugged it into the 500mA range when it was running the result was KABOOM, it knocked the non delayed fet out again DOH Note i did not have the TVS across the Drain/Source when it blew for this test.
I thought this was quiet interesting, when i was a tv engineer, there are common failure causes, things we see all the time, with CRT based tv's the common ones are as follows
1) Dry Joint's
2) Overvolting Psu's due to leaking cap's
3) arcing mains plug pins due to double adapter use or extension lead use.
3 is interesting, here in the UK we use 3 pin mains plugs of quiet large size, double adapters and extension leads are quiet poorly made, they rely on each pin of the plug to push into a u shaped brass socket and when old or over used the pins of the plug get black arc spots on, this more often than not destroys the high voltage stage transistor eventually, any noise on the mains despite filtering causes big problems once the voltage gets stepped up to 32kv, i never did fully understand why because there is filtering for the mains then the psu has filtering and regulation and more filtering and then the high voltage circuits have protection as well, yet the poor old line transistor would always go short.
It became a matter of course to check for adapters and extension leads if the line transistor was short and the LOPTX was ringing ok and dry joints were not visible or could not be found.
Anyway going back to my last FET failure above, when i disconnected the Ammeter what possible conditions could i have had on reconnection to cause failure.
Well if the fet was on for 32nS 1000 times a second then current would have been drawn and i guess an arc could have formed on the plug just as i pushed it into the ammeter so why would this cause the FET to blow.?
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Topic: Controller No5 With Protection - Looking for Explosions (Read 231543 times)
