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Registered Member #3114
Joined: Sat Aug 14 2010, 08:33AM
Location:
Posts: 608
Hey all, I built Steve Wards SSTC 5 mini a while back with good success but accidentally broke it an every since then have been rebuilding it to no avail.
I rebuilt the driver on a small bread board and i am getting an output. when it is connected to the bridge i can hear a clicking sound which im pretty sure is the GDT. The issue now is that the FET's are not switching.
If the GDT is receiving and output from the driver but the bridge is not switching on does that mean my fets are dead ?
sorry i dont have a scope :( Also i think i may have a bad 74hc chip. Would that also cause an issue with the bridge? I mean in terms of switching not actually running correctly ie. sparks)
If the 74HC14 chip is bad then it will either output HIGH constant or LOW constant, either one will cause the UCC chips to do similar, which means there will be no ticking of the GDT. So, if your GDT is ticking (magnetostriction of the windings) you can assume the 74 and UCC chips are fine unless the ticking is at the frequency of your interrupter (if one is used).
A way to check this is pull the enable pins of the UCC chips high to the 5V rail with a resistor (100 to 100k ohms) and then apply a test signal at the antenna (if you are ungrounded you can touch it with your finger and usually mains hum will provide your test signal). Then just listen for ticking, if you hear it, then your logic section and mosfet drive section are fine.
At this point if the above test is a pass then you need to look at your bridge. Power off the driver and check the resistance between the two DC bus rails. Remember that if the bus caps are discharged it will look like a short initially and then climb to a high resistance. If bleeder resistors are used across the bus caps then this is the resistance you should see in the end. If this is a short circuit then you either have a wiring problem or one or more of the bridge components is bad. You'll need to disassemble the bridge to diagnose further, but the standard procedure is to check for continuity between Gate and Source and then Drain and Source of the FETs, both should read as a capacitor reads; high resistance or open. Note that you need to discharge the Gate-Source capacitance by shorting these terminals momentarily in order to check the Drain-Source resistance.
If the FETs check out ok it is good to test the gate protection zeners to make sure they have not failed short. Then check the gate resistors to make sure they are not open circuit. Finally check the bridge rectifier feeding the bridge to make sure it functions properly.
If you still don't get output you need to check the phasing of your GDT and primary (change one at a time).
Finally, you'll need to check that feedback is being received and that a starting pulse from either noise (if running CW) or your interrupter is initiating oscillation.
Registered Member #3114
Joined: Sat Aug 14 2010, 08:33AM
Location:
Posts: 608
Sigurthr wrote ...
If the 74HC14 chip is bad then it will either output HIGH constant or LOW constant, either one will cause the UCC chips to do similar, which means there will be no ticking of the GDT. So, if your GDT is ticking (magnetostriction of the windings) you can assume the 74 and UCC chips are fine unless the ticking is at the frequency of your interrupter (if one is used).
A way to check this is pull the enable pins of the UCC chips high to the 5V rail with a resistor (100 to 100k ohms) and then apply a test signal at the antenna (if you are ungrounded you can touch it with your finger and usually mains hum will provide your test signal). Then just listen for ticking, if you hear it, then your logic section and mosfet drive section are fine.
At this point if the above test is a pass then you need to look at your bridge. Power off the driver and check the resistance between the two DC bus rails. Remember that if the bus caps are discharged it will look like a short initially and then climb to a high resistance. If bleeder resistors are used across the bus caps then this is the resistance you should see in the end. If this is a short circuit then you either have a wiring problem or one or more of the bridge components is bad. You'll need to disassemble the bridge to diagnose further, but the standard procedure is to check for continuity between Gate and Source and then Drain and Source of the FETs, both should read as a capacitor reads; high resistance or open. Note that you need to discharge the Gate-Source capacitance by shorting these terminals momentarily in order to check the Drain-Source resistance.
If the FETs check out ok it is good to test the gate protection zeners to make sure they have not failed short. Then check the gate resistors to make sure they are not open circuit. Finally check the bridge rectifier feeding the bridge to make sure it functions properly.
If you still don't get output you need to check the phasing of your GDT and primary (change one at a time).
Finally, you'll need to check that feedback is being received and that a starting pulse from either noise (if running CW) or your interrupter is initiating oscillation.
Registered Member #3114
Joined: Sat Aug 14 2010, 08:33AM
Location:
Posts: 608
so i was testing in today, setup is:
Top load 7"dia x 1" foam disk 4 primary turns mains 120v slow rep-rate
I turned it on after powering it down for some adjustments when i switched it back on the right 5ohm resistor popped and smoked... how come ? and would that fry anything else like the fets ?
also can i start the interrupter and the bridge at the same time ? I usually don't but i did this time and it popped so do i need to have the interrupter running before i power on the bridge ?
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SSTC 5 help
