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Registered Member #1361
Joined: Thu Feb 28 2008, 10:57AM
Location: Cairns, Australia
Posts: 305
Well, after shorting out the tank caps on my DRSSTC so many times, and even killing IGBT's while doing it, I decided I needed a bleed resistor. The problem is, the coil has a 12VAC input, which is stepped up to around 400V with a stepup transformer. Because of this, using a bleed resistor on the caps the whole time would waste precious power! So, I came up with a way to still have bleeders, but not while the coil is in use. Basically, I am using a relay which is connected to the logic side of the PCB, 12VDC. This pulls down the contactor, and opens the circuit between the caps and bleeder. So when the coil is turned off, the relay springs back and re-contacts the bleed resistor, emptying my cap bank in around 20 seconds!
I dunno if it's been done before, so I thought I'd share anyway. Could be useful if you dont wanna waste heaps of power in your coil
Registered Member #27
Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058
The bleeder resistor is also for your safety and relays do fail from time to time. So maybe a large backup resistor directly over the capacitors might be an idea. The chance of failure is related to the amount of cleverness by some unknown but very steep function.
Registered Member #1875
Joined: Sun Dec 21 2008, 06:36PM
Location:
Posts: 635
A transistor would work, too, if you don't want to worry about the relay failing. Not to mention, you have added an electromechanical part to a "solid state tesla coil" =)
Registered Member #1361
Joined: Thu Feb 28 2008, 10:57AM
Location: Cairns, Australia
Posts: 305
Hahaha, didn't think of that! I was thinking of adding some kind of comparator that'd light an LED until the caps were at 0V, but since it's gonna be in a box the whole time (If it doesn't fail again :P ) it's not really a big issue, I can just check with my DMM and short it manually if I need to.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Bjørn wrote ...
The chance of failure is related to the amount of cleverness by some unknown but very steep function.
It's not monotonic, either! Sometimes more cleverness makes the chance of failure go down at first, but then if you try to get cleverer still, it screws up again. I think the chance of relay failure is small in this application. He's done it right, using the normally closed contacts so that the relay will close when power is disconnected, which a transistor can't do.
Sometimes real cleverness consists of figuring out how many parts you can leave out. But then again, Bob Pease used to argue against the MIL-HDBK-217 method of computing reliability, which said that adding extra parts always reduced reliability. But Pease found that adding, say, ESD protection diodes would make the MTBF go up.
Pease also recommended that to warn of charged capacitors, you just install a neon lamp across them. (with current limiting resistor of course!) They ignite at 140V and stay lit down to 90V, and these are the kinds of voltages where shocks start to hurt. It's also very cheap, unlikely to fail, and the glowing neon is an instant visual clue to the presence of high voltage.
He convinced me, and I've started doing this on my SSTCs' bus caps.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
Further to Steve's comments, I like to put a pea neon with dropper resistor across critical fuses, so one can see at a glance in what sub-system a failure has occurred.
I wouldn't want to risk my life on a duff relay to save a few shekels.
Registered Member #1361
Joined: Thu Feb 28 2008, 10:57AM
Location: Cairns, Australia
Posts: 305
Yeah, the relay is pretty failsafe the way it is wired. If the contacts weld, it'll just stop the caps from charging altogether. If the relay coil fails, it'll stop charing too.
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Bleeder resistor idea
