Safeguards to preventing damage to a scope?

[george graves]

I've asked someone this question once before - and it must be my thick skull keeping me form understanding it....

But form what I gather, you can damage an oscilloscope by current through the ground connector on the probe?  Is that right?  Could someone explain what kind of set up would cause a situation like that that would cause damage to the scope?

As I said, someone briefly described it - but I just didn't quite get it.  So....a quick drawing of the set up would be really appreciated!

[free_electron]

the ground terminal ( outer shell of the BNC connector ) is connected to the circuit board. that same ground is also connected to the ground pin of the scope ( chassis earth )

make a fat short with the ground lead of the probe and current will flow to the earthing . blowing the ground fault interrupter , but possibly frying the thin connection point as well.... circuit board damaged = dead scope.

the same goes for shorts between ground wires. one scope ground is at 12 volts , the other ground lead connected to 0 volts of the same source.

now you again send current across the board ... if that current is large enough it may damage that connection. damaged circuit board = dead scope...

simple.

[george graves]

Thanks! 

So if you connect the ground to anything that has current to give(that is above or below ground potential) - you could fry it?  How does floating supplys factor in?

The reason I'm asking is I want to measure some signals (RPM signal from the ECU specifically) on my car - to see what voltage and frequency it is.

[Psi]

How does floating supplys factor in?

If the supply is truly floating then connecting the ground clip to either side just moves that side to ground potential.
No current is needed to do this (ok, maybe fractions of pico amps) anyway, its fine.

The problems occur when you connect the ground clip to something you thought was floating or already at earth and it turns out not to be. One common way this happens is when you have a floating circuit but you have connect it to some other device. For example an audio cable from your device to an amp/PC, the audio shield is grounded through the amp so now your circuit isn't floating any more.

If it's a low voltage thing you're measuring, and you short that to ground, you may only see a small spark when touching the clip to the circuit. You should always watch for this spark when connecting the ground clip.

If you try and connect the scope ground clip to something at full mains potential you'll get a large bang/spark and it will probably blow a fuse or circuit breaker somewhere. If you're lucky the scope will still be ok.

The reason I'm asking is I want to measure some signals (RPM signal from the ECU specifically) on my car - to see what voltage and frequency it is.

How are you planing to run the scope in the car? an extension cord inside or an inverter off the car battery.

The car is floating so if you run the scope off an extension cord to a mains socket in the house everything is fine.
You could then safely connect the scope ground clip to any part of the car since it's all floating.

However, if you run the scope of an inverter that's powered by the car battery things maybe different. It will depend whats inside the inverter. I can't remember how they do earth if at all. (it may/maynot be fine as well)

[george graves]

Well I have an old APC Uninterpretable power supply thing with a dead battery(12 volts) - I was thinking of hooking that up to the battery for power.  Not sure if the noisy voltage oif the car would fry it - so I might pick up an inverter(not sure if I trust them either)

Here's another question:  Why don't scope makers just make the scopes themselves floating - and not tie the ground to anything?  Like a multimeter would be?

[Psi]

Here's another question:  Why don't scope makers just make the scopes themselves floating - and not tie the ground to anything?  Like a multimeter would be?

Some scopes are, like portable battery powered scopes.
But when the scope runs off the mains it really should be earthed for safety.
I seem to remember a scope company wanting to make a mains powered isolated scope but their lawyers wouldn't let them.
It's the sort of thing that the companies are afraid to do in case someone dies and they get sued.

[free_electron]

simply put an isolation transformer in the power line to the scope.
but beware !!! NEVER EVER EVER connect the probe ground to a live wire ! even with an iso transformer. you could kill yourself ! the scope may be floating but if you touch that terminal you become the 'bridge' across the iso transformer .. and then the current runs through you.. and 20mA across the hart is enough to make it lock up , cramp your fingers so you cannot release the wire and you die because your bloodflow stopped..

That's why we have things like differential probes and isolated probes. Those solve the problem on the probe side.

[alm]

Here's another question:  Why don't scope makers just make the scopes themselves floating - and not tie the ground to anything?  Like a multimeter would be?

Some are, the Tek TPS2000 series is the only one in a desktop form factor as far as I know. Isolating multiple high-bandwidth scope channels is a lot harder than isolating a DMM that in many cases only does ~50 S/s, as opposed to 1 GS/s for 2-4 channels in an entry-level DSO. Both isolating a DC-100 MHz signal and isolating the ADC/memory from the FPGA is hard. I believe most designs take the former route.

simply put an isolation transformer in the power line to the scope.

I hope you're not suggesting to interrupt the ground connection. This is strongly recommended against by scope manufacturers, and is a firable/fryable offense (you may either get fired or fried) in some workplaces. Even if you consider floating a scope safe under some very limited circumstances, than it's still a bad idea to recommend it to someone that does not have a firm grasp on electrical safety.

but beware !!! NEVER EVER EVER connect the probe ground to a live wire ! even with an iso transformer. you could kill yourself !

