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Registered Member #3516
Joined: Wed Dec 15 2010, 10:40AM
Location: Australia
Posts: 32
Hi all, I parallel the o-scope probe to the full bridge output, and the circuit breaker activited. obviously it's not good. So how to measure primary waveform safely and correctly? Thanks.
Registered Member #3637
Joined: Fri Jan 21 2011, 11:07PM
Location: Buffalo, NY
Posts: 1068
You need to have isolation between the mains and your induction heater, otherwise you'll introduce a ground fault.
When a full wave bridge rectifier is working, as it swings to the negative polarity of the sine wave the bridge rectifier puts the HOT side of your mains on what is ground for your circuit...
When you put your scope probes on the full bridge you're basically putting ground whereever your ground clip is. It'll short your mains causing the circuit breaker to blow.
If you NEED to scope it, you'll have to either isolate it with an isolation transformer, or use a battery powered oscilloscope.
Registered Member #2288
Joined: Wed Aug 12 2009, 10:42PM
Location: Cambridge, MA
Posts: 179
Additionally, another option is to use two channels on your scope and measure the two outputs individually, with respect to earth (not the negative of the cap, you can't connect your scope ground to that unless you run the bridge off an isolated supply). Then set your scope to differential mode on those inputs (input2 - input1), or if your scope can't do that you can imagine the addition of the two waveforms.
But it's much easier and better to just use a floating supply for testing.
Registered Member #1451
Joined: Wed Apr 23 2008, 03:48AM
Location: Boulder, Co
Posts: 661
You need a scope that can add the channels together. Connect the ground clips together, set channel 2 to inverted, and set the channels to add. Then clip the probes in parallel with the primary.
Registered Member #4187
Joined: Fri Nov 04 2011, 08:08PM
Location: Spain
Posts: 43
Not the preffered option, but this can be done: - put the mid point of DC bus to mains ground - measure with a regular probe using a protected HV divider from + or - to ground.
you will get half of the divided voltage in the scope
I use this scheme to measure an 8kV NST output, but my probe is 600V differential, so I haven´t problems with grounds in any case.
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How to scope primary waveform?
