Scope probe ground clip sparking on chassis

[Agent24]

A while back I bought a very basic oscilloscope. It's a Dick-Smith Q-1280 (see attached manual) - don't laugh, it was cheap.

I had to get a probe on eBay since it didn't come with one, and I tested it out on a 555 timer, and then went on to use it a semi-useful way to get my bad ice tube clock running.


Today I had the idea of hooking it up to my non-working digital TV tuner PSU to see if I could spot anything obviously wrong on the outputs (yeah, unlikely, but it's all interesting, right?)


It was looking hopeful until I tried to clip the ground lead to the tuner's chassis - where I saw small blue sparks and got a shock to my finger

Of course I didn't proceed any further for risk of damage to anything, and am now wondering what would cause this?

The tuner is not grounded, fed only by a 2-pin mains lead, while the oscilloscope does have a 3-pin plug. Interestingly this happens even with the tuner's main switch OFF.


Is this kind of thing normal? (doubt it) or is there likely a fault with the tuner and\or scope? I wonder if this would explain why the tuner doesn't work.

[Toasty]

Re: Scope probe ground clip sparking on chassis

The scope has a transformer and is therefore isolated on the HOT side, but the Mains ground is attached to the scope chassis.

What does the tuner use for power? Is there a transformer or is it a switching supply? You should be using an Isolation Transformer when testing anything connected to the Mains, especially with a scope.

In the US, Neutral and Ground are typically the same potential. Tied together at the circuit box. Are you 120V or 220V?

Pics if you can.

[joshnz]

Re: Scope probe ground clip sparking on chassis

The scope has a transformer and is therefore isolated on the HOT side, but the Mains ground is attached to the scope chassis.

What does the tuner use for power? Is there a transformer or is it a switching supply? You should be using an Isolation Transformer when testing anything connected to the Mains, especially with a scope.

In the US, Neutral and Ground are typically the same potential. Tied together at the circuit box. Are you 120V or 220V?

Pics if you can.

Yes toasty the neutral and earth are tied together here in New Zealand 230v at 50HZ.

That is very interesting Agent24 maybe power switch switching neutral and getting current from the cap that goes between primary and secondary.

[randtek]

Re: Scope probe ground clip sparking on chassis

As Toasty said, you need an Isolation Transformer when using a scope on most A/C equipment, especially true for an older scope.

[Agent24]

Re: Scope probe ground clip sparking on chassis

What does the tuner use for power? Is there a transformer or is it a switching supply? You should be using an Isolation Transformer when testing anything connected to the Mains, especially with a scope.

Pics if you can.

Tuner uses an inbuilt SMPS. There's photos of the thing here: https://www.badcaps.net/forum/showthread.php?t=16541
I will make sure to get an isolation transformer before fiddling with anything else running from mains!

Is this one big enough? http://www.trademe.co.nz/business-fa...-417549572.htm

That is very interesting Agent24 maybe power switch switching neutral and getting current from the cap that goes between primary and secondary.

Entirely possible, the lead for the tuner can be inserted to the socket either way (one of those figure-8 plugs) and the power switch only switches one side of the line.

[Toasty]

Re: Scope probe ground clip sparking on chassis

Might be a failed Y cap.

DMM set to AC volts, 300 or better - Measure between scope ground and tuner chassis.

Can you flip the plug over on the back of the tuner and measure it then?

>>Is this one big enough?<<

LOL!

[Agent24]

Re: Scope probe ground clip sparking on chassis

The plug in one direction I measured 0 volts, the other, it starts at a high value, and quickly comes down to fluctuate around 0.7-0.5 volts

Either case regardless of the setting of the switch.

[Toasty]

Re: Scope probe ground clip sparking on chassis

It sounds like a leaky cap. Mark the plug and the unit so you only use it the way it gives the 0 volt reading.

[Agent24]

Re: Scope probe ground clip sparking on chassis

Just read up on what a Y capacitor is... after a few rather useless definitions such as:

What is a Y capacitor?
This is used to describe safety approved capacitors whose failure in a circuit will present danger of electrical shock.

Oh, really... yeah I wonder what that feeling was in my finger.....


So if I got this right, a PSU has two Y capacitors going from each side of the AC line to ground, to filter noise. If the one on the active side shorted\leaky, it would cause the chassis to become live.

Since the tuner has no earth, not much happens until it is connected to the earthed scope probe.

I guess then I can figure out which one is faulty by finding out which capacitor is connected to active when the chassis measures 0.5v with respect to earth.



Finally, since signal and power ground on the tuner is also connected to chassis - would the presence of voltage from the leaky capacitor cause a fault with the operation of the system or damage to the components?

[Toasty]

Re: Scope probe ground clip sparking on chassis

From Kemet.com:
X capacitors are across the line. { Think X = cross }
If they short-circuit, the risk is fire.

Y capacitors are from line to chassis. { Think Y = upper points are power lines, lower is ground. }
If they short-circuit, the risk is a shock to the user. => as is your case

In operation, the tuner does have an earth connection through the cable connection shield.

The leaky cap could cause operational problems. However, as The_Unique said in the other thread, you may have damaged the MCU/CPU/IC's with those previous bad caps from excess ripple.

Also, the little blue circular caps are a concern also. Typically the 'box' type fail open. The blue ceramic ones are more likely to fail shorted.

T

EDIT: Did this perhaps sustain a lightning strike nearby?

