measuring voltages up to 400 volts safely with scope

[davy peleman]

i have a portable scope: velleman hps50. the manual says it's cat.2 (up to 600v) but then a little later it says the maximum voltage is 100 Vp. now i'm confused. i need to measure ripple on the output of an isolation transformer that's in a tube signal generator. how can i do that??? wich probes and attentuator do i need??? i'm new and this is confusing me. i'm very concerned about safety. so can somebody please explain how to proceed.
thanks

[Vtile]

Please ask this in the beginners section. Metrology section is more of an questioning ones sanity when the measuring device is affected when neighbor arrives home with his horse.

https://www.eevblog.com/forum/beginners/

edit. Or in https://www.eevblog.com/forum/testgear/

[mikeselectricstuff]

Buy a x100 probe with a suitable voltage rating.
Cat ratings are mostly aboiut not killing you when exceeded, not functioning correctly

[Simon]

This is why most scopes use a 10x probe, it divides the input signal by 10 making it much safer for the scope. The cat rating is a bit different, it's like saying it will withstand 600V for "x" time and insulation wise will withstand 600V but is not designed to work with 600V.

You need a 10x probe that is rated for the voltage you want to measure. 10x probes also work with higher frequencies for reasons I can't even remember but studied not long ago .

[davy peleman]

ok thanks

[Vtile]

Please do note that while your Velleman scope might be separated and the source might be separated from the ground potential, with non-insulated BNC socket the metal parts can be in any contact potential !!

[Simon]

yes indeed you can now connect the ground of your scope to any high-voltage without benefiting from the usual "everything that is exposed metal is ground" proceed with caution. It might be a good idea if possible to ground whatever you are connecting the ground clip to assuming something else in the circuit has not already been grounded thus creating a short.

My favourite method of measuring high voltages is to use two probes and the maths function on the scope to create a 3rd channel which is effectively a differential channel. This means that all of the metal is still grounded (using a mains grounded scope) and there is minimal chance of chassis voltages that are dangerous. I would do this for example to look at the mains rather than connect my ground clip directly to something that could be live.

[davy peleman]

i think i'm gonna wait because it's all too confusing to me. if the scope is isolated and so is the power supply how can i hurt myself??? i'm restoring a tube signal generator and i need to know if the plates of the tubes are getting clean dc.  i'm gonna do measurements after the rectifier now, so i will know which side is ground and wich side is V+. that will be much safer and it will also give me an indication of the ripple, right??? i just need to know  if the tubes are getting clean dc on the plate and grid. the voltages are around 200 volts.

[Simon]

Well if they are truly isolated both on the device under test and test instrument you should be okay. Providing there is absolutely no chance that any part of the circuit can become ground. I'm not an expert on high-voltage so I don't know if perhaps it is a good idea to wilfully ground but negative side just so that the thing is not floating.

You can see the ripple on an oscilloscope, if you are using a multimeter it won't help you see the ripple.

If there is a rectifier I assume there is a transformer? In this case it is mains powered is it not? Therefore the metal chassis should be grounded by the mains connection that is all we are really asking for. Again if you are talking two hundred volts perhaps it is a good idea to use the differential probe method I described. Failing that if you're really careful you should be okay but is always difficult to offer advice about high voltages as nobody wants to be responsible for an injury and we don't know how competent you are. No offence there but high-voltage really does not care how innocent you are and how much slack you should be given, it kills indiscriminately.

[davy peleman]

maybe this project should be delayed untill i have better knowledge, but i'm so close. it's working and i just need to check if the problem is power supply ripple or a bad tube. and since i do not have a tube tester... i'm gonna order new tubes anyway so maybe i should just try replacing the tubes and see what result it gives me. i know that there is a lot of responsability involved giving that kind of advice. i know it's very dangerous and i'm a father so i don't wanna die. the chassis is not groundend. de generator  has a 2 pin socket it is an old tube device. i'm powering the generator with a variac wich has a light bulb in series that goes to  an isolation transformer (also without ground connection). the isolation transformer is a toroidal transformer that came out of a medical device it has no ground connection whatsoever.

[Simon]

While I don't really work on high voltage stuff which is why I'm hesitant. Given all the isolation have in-line it might be a good idea to ground what ever you deem to be the negative side of the circuit to earth and only use the oscilloscopes earth clips on that rail. That way you know that any oscilloscope chasse or any other metal object is at earth potential.

It would of course be a good idea if somebody else can back me up on that or give contrary advice. At the end of the day it doesn't matter if you are really careful that I don't like working like that myself.

