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Open circuit Half Bridge Tripping GFI

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ScotchTapeLord

Mon Mar 16 2009, 11:40AM

Registered Member #1875 Joined: Sun Dec 21 2008, 06:36PM
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Posts: 635

Hello, I just finished a little half bridge of irf740s and 1uF caps fed by rectified variac (have only tested at low voltages). I have a small motor run capacitor for smoothing. I tested the bridge output as an open circuit at about 80kHz and saw a square wave rising above and dropping below 0v. Upon further inspection I found the square wave to be following a sinusidual, 60 Hz wave... I raised the voltage and then tripped the gfi. I'm not sure what's going on... Shoot-through? I don't know how else I could have pulled any current. I tried again with a lower frequency of 1.5kHz and the wave was quite even, square, and generally beautiful... And I was able to raise the voltage without failure. I'm thinking about replacing the full wave rectifier and adding more smoothing, but since some people feed their drivers with rectified unsmoothed mains, I don't know if it will help. Should I use more than 1uF for the pseudo-gates? Would putting balancing resistors across them help? I am sorry for not having the specs at this time, I was just wondering if anyone else has heard of this problem. I am driving it with GDTs using a tlc555 to feed bipolar push-pull transistors (richie burnett's design, except with a 555... And a half bridge.)

Zum Beispiel

Mon Mar 16 2009, 12:37PM

Registered Member #514 Joined: Sun Feb 11 2007, 12:27AM
Location: Somewhere in Pirkanmaa, Finland
Posts: 295

Your scope probes are grounded to mains ground. Therefore scoping a mains fed bridge is generally a bad idea. You need to feed the bridge from an isolation transformer, if you want to scope it's output.

ScotchTapeLord

Mon Mar 16 2009, 01:05PM

Registered Member #1875 Joined: Sun Dec 21 2008, 06:36PM
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Posts: 635

Doh. Doh! That explains it! I grounded the bridge... But why did it work at low frequency, I wonder. Instead of an isolation transformer... Can I just use a two-to-three prong adaptor on my o-scope? :P (seems easier, and cheaper)

Shaun

Mon Mar 16 2009, 03:18PM

Registered Member #690 Joined: Tue May 08 2007, 03:47AM
Location: New Jersey, USA
Posts: 616

Yes, floating your scope using a 3-to-2 prong "cheater" adapter will probably work, but it is not recommended. You could end up hurting your scope, or more importantly, yourself.

Sulaiman

Mon Mar 16 2009, 06:58PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

PLEASE do not operate your 'scope without the earth connection
it's a 'trick' used by older TV servicemen and it's VERY dangerous.
Ideally you would use an isolation transformer before the rectifier(s)
Isolation transformers are pretty expensive.
IF you have two similar mains transformers of sufficient power rating you can use them back-to-back, e.g. (mains/12V) - (12V/mains) etc.

At work I'd use an isolating differential probe, ideal but $$$$$$$$$$$$$

Shaun

Mon Mar 16 2009, 07:26PM

Registered Member #690 Joined: Tue May 08 2007, 03:47AM
Location: New Jersey, USA
Posts: 616

Someone here once mentioned a great was to make a powerful isolation transformer: Take two MOTs, parallel their secondaries, and plug one of the primaries into the mains.

That puts isolated mains voltage on the opposite primary, and it is capable of as much power throughput as one of the MOTs (~1000W).

Such a device would be a great tool to have for any HV related experimenting.

Tom540

Mon Mar 16 2009, 08:39PM

Banned on 3/17/2009.
Registered Member #487 Joined: Sun Jul 09 2006, 01:22AM
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Posts: 617

You can put the scope in differential mode and do it that way using two normal scope probes. Tie both channel grounds together, put scope in ADD mode and select the invert channel two button. Then use channel two as ground and channel one and your probe.

ScotchTapeLord

Mon Mar 16 2009, 09:51PM

Registered Member #1875 Joined: Sun Dec 21 2008, 06:36PM
Location:
Posts: 635

I do have MOTs lying around... But is it okay to float the oscilloscope if I only run the bridge at a few volts? Why exactly is it dangerous in the first place?

aonomus

Mon Mar 16 2009, 11:18PM

Registered Member #1497 Joined: Thu May 22 2008, 05:24AM
Location: Toronto, Ontario, Canada
Posts: 801

Read

An oscilloscope is essentially a voltmeter that tracks change over time, and all voltmeters require 2 points to measure a voltage between.

The main danger is visible in that first diagram, as the top mosfet switches on (and the bottom does not), TP2 (lets assume scope ground) will be floating at +175V, and scope ground includes its casing and in particular its earth or ground pin. Essentially connecting scope ground to TP2 would be like having +175V short to the casing, in which case fuses blow, the most practical fuse being that flimsy probe lead....

And in the case of people removing or circumventing the scope ground, the casing becomes charge to +175V, and you could get electrocuted.

Note that if you do use the differential method as Tom540 suggested, that either your scope inputs are rated for the voltage (not all are), or that you use the 10x attenuation mode on your probes. Be sure to very clearly protect your ground clips from accidentally shorting out to anything.

And an addendum, my old Philips oscilloscope does not have a ground pin, which makes it particularly risky for me as I can hook up a bridge and view gate signals, but I have to be mindful about it because there is alot of other grounded equipment beside and ontop of the scope. This also would include any metal shelving (which I don't have, but others may).

ScotchTapeLord

Tue Mar 17 2009, 12:30AM

Registered Member #1875 Joined: Sun Dec 21 2008, 06:36PM
Location:
Posts: 635

I think I'll just assume it works and hook up my primary. :)
Thank you for all the information you've given me!

I'm not sure if this belongs in the Tesla Coils section, so to make it more relevant I will just say that I intend on using the bridge to run a mini sstc... Expect results soon!

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