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What did I do wrong?

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bryan

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Trying to fix an electronic ballast for fluorescent tube ,so i wanted to scope out some of the electronics on the board. Connected the osilloscope ground clip to the - on the bridge rectifier that supplies the circuit. Bang and the GFC trips. The power to the electronic componets for the low power side of the circuit is isolated by a small transformer.

Any idea what I did wrong??
 
Which device scope or ballast was plugged into the GFCI? The GFCI looks at in and out current between mains hot and neutral. If there is an imbalance the unit trips. You managed to provide an additional current path to ground so the current In/Out became imbalanced. That would be my take.

Ron
 
The power to the electronic componets for the low power side of the circuit is isolated by a small transformer.
Probably not, that's most likely a line filter not an isolation transformer.
 
I second Brevor. You need a line isolation transformer for your scope or a battery operated scope for this test. E
 
Your scope common is likely tied to the AC safety ground, thus there was a dead short between the power line and the ground through the circuit bridge rectifier.

As has been stated you need an isolation transformer for the test. A battery operated scope would keep the breaker from blowing but it would still expose you to direct line voltages which is not safe.
 
Or a differential input active probe or a scope with differential inputs...... :)

Ron
 
Probably not, that's most likely a line filter not an isolation transformer.

The ballast has a high voltage side (Mosfets etc) and a low voltage side for some of the discrete logic and protection circuits. The low side is what I am assume is a small isolation transformer that drops the mains to 12v.
 
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