.

[Agent24]

Re: Scope probe ground clip sparking on chassis

Wouldn't have a clue, I acquired the tuner from someone else as faulty and thought I'd have a go at it.

[Bob Parker]

Re: Scope probe ground clip sparking on chassis

"Is this kind of thing normal (doubt it)?"

I've encountered this kind of thing due to mains filter caps in unearthed equipment quite often. If it was me I wouldn't worry too much about it, but I suppose now I'll get flamed for telling you to take dangerous risks.

[Agent24]

Re: Scope probe ground clip sparking on chassis

"Is this kind of thing normal (doubt it)?"

I've encountered this kind of thing due to mains filter caps in unearthed equipment quite often. If it was me I wouldn't worry too much about it, but I suppose now I'll get flamed for telling you to take dangerous risks.

Can you explain how they would cause the problem? I don't understand, unless you mean if they were leaky too?

[Bob Parker]

Re: Scope probe ground clip sparking on chassis

Can you explain how they would cause the problem? I don't understand, unless you mean if they were leaky too?

I hope you can view the attachment. It's the primary side wiring of a Teac DVD player power supply but reasonably typical of that kind of domestic gear. As you can see, it has a two conductor mains lead. Notice the two capacitors with red arrows pointing at them. Each is connected between a mains conductor and the non-earthed chassis. Capacitors pass current when an AC voltage is applied to them. They don't need to be leaky.

Do I need to explain in any more detail?

[Agent24]

Re: Scope probe ground clip sparking on chassis

I think I understand now, it's when you said 'mains filter capacitors' I thought you meant the big electrolytic(s) after the bridge rectifier.

So, if my brain is working correctly: the Y capacitors are like a voltage divider, with the output connected to ground, however because a 470pF cap at 60Hz has reactance of 5.6 Megohms, they won't be passing much of the actual mains current, but any high-frequency crap will see a much lower resistance, and thus get filtered out.

So then the 0.5v I measured across chassis and ground is the result of a small current being passed by the capacitor?


Is this a reason why isolation transformers should be used?

(Note: if none of that made sense, I blame lack of sleep)

[Bob Parker]

Re: Scope probe ground clip sparking on chassis

Sorry, I should have called them something like mains bypass caps to distinguish them from the big electrolytic cap. Yes, their function is to reduce high frequency noise going up and down the mains lead.

I'd have expected you to be measuring a considerably higher voltage than 0.5V, considering that you saw small sparks and got a minor shock.

It's pretty risky taking measurements around the "hot" (primary) side of a switching power supply. If you really have to do it and you're connecting things like an oscilloscope, an isolation transformer is absolutely essential. I know a guy whose oscilloscope probe lead exploded when he didn't use one. It could have been worse.

For measurements on the rest of the circuitry, personally I'd just clip the oscilloscope's ground lead to the chassis and measure away.

Now get some sleep!

[Krankshaft]

Re: Scope probe ground clip sparking on chassis

Anything on the primary side of the supply needs an isolation transformer. The main filter cap, bridge rectifier, switching transistor (whos heat sink may be live with respect to ground if not insulated), and the VCC pin of the PWM. Are all on the unisolated or HOT side of the PSU before the switching transformer.

Everything after the secondary the schottky diodes, output filter inductors, and output filter caps are on the cold side. Good boards will even have a white silkscreened line with the words HOT and COLD demarcating the sides of the board.

While technically it's safe to probe the COLD side without an isolation transformer just hook the unit under test into one anyway no matter what you're measuring.

The reason why you got a shock is because at the service panel the neutral and ground lead are tied together. The ground wire is a connected to a ground rod in the soil. You are standing on the ground so you've already completed half the circuit. All it takes is to touch a live conductor and you will get a shock.

An isolation transformer breaks that bond between neutral and ground making it safer for you and your equipment. In order to get a shock now you would have to touch two points in the circuit.

[Agent24]

Re: Scope probe ground clip sparking on chassis

I'd have expected you to be measuring a considerably higher voltage than 0.5V, considering that you saw small sparks and got a minor shock.

The meter did start at about 55v then quickly drop to the lower value, if that means anything.

Could it be that the current is very small, and the meter be 'sucking it up' and causing a false low reading? I am not sure what the input impedance is on this thing actually.

The reason why you got a shock is because at the service panel the neutral and ground lead are tied together. The ground wire is a connected to a ground rod in the soil. You are standing on the ground so you've already completed half the circuit. All it takes is to touch a live conductor and you will get a shock.

An isolation transformer breaks that bond between neutral and ground making it safer for you and your equipment. In order to get a shock now you would have to touch two points in the circuit.

And the picture becomes much clearer now (or maybe I'm just more awake!)

[Toasty]

Re: Scope probe ground clip sparking on chassis

Gotta love how everyone keeps repeating what's already been said...

Do the previous voltage check with the meter set to DC. I'm curious if the leak is on the rectified side of the supply.

Toast

[b700029]

Re: Scope probe ground clip sparking on chassis

The meter did start at about 55v then quickly drop to the lower value, if that means anything.

Could it be that the current is very small, and the meter be 'sucking it up' and causing a false low reading? I am not sure what the input impedance is on this thing actually.

This sounds like stray voltages caused by capacitive coupling - the amount of leakage current is tiny, but enough to build up a charge to high voltage and cause a shock if there's no ground.