Phillips pm3212 scope grounding/earthing/tips?

[apbosh1]

Hi,
Why does the signal seem erratic sometimes until i touch the body of the scope?? i feel stupid asking this question. the mains lead is 2 pin so no contact with mains earth i've watched daves vids on mains earth reference and floating devices and it makes perfect sense to me on his white board, but is this anything to do with the interference i get. it displays really nice when i touch it and gets messy when not, it seems odd im on a chair with feet off the ground.
i have it plugged direct into the wall outlet and there is a lot i know of that interferes with it in my flat but i think this is a different issue.

i have just got this scope second hand (exciting stuff for me!) im a beginner with oscilloscopes really although i have done alot of reading/ytvid on them this is my first hands on, i like it. I'm using it and getting expected results and making good progress but i think im probably missing some basic not to do things.

Thanks for any help and dont hold back i have a good understanding just not much real life experience....yet.

[alm]

Are you sure this is the original plug / power cord? My guess is that someone cut off the plug / cord and put on a new one, because I find it hard to believe this scope was shipped with a two prong cord. The channels are not isolated from the metal case, and I don't think the unit is old enough to pre-date grounded sockets in labs. Check if their is a ground connection either in the power cord or inside the scope where the power cord enters the case.

[firehopper]

if its only a two pin plug, DONT USE IT till it is replaced with a proper 3 pin grounding plug. thats dangerous as it is now.

[sync]

Are you sure this is the original plug / power cord? My guess is that someone cut off the plug / cord and put on a new one, because I find it hard to believe this scope was shipped with a two prong cord.

Some of the PM32xx oscilloscopes are double-insulated and not earthed by the power cord.

[Tepe]

if its only a two pin plug, DONT USE IT till it is replaced with a proper 3 pin grounding plug. thats dangerous as it is now.

Dogmatism is so cute.

According to the manual for the PM 3212, its power supply is double insulated, safety class II, IEC 348.
Hence the two prong plug.

[alm]

I'm surprised. This would never make it on the market these days, let alone get UL listed. I'm surprised Philips were so loose with safety back then, I don't think the big US brands ever made a scope like this. Their floating scopes would feature extensive isolation to prevent the user from coming into contact with the instrument ground. The safety of this instrument is essentially dependent on one line in the manual: "Warning: The frame ground (and the probe's ground lead) must not be connected to live potentials". So this scope should be safe to use (the power supply is designed for this use, unlike other scopes), but I wouldn't use it near devices with exposed mains wiring/traces, for example power supplies.


But to actually answer the question at hand: how are you connecting to the signal? Are you using a probe, does it have a ground lead? Is the probe connected to a signal or is it loose? What is the signal source? It sounds to me like you might only connecting the probe tip, not the ground lead. It could also be a bad connection somewhere. Does the same happen when you connect the probe (or a piece of wire) to the probe calibration output on the scope? Your body will have a substantial capacitance to ground, even with your feet of the ground. At higher frequencies this can effectively ground the frame of the scope.

[apbosh1]

Thanks all for your response Yes its the original lead with a molded plug and it seems correct from other responses although i downloaded the manual and it doesn't say anything regarding this unless i missed it.
i am using a x10 probe with ground clip, it came with the scope. it will measure the calibration point accurate (as good as) without the ground clip connected anywhere. must be grounded from the bnc connector internally??
When the probe is connected fully to a device it seems to work well very clear and stable once levels adjusted but with noisy messy signals open probe or whatever, it has a big impact when i make contact with the case it stabilizes. perhaps just the nature of it all! and it does read well now im getting used to it.
However - what do i need to look out for with this unusual floating scope setup? if i can call it that. i've got plenty to do with low power stuff until i fully grasp the system and attempt any possible mains powered devices so no worries there, last thing i want is a belt off the mains.
Many thanks. Andy

[apbosh1]

