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5+ foot arcs, and then board problems.

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Feathers

Tue May 28 2013, 05:32PM

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

Hi.

I've been getting my 6" cm300dy-12H full-bridge coil running, and up untill now its been great!

I had trouble with driver boards in the past, but put together a UD 2.5, and all has been well, up untill yesterday.

The coil was running with the OCD set at 600 amps for about 15 cycles at 240 volts accross the bridge (5+ foot arcs! photos to come!), when it arced up and hit the rails for my garage door. The coil has arced to the strike rail and other objects on the RF ground (braided aluminum tether leading to a 6' ground rod just outside of the garage), but when it hit the rail something went wrong and the coil began sputtering, then quit.

My worst fear is that arc current found its way into the garage door opener, and subsequently into the mains, back to the coil, and killed my driver or IGBTs (I've got a variac going straight through a rectifier to charge the bus, so it is not mains isolated). However, I'm just not sure what happened.

The IGBTs appear to be okay, and the driver powers up and displays signal. The gate drive section appears to be working fine, and responds to input from the interrupter, so I think my problem lies in the feedback circuit. I've been poking around with a scope and this is what I've found.

I'm running a 20v p-p 75kHz sine wave to the comparator, and just past the chopping diodes (the schottky diodes rather than the standard, as eric suggested), everything looks okay, a clipped square wave biassed to ~1.6 volts. But, while (as far as I can tell) the reference pin of the comparator should exhibit the exact same ~1.6v bias (1Kohms to the pin, then 470ohms to ground), it is sitting below the chopper output at around 1 volt.

Untitled by HighVoltageFeathers, on Flickr

Untitled by HighVoltageFeathers, on Flickr
I've de-soldered the bias resistors, tested, and re-installed them all within 5% of what they should be. I'm just not sure what the problem could be, thought I might see what you guys think.

I have a boards worth of new parts comming in from mouser, but I've got a "young makers" photoshoot with the kansas city star on friday and I need to have the coil working again.

Any input welcome. Thanks!

Steve Conner

Tue May 28 2013, 05:42PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

If the bias resistors and C33 check good, the most likely explanation is that the comparator chip is toast.

Feathers

Tue May 28 2013, 06:54PM

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

I've replaced the comparator chip with a brand new piece, I'll check the cap for resistance.

Feathers

Tue May 28 2013, 07:41PM

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

Ahah! Faulty comparator straight out of the package from mouser!

Replaced it with one out of an old board and all is well.

Thanks!

Aragorn

Tue May 28 2013, 09:15PM

Registered Member #18516 Joined: Sat May 18 2013, 09:09AM
Location: Lancashire, UK
Posts: 38

Any thoughts on what killed the comparator?

HV Enthusiast

Tue May 28 2013, 10:01PM

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

How is the garage door opener?

Feathers

Wed May 29 2013, 12:14AM

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

I have a few ideas.
This thing is humming fine now, pushing out 4+ foot of spark at 250 volts and 600APk (tons of on time, primary is heating up.)

I think Mouser sent me two faulty comparators. The one I had in the coil failed partway through, and the other one - brand new out of the tape and ESD bag from mouser - didn't work, so I thought it must be some other component, but the problem just didn't make sense.

Swapped in a comparator off of an old board, and its working great again.

The garage door opener is at least as old as the house (20 years). I think it would have to be physically destroyed to keep it from chugging right on forward.

Video coming soon!

Physics Junkie

Wed May 29 2013, 01:44AM

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

Are you running more than 200uS on time?

HV Enthusiast

Wed May 29 2013, 02:31AM

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

Regarding bad comparators . . . make sure you aren't overheating the parts.

Make sure your soldering iron is set to the temperature required for the SMT parts which in some cases can be considerably lower than you think. Overheating the parts can definately damage them.

Also, some of those ultra-high speed comparators can have inputs which are more sensitive to ESD than traditional logic chips and general purpose comparators. So make sure you practice good ESD prevention - using ground straps etc... when handling these parts.

Feathers

Wed May 29 2013, 06:42AM

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

Good tips, didn't know that regarding ESD. I'm careful to keep iron temps around 325* with the SOIC chips.

I've been running on times up to 240us, hopefully I'll get a proper power supply rigged up and more voltage to play with soon to bring that number down.

Thanks for the help guys.

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