Oscilloscope AC 3 prong plus Ground causes Regulators to short or blow up

[danny davis]

Why does an oscilloscope with a 3 prong plug ground terminal causes IC chips and Regulators to short out or open circuit?

What is wrong with the design of the circuit? because when measuring voltage or waveforms with an 3 prong plug oscilloscope it causes shorts or the chips to open or short?

When cutting the ground terminal on the 3 prong plus making it a 2 prong plus just hot and neutral it fixed the problem, why?

 

[BradtheRad]

My oscilloscope is an old Tektronix tube model. It operates on mains AC. The plug has 3 prongs.

I do not like to hook it up to anything that is in contact with mains AC. The reason is that sometimes I have allowed the hot and neutral wires to get reversed. I get a spark and a bang. Often it ruins something. If I'm lucky it's only a burned metal contact somewhere.

It happened because I allowed mains AC to be short circuited through my oscilloscope ground wire.

You must make sure that the device under test does not have a hot ground. If it is hot then I believe that is the cause of your trouble.

No one will recommend that you test for proper polarity by touching one hand to oscilloscope ground and the other hand to the device ground, but that is what I have done sometimes, very cautiously.

However...

Do you mean that it helped for you to remove the ground prong? There's still a chance something in the wires was reversed, so that heavy current found a path to ground, regardless whether went to the neutral prong or the ground prong.

 

[pplus]

As i understand your scheme don't have galvanic isolation from the line.When you connect an oscilloscope, you close the phase or neutral to ground. When cutting the ground terminal your oscilloscope will be a high voltage, which is dangerous. I would recommend using a transformer 1:1 (not autotransformer!) for electrical isolation. This will prevent your circuit burn.

 

[danny davis]

Do you mean that it helped for you to remove the ground prong?

YES IT stopped shorting out the power supply regulators

When cutting the ground terminal your oscilloscope will be a high voltage

Why is the oscilloscope at a high voltage? and I shouldn't touch the oscilloscope with my hands right?

When you connect an oscilloscope, you close the phase or neutral to ground

What is wrong with the design of the circuit to cause this issue?

Why do I have to use an transformer 1:1 for electrical isolation?

What kinds of circuits/designs do u need to use an transformer 1:1 for electrical isolation?

 

[betwixt]

I'll reword the solution already given:

You have a direct electrical connection between your oscilloscope probe and the ground prong on the oscilloscope power plug.
You also have a direct electrical connection betwen part of your circuit and the power ground, probably through the ground of your power supply.
When you connect your osciloscope probe to your unit, you complete a short circuit through the probe, oscilloscope ground, power sockets, your units power cable and back to your circuit.

Solutions:
1. if you have a dual-beam oscilloscope, disconnect the ground of the probe and measure differentially (add and invert) across the probe tips.
2. disconnect the oscilloscope ground connection at the power plug. Note that this may make the oscilloscope chassis and ground 'float' to high voltage.
3. same as solution 2 but use an isolating power transformer to ensure any high voltages on the oscilloscope are not dangerous if touched. The oscilloscope chassis will assume the voltage of the ground clip on the probe.

If you use solution 2 there is a risk of danger to yourself and the equipment, at the very least, reconnect the ground through a resistor of say 470K Ohms and in parallel with it a capacitor of say 470pF. both must be rated t withstand the highest voltage the mains power and your equipment may ever encounter, I would suggest 1KV at least. Reconnect the ground again when you have finished your work.

Brian.

 

[danny davis]

What can be changed on the Circuit design to allow a 3 prong plug on the oscilloscope?

Cause right now the circuit will only allow a 2 prong plus oscilloscope

Is there something wrong with the power supply design of the circuit/designs or what component or design flaws makes a 3 prong plus oscilloscope , short out or cause components to open?

This is a circuit design issue , but is the design concept that is cause this?

Some circuits allow a 3 prong oscilloscope other circuits a 3 prong oscilloscope will short out IC chips and regulators, etc.

