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Registered Member #10052
Joined: Thu Feb 07 2013, 11:31PM
Location: Overland Park, Kansas
Posts: 78
There is a 100uf cap filtering the input to all 3 regs after the rectifier, 330uf caps on the outputs of the 5 and 9 volt regulators, and 2 100uf paralelled on the output of the 24 volt regulator.
I had the the 100uf caps on the outputs of the 5v and 9v regs, so they weren't doing as much smoothing, and had the 10v 330uf caps on the output of the 24v reg, so they were being overvolted.
Right now I'm looking at a small plug-and-play DC-DC converter meant to replace the LM78xx linear regulators, and operate at up to 36 volts. This would be great peace of mind, what without worrying about heating up no matter what Vin ends up being.
I can't believe I missed these problems, having examined everything so closely, but hopefully this rebuild will go well and I'll have a working board.
Registered Member #2292
Joined: Fri Aug 14 2009, 05:33PM
Location: The Wild West AKA Arizona
Posts: 795
HighVoltageFeathers wrote ...
Thanks everybody.
I was wondering about the voltage. Just out of curiosity I put the transformer on the scope and it peaked unloaded at 42 volts (no real surprise here I guess).
I'm looking to Eric Goodchild's twin coils, and Phill Slawinski's drsstc 5, saw that they both used the Radio Shack 25.6v transformers, and mistakenly assumed it would work for me. They must have been using a resistor or different regulators.
I know that the gate drive half of the board (drivers and mosfets) will probably have to be replaced, and am pretty certain now that I smoked the regulators (that 5v got HOT).
Just crossing my fingers now that the 5 and 9 volt regulators didn't freak out and overvolt all the logic circuits when they failed.
Does anybody have insight into the modes of failure for these regulators? I'd hope that they are designed not to pass any voltage much higher than they're meant to regulate if they fail, but honestly have no idea.
Thanks for all the help guys. Noobster here appreciates it!
Just an FYI, when I used a 25.6VAC transformer the 9Vreg blew up horribly, the one I use now happens to be a 20VAC output. 25VAC will overvolt the 5V and 9V regulators as others have stated. 18VAC is also too low, you will get lower than normal output voltage from the 24V reg.
It's a fine line and usually 20VAC is the best choice.
As for your gate drive section, it looks like only the resistors cooked, you may want to check your GDT out and the connections to and from the GDT. A shorted primary or secondary will do exactly what you experienced.
As an added test try disconnecting the GDT completely on your next test and just look at the output of the gate drive section on a scope to verify all is well.
Registered Member #10052
Joined: Thu Feb 07 2013, 11:31PM
Location: Overland Park, Kansas
Posts: 78
Goodchild wrote ...
It's a fine line and usually 20VAC is the best choice.
As for your gate drive section, it looks like only the resistors cooked, you may want to check your GDT out and the connections to and from the GDT. A shorted primary or secondary will do exactly what you experienced.
As an added test try disconnecting the GDT completely on your next test and just look at the output of the gate drive section on a scope to verify all is well.
I'll pick up that transformer then. Thanks!
Yeah, the resistors on the GDT output were de-soldering themselves. I checked for shorts, but I think the problem was fried logic circuitry that got one of the drive mosfets stuck on. The GDT was humming for about 3 seconds, then stopped, and the resistors started smoking.
Testing without the GDT is probably a good idea :P
Registered Member #15
Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068
If you know exactly what the range of input load current is, you can select a dropping resistor which you can place in series with the input to the regulators and drop the excess voltage across the resistor.
However, you need to know exactly what the minimum load current is and maximum load current is so you can properly size the resistor.
Registered Member #10052
Joined: Thu Feb 07 2013, 11:31PM
Location: Overland Park, Kansas
Posts: 78
With all this trouble, I was considering just sticking a diode string on the Vin for the 5v reg (and maybe 9v) to drop the voltage, and maybe a zenner on the outputs (5 and 9) so this doesn't happen again!
Registered Member #15
Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068
HighVoltageFeathers wrote ...
With all this trouble, I was considering just sticking a diode string on the Vin for the 5v reg (and maybe 9v) to drop the voltage, and maybe a zenner on the outputs (5 and 9) so this doesn't happen again!
there are many ways to skin a cat. Just be sure you have a good understanding of what your load currents are - from minimum to maximum (and then add a little margin). You want to make sure whatever claptrap you add to your circuit will ensure that the minimum. headroom voltage is sustained, through all loading conditions. And recognize and series pass element you add, whether it is a resistor, diode, etc... will dissipate power, so understand what that dissipation is so you can select the proper component and heatsinking provision, if necessary.
Registered Member #3908
Joined: Tue May 24 2011, 09:40PM
Location: Gilbert, Arizona USA
Posts: 68
HighVoltageFeathers wrote ...
I'm looking to Eric Goodchild's twin coils, and Phill Slawinski's drsstc 5, saw that they both used the Radio Shack 25.6v transformers, and mistakenly assumed it would work for me. They must have been using a resistor or different regulators.
I built a copy of Eric's large DRSSTC, and power for the UD2 is provided by a 20VAC 2A transformer acquired on ebay. It's run for hours without any issue.
Output from the bridge rectifier is ~29vdc. In my app it also powers a pair of 12v green lighted ghetto fans wired in series on the MMC, besides powering the UD2. The three voltage regulators on the UD2 are remote mounted to a heatsink bar. I also prefer a Motorola MC7805 regulator, considering it's ceiling is 35v.
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UD 2.1 Nightmare - Smoking boards.
