Scoping the primary of a switching power supply. fuses blow

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mmalf_mi
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Scoping the primary of a switching power supply. fuses blow

Post by mmalf_mi »

Do I need to power the TV through an isolation transformer? Fuses blow when I hook the scope ground to the chassi ground side of the switching power supply. Been getting back into service work again and I forgot the right way to scope the primary of the switching power supply. I think years ago, the scope I used had the power cord ground cut off. Sort of a rookie question, but I am humbled.

Thanks
Mike
Bigglez
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Re: Scoping the primary of a switching power supply. fuses

Post by Bigglez »

mmalf_mi wrote: Do I need to power the TV through an isolation transformer?
Yes, for your own safety and to allow arbituary "grounding"
of hot circuits to drive your scope (or other test equipment).
mmalf_mi wrote:I think years ago, the scope I used had the power cord ground cut off.
Dumb idea...
rshayes
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Post by rshayes »

With few exceptions, you will need an isolation transformer to look at waveforms in a line powered switching power supply.

These usually uae a bridge rectifier feeding a storage capacitor. During the positive half cycle, the rectifiers connet the hot lead of the power source to the positive terminal of the capacitor, with the neutral lead of the power source connected to the negative terminal of the capacitor. These connections are reversed on the negative half cycle.

The neutral lead of the power source is connected to the safety ground (green wire) at the point where the power line enters the building. The safety ground is connected to the case of the oscilloscope. In turn, the case of the oscilloscope is connect to the ground lead on the probe. Normally, you would connect this to the ground of the circuit that you are examinimg.

The closest thing to ground on the primary side of a switching supply is the negative terminal of the storage capacitor.

During the positive half cycle, this point is conected to the neutral power lead by the rectifier, within about a volt or so. During the positive half cycle, this point is connected to the hot lead of the power source, which can be as much as 176 volts below ground. This results in close to a direct short across the power line with the fuse and rectifier in series. Hopefully the fuse blows before before other damage occurs.

An isolation transformer breaks the direct connection between the hot and neutral line conductors and the safety ground, and allows the primary circuits of the switching power supply to operate with a ground defined by the oscilloscope probe lead rather than the input power leads.
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Lenp
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Post by Lenp »

Mike
As was previously said, an isolation transformer is a must. If you don't have one but do have a junkbox full of other transformers and have two that are like rated with VA ratings to handle the load just connect one as a step down and the other as a step up. The secondary voltage doesn't matter as long as they are the same, or really close. What really matters is the VA rating.
Example #1 120V ->24V connected to #2 24V ->120V
Make sure there is no continuity between the input 120V and the output 120V. In effect, you have double isolation!

Hope this helps..
Len
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jollyrgr
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Post by jollyrgr »

Has anyone ever used a UPS (unplugged and running on battery) to power a TV while using a scope? Or how about a 12V power inverter (or a modified UPS) running off a linear power supply to feed the TV?
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mmalf_mi
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thanks all !

Post by mmalf_mi »

Nice to have people. Hope I can pay it back someday.

Mike
Robert Reed
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Post by Robert Reed »

Mike
If you service on even a limited basis, you may consider browsing around on E-Bay for a used Isolation transformer. I picked up a 1000 VA new one last year for < $50. I would assume with the average TV set today, you could get by with 200VA or less and should be much cheaper than mine. Even aside from service work, these are nice to have around.
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