Ok i have been doing some measurements and all is good measured 1.8khz from a 555 out on a random sensor board i found also the triggering cap and looks good to me (sorry if my terminology is wrong or amateurish i don't get to talk electronics with any experts unfortunately but hope you can tell what im saying) , expected wave forms nice and clean. but disconnect the probe and i get a messy signal. (other attempts at the same measurement the beam returned to a solid line!!)
it seems to be based around 50Hz sine 14mV pp (or from -peak to +peak did i get that right?). got to be the mains surely! uk 50 Hz. my annoying old fridge is running and its noisy to the ear let alone any disruption in the supply , spikes etc (plug is behind cant drag it out now)
anyway if i touch the scopes case the signal drops to just 4mV pp roughly so......
is there a way to say that in Db ie - 3db or is that just unnecessary.
i can see a modern digital scope coming sometime in the near future best i get some overtime in.
Thanks
Andy. 

[alm]

If you disconnect the probe, then it will essentially act as a huge antenna. The shield of the coax is connected to the scope ground, and in most scopes connected to earth. This scope is floating, but by touching it you are creating a relatively low impedance to ground for AC.

Note that you can always tie the scope to a solid earth connection, just because it's double isolated does not mean it has to be floating. I don't see much point in chasing the signal you get if the probe is disconnected: if you want a meaningful signal, connect the probe to something or even short it.

As for safety: I would not use this scope on mains circuits or circuits involving > 50 V without a solid ground connection. The problem with a floating scope, and why most companies never made one like this and strongly recommended against floating any of their scopes, is that you might clip the probe ground lead to mains without noticing. This works fine until you touch one of the many metal parts of the scope, then you suddenly get shocked.

The advantage is that within low voltage circuits, you can essentially clip the probe ground lead anywhere (*) within low voltage circuits without risking shorting anything. See Dave's how not to blow up your scope video for some information (though I don't think he discussed floating scopes). Note that as soon as you attached one ground lead, the scope is no longer floating relative to your DUT, so you should be careful only clipping the second ground lead to a node at the same potential as the first ground lead.

* The scope ground will still present a huge capacitive load to ground. Some circuits (eg. FET drivers) can get unhappy when you add a large cap to ground to their output. Worst case is a switching power supply where the switching MOSFET fails to turn off because of the capacitive load. Fireworks will ensue. This is quite rare, however.

[Tepe]

Thanks all for your response Yes its the original lead with a molded plug and it seems correct from other responses although i downloaded the manual and it doesn't say anything regarding this unless i missed it.

It's on page 11:

However - what do i need to look out for with this unusual floating scope setup? if i can call it that.

As Alm also quoted, the manual contains this:


I guess you could float the DUT with an isolation transformer.

[G7PSK]

How is this problem overcome with battery powered scopes, I know the are normally supplied with fully insulated bnc probes, but how do they get over the need for grounding to stop erroneous pick up or are they just deliberately made insensitive.

[AndrejaKo]

Well my battery powered scope (Owon SDS7102) has a ground screw at the back and the manual says to connect the screw to ground before using scope on battery.

Running it on battery while it's floating or running it from AC power doesn't affect the 50 Hz noise reception for me. It picks up 50 Hz noise as soon as the probe is disconnected and stops picking it up as soon as the probe is connected to a circuit.

 I'm also interested in finding out how this works out in more "serious" scopes. I know some AC powered scopes will pick up mains hum when probe is left open. I also don't see why power noise would be "erroneous pick up" since it is actually there.

[G7PSK]

When I say erroneous I mean unwanted or noise signals they may be there but you dont want them spoiling what you are looking at.

[apbosh1]

Thanks again everybody that definitely helps. Alm much appreciated great info just the ticket! and tepe i do remember looking at those sections of the manual no excuses really! but wasn't sure with it. sounded a bit medievil like case shall be connected, and and borne in mind  its probably the correct english but its been good to here from you all as well.
i dont really like the thought of accidently clipping to live and then the whole scope body is one big danger zone no high power stuff for me yet! still its good to learn on for now i can see an upgrade coming fast.
I was lucky when i bought this the retired engineer for a copier company included a mother load of components and cases instant lab deal of the year and a good guy.
Cheers.
Great forum i'll be back. maybe even help someone else sometime.

