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UD 2.1 Nightmare - Smoking boards.

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Feathers

Sun May 05 2013, 05:35AM

Registered Member #10052 Joined: Thu Feb 07 2013, 11:31PM
Location: Overland Park, Kansas
Posts: 78

I've soldered together 2 UD2.1 boards, and have more recently been getting my coil ready to fire up.

All I intended to do was to see that the board powered up uneventfully. Unfortunately, neither one did.

I have a 25 volt transformer I bought from Radio Shack. These are the kind of transformers I've seen used before.

I had the GDT plugged in, and the two CT's plugged in.

All I was going to see is that the board powered up uneventfully, then plug in the interrupter, and see that it was receiving the signal.

Something went horribly wrong when I plugged into the mains. At first, the power and OCD lights flashed once, but went off, so I unplugged it.
I plugged it back in, and the power light came on, but not the OCD light. I turned on the interrupter at minimum on time, and I could hear the GDT buzzing, and the signal light came on. Then the buzzing stopped, and I noticed one of the gate driver mosfets getting hot. From there, the gate driver on the opposite channel started smoking.

I concluded that this must be an some error on the board, so I spent 4 hours double and triple checking my 2nd board against the schematics, wired it all up, tried again, and nearly the exact same thing happened. Its probably worth noting that the 5 volt regulator got extremely hot as well.

I'm heartbroken, and can't begin to imagine whats going on. I'm probably making a stupid mistake, and was just hoping that somebody with experience with these boards might spot a problem.

Here are pics of the setup, and the board.

Untitled by HighVoltageFeathers, on Flickr

Untitled by HighVoltageFeathers, on Flickr

Untitled by HighVoltageFeathers, on Flickr

Untitled by HighVoltageFeathers, on Flickr

Untitled by HighVoltageFeathers, on Flickr

I'm desperate. Any advice is welcome. Thanks everybody.

Dr. Dark Current

Sun May 05 2013, 09:57AM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384

Well I'm not knowledgable of the circuit, but using a 25 volt bridge-rectified transformer to power the standard 78xx regulators is asking for trouble (you will overvolt them).

HV Enthusiast

Sun May 05 2013, 01:37PM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068

Dr. Dark Current wrote ...

Well I'm not knowledgable of the circuit, but using a 25 volt bridge-rectified transformer to power the standard 78xx regulators is asking for trouble (you will overvolt them).

Yes, I agree. Thats too much voltage for the 78xx regulators.
I think most 78xx series regulators are good for 30V max. input.

With a 25VAC transformer, you are getting nominally 35V (minus bridge rectifier voltage drops) on the 78xx. Also, the higher voltage will lead to increases power dissipation in the 78xx as well.

An 18VAC transformer should work well for this. It might not give enough headroom to regulate the 24VDC, but for the given application the gate drive for the CM's doesn't have to be regulated once the voltage is well enough above the saturation threshold. The 7824 or whatever you are using for is more or less just a overvoltage protection device at that point.

If you want to the 7824 to regulate 24VDC, than you'd probably need about a 19-20VAC transformer. From all the photos i've seen of others coils, it seems everyone uses the Marlin Jones 18VAC, 1A control transformer that they sell for this particular application.

loneoceans

Sun May 05 2013, 02:46PM

Registered Member #4098 Joined: Fri Sept 16 2011, 09:26PM
Location:
Posts: 236

I've also used a 25VAC radioshack transformer but I put a 20W 15 Ohm resistor in series with the output to board input rectifier, heat sinked my 5V regulator and added a small fan. Supposing it draws 1A in total, that should drop the voltage down to reasonable levels. It runs my DRSSTC 2 with no problems at all and no heating, but I do agree that an 18-20V transformer is probably a better idea. Good luck and your bridge looks very good!

Physics Junkie

Sun May 05 2013, 03:56PM

Registered Member #7267 Joined: Tue Oct 16 2012, 12:16AM
Location: Detroit, Michigan
Posts: 407

Like EVR said, use the 18V 1A transformed from MPJA. I use that on the UD and when rectifies it gives 26V. Rather good for this board

Mads Barnkob

Sun May 05 2013, 04:29PM

Registered Member #1403 Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968

Remember that a typical 7824 requires minimum 27VDC as it at 1A load has a 2 Volt voltage drop.

Feathers

Sun May 05 2013, 04:30PM

Registered Member #10052 Joined: Thu Feb 07 2013, 11:31PM
Location: Overland Park, Kansas
Posts: 78

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!

HV Enthusiast

Sun May 05 2013, 05:08PM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068

The pass element for these regulators is just a transistor. If you exceed the voltage across this internal transistor, you will blow the MOSFET. There may also be housekeeping voltage regulators internal that may fry if you apply to much voltage across the input.

Also, check out the datasheets of various 78xx regulators. Some have lower max. input voltages and some have higher max. input voltages. There are also high voltage input type 78xx regulators which can accept 35-40V, but not as common.

Feathers

Tue May 07 2013, 01:17AM

Registered Member #10052 Joined: Thu Feb 07 2013, 11:31PM
Location: Overland Park, Kansas
Posts: 78

I checked out the regulators, and the 5v and 9v are fried and shorted! I also realize that I'd been careless with my capacitor placement, and had some of the 10 volt electrolytics seeing a lot more than 10 volts, and probably failing.

I'm replacing all the regulators, the 5 capacitors, and all the ICs on the board. Overkill? Not sure, but for me it'll just be peace of mind.

Thanks for the help guys.

HV Enthusiast

Tue May 07 2013, 12:15PM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068

That will do it, although i think electrolytics generally fail open. Tantalums are the ones that fail short.

Were the capacitors on the input or output of the regulators?

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