That's all fine, except that said circuit may not be expected to be live, either due to operator error or due to a fault. I'd rather have my scope emitting smoke when I make a mistake than my head. A scope with ground connection is a lot more foolproof than one without: it may show that your power supply is actually not short-circuit proof and kill the regulator, but it is less likely to kill the operator.

[vk6zgo]

BWD in Australia  made a special Oscilloscope for use in these situations.(they are in fact mains rated)
You still see them on EBay at times for a much higher price than you would pay for any other BWD or anything in fact with its otherwise
modest specs.

In the car stuation,it is easiest to run your 'scope from a mains extension cord.
DO NOT damage the power cable with the car door or similar,as you do not want the car to be "HOT" at mains voltage!
Yes,I know it is clumsy & inconvenient,but it is floating with repect to the car voltages,if it is really that important to you (but see my next point).

In a car,the best procedure is to connect your probe earth clip  to a car "earth" point (the car chassis/body & anything electrically bonded to it is  by convention.regarded as "earth" although the whole car is insulated from the actual Earth).
Any data regarding ,waveforms,dc voltages,etc on the Internet or elsewhere will follow this convention in any case.

[george graves]

Excellent - thanks all.

Any thoughts on using an off-the-shelf consumer inverter?

[EEVblog]

*scurries to the whiteboard*...

Dave.

[sonicj]

Excellent - thanks all.

Any thoughts on using an off-the-shelf consumer inverter?

I've done it. The output of the inverter was completely square & my Rigol DS1052 didn't seem to mind.
-sj

[grumpydoc]

> simply put an isolation transformer in the power line to the scope.

No, please don't - you'll risk making the entire chassis of the scope live. I know that you then go on to point out that connecting the 'scope ground to the live supply is a bad idea but it's too easy to connect to a point that you thought was OK but isn't.

It might not be you that touches the scope chassis.

When using an isolation transformer it feeds the device under test, NOT the 'scope. The 'scope can then stay grounded and not be a nasty surprise for anyone.

[EEVblog]

I just shot a video on this, editing now...

Dave.

[mikeselectricstuff]

The correct method is to isolate the thing you are testing and then ground the scope to the  signal ground.
e.g. if looking at the mains side of a SMPSU you run the PSU in an isolation transformer and ground the -ve HVDC rail.
For safety it's good practice to  make a seperate connection from the unit ground to mains earth, so you're not relying on the scope ground lead.

[free_electron]

There is nothing wrong with using an iso transformer PROVIDED YOU KNOW WHAT YOU ARE DOING.

There are even fully isolated bnc cables and probes for such circumstances.  There simply are situations where you can't go without. Take an H bridge in a motor drive: 400 volts dc( im talking frequency drive ).
Your task is to visualize the gate drive voltage of the top transistor.... You cant do it because the scope does not have enough dynamic range to show a 2 volt signal superimposed on a 400 volt offset.
The solution ? There are two : use a high voltage differential probe or float the scope...

Tektronix makes a scope that has separated and isolated inputs. 4 channels. The input bnc is not connected to chassis ground. Each input channel has its own psu digitzer and memory and communicates over an optolink with the main processor.

[EEVblog]

I just shot a video on this, editing now...

As usual, was going to be 5 minutes, but ended up being 25min!

Dave.

[george graves]

You rock Dave! 

[T4P]

I just shot a video on this, editing now...

As usual, was going to be 5 minutes, but ended up being 25min!

Dave.

Informative videos are good .... 

[saturation]

Most automobile waveforms, daresay all, are < 100kHz, and easily viewed with cheapo scopes like the Velleman class.  Rather that struggle with your bench scope, extension cords etc., buy a true floating DSO that is capable of reading auto waveforms, the Vellemans' will run you between $100-200 US, tops.

Thanks! 

So if you connect the ground to anything that has current to give(that is above or below ground potential) - you could fry it?  How does floating supplys factor in?

The reason I'm asking is I want to measure some signals (RPM signal from the ECU specifically) on my car - to see what voltage and frequency it is.

[EEVblog]

Dave.

[george graves]

OK - totally makes sense now!  Now I see says the blind man.

So you really think that the probe ground lead would blow before a PCB trace on the scope does?

[sonicj]

So you really think that the probe ground lead would blow before a PCB trace on the scope does?

The BNC has a low resistance connection to the metal chassis. The metal chassis is the connection to ground. The clip lead will eventually pop like a fuse, but likely not before you **** your pants.

We should ask photon to test this for us. 
-sj

[george graves]

Got it - thanks!

[free_electron]

USB isolators are not that rare. analog devices has a single chip one . little 14 pin thing. I used one a couple of weeks ago on a design i'm working on....
Maybe an idea for a product to add to the eevblog store. I can send you the layout and schematic for it.