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Possible ground loop problem?

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nrhoades

Fri Aug 24 2007, 02:24AM

Registered Member #610 Joined: Wed Mar 28 2007, 09:44PM
Location: Middletown, RI
Posts: 110

Tonight I made an H-bridge with IRF740's. The gates are driven with a GDT at 15V 50kHz, have 15V zeners, and in addition each MOSFET has extra reverse recovery diodes. The waveform looks fine.

I attached a x10 probe to a low-side gate, and attached the ground clip to the source. My circuit has a very low parasitic inductance. I used a 400ohm power resistor as a load.

I hooked the power rails up to an isolated regulated 30V 7.5A power supply, and everything worked fine. The bridge drew about 2A.

Next I used a 20A variac tuned to 30V, and this was rectified by a capable diode-bridge and capacitor. (So basically, an "un-isolated, un-regulated power supply with a very small ripple). I turned the power on and the probe's ground clip started melting.

I thought maybe that I had a bad ground loop. I DID NOT use a ground wire with either setups. However, I believe that neutral is already been tied to ground in the wall. I verified this by checking my outlet and I saw 120VAC between ground and hot, and 0VAC between ground and neutral (I live in the US in an apartment.) I think my probe is plugged into the same line with the same ground/neutral/hot wires running to the outlets.

Is it possible that this is actually a ground loop problem, or is it something else? Thanks

Sulaiman

Fri Aug 24 2007, 07:05AM

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

This is a common problem caused by not using an isolation transformer !
The same problem is encountered when 'scoping older TV sets.

Think about the bridge rectifier;

When the 'live' wire is positive with respect to neutral the neutral is connected to your scope earth
both through the house wiring and through the bridge rectifier.

When the 'live' wire is negative with respect to neutral the live is connected to your 'scope earth via the bridge rectifier
and the 'scope is connected to neutral via the house wiring.

Some people disconnect the earth wire of the 'scope to overcome this problem.
This is a VERY BAD practice as the case of the 'scope will be at 'live' voltage each half-cycle of the mains
Potentially LETHAL.

You need to have an isolating transformer between the mains and your equipment.
Do NOT use the isolating transformer for your 'scope instead as the 'scope case will be 'live'

nrhoades

Fri Aug 24 2007, 01:09PM

Registered Member #610 Joined: Wed Mar 28 2007, 09:44PM
Location: Middletown, RI
Posts: 110

I understood your response except for this last line...

Do NOT use the isolating transformer for your 'scope instead as the 'scope case will be 'live'

Other then that, I'm taking that I need to buy an isolation transformer, and place it before the mains of any equipment that I am using, especially if my device-under-test is not isolated.

Marko

Fri Aug 24 2007, 01:19PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

I understood your response except for this last line...

You isolate the scope, so it's case floats.

Then if you scope something that is not isolated, you place the scope 'ground' to - of your bridge rectifier, but you forget that - of bridge rectifier is hot for one half cycle.

You touch the scope case and you die.

nrhoades

Fri Aug 24 2007, 02:25PM

Registered Member #610 Joined: Wed Mar 28 2007, 09:44PM
Location: Middletown, RI
Posts: 110

You isolate the scope, so it's case floats.

The risk of death is one option...

The other place I could put the isolation transformer is before the variac? The problem I see with this is that the now floating 'bridge' ground would be pulled to 'sccope' ground through that little ground clip, which doesn't sound good.

Also, the isolation transformer's current capability would be a possible bottleneck.

Wait a minute... is "wall wart" lingo for an isolation transformer?

uzzors2k

Fri Aug 24 2007, 02:50PM

Registered Member #95 Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308

nrhoades wrote ...

Wait a minute... is "wall wart" lingo for an isolation transformer?

Nope, a wall wart is a small transformer used to power radios, game consoles and such. However two identical wall warts can be built into an isolation transformer. Remove the rectifiers, and put the secondarys together. This will create a 1:1 transformer. Notice that this will only work with iron core mains transformers.

While on the topic, would two MOTs put together like this be safe, or would it be best to replace one's secondary with the other's primary? I've been thinking of making a cheap variac with a MOT isolation transformer and SS variac in front.

...

Fri Aug 24 2007, 04:41PM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

You could use 2 mots back to back, and as long as the hv terminals are well insulated it should be safe. Of course it would be better (at the least, lighter

) to make a core with 2 primaries...

nrhoades

Fri Aug 24 2007, 06:44PM

Registered Member #610 Joined: Wed Mar 28 2007, 09:44PM
Location: Middletown, RI
Posts: 110

I am scared of my variac... I have yet to use it to source large currents, and I don't know how much "buzz" it should make before an alarm goes off in my head.

Brand new...

Hazmatt_(The Underdog)

Fri Aug 24 2007, 07:44PM

Registered Member #135 Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735

Some of those Chinese variacs come with really shady construction! If you plug into the one I have the hot lead is always on because the switch and autoformer are connected through the neutral not the hot. So if you have any type of ground fault you get the full 120V out between the two!

I wouldn't ground isolate the 'scope and allow its chassis to be at 120V, that's really dangerous. I would either isolate the source and keep my scope grounded, OR use both channels of the scope with INVERT + ADD, then its ground isolated.

Steve Conner

Fri Aug 24 2007, 08:00PM

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

Well, if they put the fuse in the variable voltage output (which they should have done) then it should pop before the variac suffers any damage, so you don't need to worry.

If the fuse is in the input side, you should add another 20A fuse or circuit breaker on the output side for peace of mind.

The reason why I say this is, that if you're trying to drive a near short-circuit load with the knob turned down low, due to transformer action you can have much more than 20A flowing in the output side without blowing the input fuse. This typically leads to melting of the wiper and a big hole burnt in the windings, and is probably how most variacs meet their doom.

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