What is wrong with the circuit design?

 

[BradtheRad]

Why do I have to use an transformer 1:1 for electrical isolation?

What kinds of circuits/designs do u need to use an transformer 1:1 for electrical isolation?

This is the usual advice... because it's the safest advice... when hooking up wires between your oscilloscope and an AC appliance. Many people have this problem.

A 2-ampere 1:1 isolation transformer should be adequate to power your oscilloscope. (Of course it's wise to confirm its power rating.)

A new one is expensive, but you can find a used one for a bargain on Ebay. (Although shipping will raise the overall price.)

The other solution is to get an oscilloscope that has internal isolation from mains AC, or else is not powered from mains AC at all.

 

[betwixt]

You are learning the problems of having multiple grounds in a system.

It isn't necessarily a design problem, in some instances, for example some SMPS, connecting the oscilloscope shorts out the AC mains and blows the building fuses! It sounds as though in your case there is a connection between part of the PSU that is above ground potential directly back to the power plug. With everything unplugged, try using a test meter to measure resistance between the ground prong and other parts of your circuit, particularly around the regulators and I think you will find a short circuit. That may not indicate a problem but if the short is anywhere other than to the regulator grounds it will account for the damage to them.

For more information you will have to show us the schematic of the circuit around the power supply.

Brian.

 

[pplus]

A 2-ampere 1:1 isolation transformer should be adequate to power your oscilloscope. (Of course it's wise to confirm its power rating.)

I would suggest to isolate the circuit, not the oscilloscope if possible.
1. Housing oscilloscope remain at ground potential, and not at a floating voltage.
2. Power transformer should slightly exceed the power consumption scheme. Load circuit in debug time can be reduced to a minimum.
3. High output impedance transformer can prevent burn power circuit elements. The fuses must prevent burning the transformer.

 

[Kripton2035]

the safetiest way is to use a differential probe... it's made for this situation.

 

[danny davis]

So a 3 probe oscilloscope will short out components in a power supply because why? there is resistance between the ground plug and the component?

So when doing AC and DC voltage measurements on a power supply its best to use a DVM meter? or use an oscilloscope with a 1:1 transformer or use an oscilloscope in differential mode?

 

[betwixt]

Please read what we have already explained.

Imagine this scenario: your scope probe is connected to scope ground (which is normal), your scope ground is connected to the ground pin on the power plug. The power plug ground is connected to the AC outlet ground which also has all the other power outlets connected to it as well. So there is a direct connection between the scope probe ground and the ground wiring of the building. Now look at your equipment, it also has a ground pin on the power plug which connects through the power cord to your power supply ground. Perhaps now you can see why connecting the probe ground to something in your PSU which is not a ground potential shorts it out. You have linked them together via all the cables and sockets, causing a short circuit.

There is ALWAYS a danger of this happening with ANY grounded test equipment. There is also a danger of ungrounded equipment becoming live and dangerous, sometimes you have to live with that and just be careful. The safest way to connect AC powered test equipment is though an isolating transformer. It lets the power through to make the equipment work but has no direct connection back to the AC source and therefore cannot cause the problem you are seeing.

Brian.

 

[danny davis]

I connect the oscilloscope ground on my probe to power supply ground

im using the oscilloscopes probe ground and connect it to the power supplies ground

so that creates a short circuit?

 

[betwixt]

OK, look at it a different way: Connect a DVM between your scope probe ground and the ground on the power supply. If there is any voltage there, it's what you are shorting out. Then turn everything off, leave it a moment for any residual charge to die away then measure the resistance between the ground on the probe and the ground in the PSU, if it's more than about 1 Ohm, something isn't connected properly in the ground wiring.

I fully agree that if they are properly wired, they are at the same potential but that also means the regulators would not be affected by connecting them via a second route.

You really have to show us the schematic for more help.

Brian.

 

[danny davis]

Thanks for the help

So there shouldnt be voltage from the oscilloscopes probe ground and the psu ground? it should be zero volts right