[Greenray]

Hi.
Yes I know that ths is an old topic, but we need to remember some thinghs.

The PM 3212 were been designed many years ago, when same safety rules was different to the rules of today.
So, the  ground chassis was not connected to the Hearth pin of the main supply cable, why?

The principal reason was to avoid the risk that occurs when we need to see on our scope a wave coming from the main supply. Do do this we need to connect the GND clips of the probe to a some reference line and the "warm" point to the "NEUTRAL" or to "LINE".
Doing this we can connect the GND to the HEART and in this case nothing happens and we will see the sine wave on our scope.
But if we reverts the connections, we will have the chassis at "POWER" line (and the probe to a safe line).
This is an hazardous risk, but when we do this kind of measurement need to think to what we are doing.
This risk still also today if we need to do these measurements. To avoid this risk, it is possible connect GND to HEART and two high impedance probes to the remaining two lines, using the differential mode for the Y channels.

The second thing that I want to say is that we live into a world "not perfect" where we know that the resistors are not only resistors, but also small inductors with a small part of capacitance. The same is for the capacitors that collect the property of the resistors and the inductors. Obviously, also the inductors are showing their small but real characteristics of the other kind of componenes.
In the same way also the Transformer into the Power Supply of our PM 3212 is not perfect and as a small capacitor+ resistor, transfer on his chassis Ground a very small part of the POWER AC LINE (220/240 VAC).
Having no connection between Chassis GND and HEART safety line, we can feel the effect of this.

Yes, one solution can be to change the 2 wires cord with a cord with 2 wire + HEART, but BE VERY CARE!!!!!
Don't use in this condition your equipment for measurements on the 220/240 AC Power line!

My opinion is the absence of the Heart wire in the power cord was not an error but a preponderate choice.
Now we can disagree this choice, but it was not a "non sense".

We can also consider other ways to connect Chassis GND to HEART cable, maybe using a fuse, a PTC, a Fuse Resistor or other. In these ways we will work with our chassis connected with a relatively LOW impedance to Heart, but in case of accident, the connection can be quickly unconnected. 
Sorry for my poor English.

[Vtile]

This is rather old topic, but I'm new on forums and the subject is something that is still relevant. 

If I'm understood correctly in electronic repair shop you power the device under the test to mains through isolating safety transformer to galvanically isolate the D.U.T from the main supply to allow "one mistake" by floating the DUT in that way. Then you take the non-floating instrument of current safety diciplines and measure the device in seemingly safely way.


Now in second scenario. You are repairing a papermill that have same dimensions as your own and neighbours home combined. You can not just unplug it and then put a isolating safety transformer between the Schuko or any other powerplug and the wall outlet. No, you float the measuring instrument or better you take instrument that is designed to float, like the one in OPs post. To make it even more quirk situation the papermill is propably also floating, but have isolation failure somewhere that you are hunting down. Not for hobbyist and only with pretermined rules between professionals. <- See, don't do it closure.

What I did want to say, that there is definedly a reasons to do seemingly odd safety solutions at times for specific purposes.

[karoru]

I'm surprised. This would never make it on the market these days, let alone get UL listed. I'm surprised Philips were so loose with safety back then, I don't think the big US brands ever made a scope like this. Their floating scopes would feature extensive isolation to prevent the user from coming into contact with the instrument ground. The safety of this instrument is essentially dependent on one line in the manual: "Warning: The frame ground (and the probe's ground lead) must not be connected to live potentials". So this scope should be safe to use (the power supply is designed for this use, unlike other scopes), but I wouldn't use it near devices with exposed mains wiring/traces, for example power supplies.

These scopes were made and advertised mainly for TV repair shops, where technicians powered TV under test via isolation transformer. You don't want to ground any point of transformerless CRT TV so it IS a safety feature as long as you know what are you doing.