Microwave transformer hum when measuring

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brent1369

Joined May 2, 2020
16
I have a hantek6022be oscilloscope connected to a laptop to isolate it from ground. I connected a 1G ohm resistor in series with my oscilloscope to attenuate the signaal by 1000x. It all works good but when I connect it to a microwave transformer it makes an arcing noise. The weird thing is when I exchange the two oscilloscope wires connected to the secondary of the transformer with each other it does not produce this arcing noise. I have exchanged them several times but it always comes out the same.

How could it be by just merely switching the probes positions that it does not make this noise?
 

MaxHeadRoom

Joined Jul 18, 2013
23,639
Why do you need to 'scope the secondary?
It should be a straight forward 50/60Hz sine wave
Where is the sound emanating from exactly?
 

Thread Starter

brent1369

Joined May 2, 2020
16
I meant connected to the secondary 2kv windings.
The sound is coming from the transformer.

with further testing I realized that this arc sound also only happened when the laptop was being charged
 

KeithWalker

Joined Jul 10, 2017
1,919
The 2KV AC will be referenced to ground. If you connect the "hot" side of the secondary winding to your scope common terminal, you are applying 2KV to it and to the common of your laptop. YOU ARE LUCKY TO STILL BE ALIVE!
 

Thread Starter

brent1369

Joined May 2, 2020
16
i thought the 2kv was between the two secondary cables. or is one of the secondary cables 0V and the other 2kV.(in reference to primary ground)
 

KeithWalker

Joined Jul 10, 2017
1,919
i thought the 2kv was between the two secondary cables. or is one of the secondary cables 0V and the other 2kV.(in reference to primary ground)
One side of the secondary winding is connected to ground. When you connect the hot side to the ground of your scope, the transformer is being overloaded to ground by the capacitance to ground of your scope and laptop. You are lucky that the laptop charger is isolated, otherwise you would have had a direct short across the 2KV winding.
The dangerous part of that is, if you happened to touch the common of your scope or laptop and mains ground at the same time, you would be toast! You can still get a bad shock through the capacitive coupling to ground.
The moral is: Don't mess with lethal things that you don't fully understand. Consider yourself lucky that you learned it on these pages and not on a respirator!
 

Thread Starter

brent1369

Joined May 2, 2020
16
I measured between common and ground and got 6V. u can also hear the noise in that video, but the arcing can't be heard very well.
 

Delta Prime

Joined Nov 15, 2019
1,150
but when i disconnected the laptop charger my 600v multimeter got overloaded, oops lol
This is not something to laugh about!
And surely you would have loved ones that will miss you think about them if not yourself . By the way it will not only kill you it will hurt the whole time you are dying!!
 

KeithWalker

Joined Jul 10, 2017
1,919
but when i disconnected the laptop charger my 600v multimeter got overloaded, oops lol
Do you really have a deathwish? I can almost smell the smoke from here!
Please be careful. You only get one try at life!
The next time you want to measure a high voltage, check first with an ohmmeter to find out which side is grounded - with the power OFF! Don't ever connect the high side to the common of your measuring instrument's . If it is grounded, it will kill what you are testing. If it's not, it can kill you!
 

KeithWalker

Joined Jul 10, 2017
1,919
the secondary of the transformer is not grounded I could not measure it
What are you measuring between? You should measure the resistance between each end of the high voltage secondary and the microwave ground, which will be all the metalwork, which is connected to the power input ground. The hot end of the winding is connected to the magnetron through a high voltage capacitor.
 

MaxHeadRoom

Joined Jul 18, 2013
23,639
As per post #14, it seems like a rather futile exercise?
As to post #15 , Measuring HV such as this, generally just requires a suitable meter
 

Thread Starter

brent1369

Joined May 2, 2020
16
sadly the 1Gohm 10kv resistor did not like the inductive kickback and died. I guess ill add a capacitor or something next time...
 
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