[davy peleman]

how did guys like Tesla never killed themselves with all that high voltage stuff. i know he was a genious and that he really knew what he was doing but really. i already had the pleasure twice to be bitten by mains. once the washingmachine was leaking water and i was so stupid to put my hand in that water while kneeling down. thought the breaker was off by then, but i was not, so i was bitten. must have been lucky twice because it did't hurt all that much i was just a bit shaken. also i can not touch any pinballmachine without getting schocked. my body resistance must be very low. so i'm unable to play any pinball machine. all my friend find it funny they play and i watch

anyway thanks alot. your are absolutely right. i'm not doing anything untill i'm really confident. will just replace the tubes.

[Simon]

Tesla worked with stupidly high voltages but much much much smaller than safe current. It is in fact the amps that kills you and while it requires enough volts to drive those amps if you end out in a situation where you have not enough amps to kill you even though there is a high voltage present you won't die. Also higher frequencies take to travel across your skin and not go through you and therefore not disrupt your heart giving you a heart attack which is essentially how voltage and current kill you.

It's usually about your ground reference. The classical question is, if you are sat in a bathtub full of water and somebody throws a live toaster into it when you be electrocuted? The answer is most likely not although of course there are always exceptional circumstances where you could be killed and this is why we are the most dubious when giving advice and answers about working on high-voltage.

[davy peleman]

so it is a myth that a live device thrown in water makes the water live??? i surely was bitten that day with the washingmachine. oh man it's so confusing. if i could i would go to evening school to really learn how it all works. but as hardworking father during the week i have no time to go to evening classes. my father in law had a tv repair shop but also he's hesitant to give advice. he always says i should find another hobby that is less lethal. i his time he also got bitten by crt flyback transformers, and loaded caps. he always says that it could hurt for days. he has the knowledge but don't wanna share because he's fearing the worst.

[Simon]

It depends, it's not about making the water live. Yes the water is live, but live with reference to what? It's like birds that sit on powerlines, they don't get electrocuted do they? Now if they were to put to one leg on one line and one leg on the other they will get fried, fortunately for them the lines are always too far apart. It's always about your point of reference which is why people are so adamant that all exposed chassis should be grounded. When you can you choose the ground reference in order to make the situation as safe as possible.

[davy peleman]

the bathtub is enameled so it's isolated is that the reason why??? is it like standing on a plastic chair and touching live and you don't get electrocuted???

[Simon]

Well, the bath tub could be completely earthed if it's an old one. So you would already be grounded and the electricity would just flow to ground. The other thing is that unless your sticking your fingers into the live toaster it is unlikely any current will flow through you as it is more interested in flowing only through the water that is between the low and neutral parts of the element in the toaster. The rest of the water in the bathtub is hardly part of the circuit so to speak. Obviously if the bathtub is earthed then there could be a path to earth through the water. So there are various scenarios and it depends really on how close you are to the toaster.

[tautech]

OP. Get a 100:1 probe and know you and your scope will be safe.

Even though most 10:1 probes are rated to 600V don't push the boundaries when it comes to personal and equipment safety.

Learn and understand how probe voltage derating with frequency might affect you in what voltages are safe for a specific probe. Oscilloscopes cost money.....the probes and attachments to allow a multitude of measurements CAN cost more !

[davy peleman]

ok will do that.

[Vtile]

What Tautech says..

Now what I quickly did search your scope model it seems that it does have USB connection and maybe also external power supply options. If you attach that USB, the scope should be threaten as any grounded scope I'm also pretty sure that is the case with the external power. When it is battery operated in insulated work surface it should be floating as is said.

Why I said in the previous post that the source and Velleman scope might be separated from ground potential (ie. radiators, waterpipes, concrete floors, audio and other equipment, (protective)earth pin in wall sockets etc.) is those external options (USB & power) and if your "isolation transformer" isn't isolated due to fault or some mystery repair.

I think it should be good for you to read about floating supplies a bit and why they are said to be floating.

You need a multimeter you can trust with this kind of potential levels to check the potential (voltage) is where you think it is and in magnitude you think it should be, before attaching more delicate instrument to the circuit. The meter doesn't need to be brand new, but I would stay away from those few dollars new Chinese (or any other country of origin) meters.

Your hesitation is healthy.

[davy peleman]

i never connect the power adapter or usb connection when using the scope. i know usb has a ground. but then again i only have a laptop and i will not plug the scope into it when the adapter is plugged in. so i really try to keep everything floating. that i'm pretty sure off.

like i said i will educate myself some more before attempting to measure ripple on a 200 volt supply. and i will buy a differential probe with 100:1. thanks again everyone for keeping me from killing myself.

take care, davy

[Vtile]

Yep. You can skin the cat in variety of ways even at this case. Some are more dangerous methods than others. Differential probe is definedly a good option, just don't trust it blindly, there is still hazards lurking on your workbench with this kind of measurements. 

[davy peleman]

ok got it

[max_torque]

With the following assumptions:

1) your scope is NOT ground referenced. ie, it is fully floating, and you have nothing connected that could be a "bridge" to ground (ie a charging or data lead etc)

2) You ensure your own personal safety by the following precautions:
    a) some decent high voltage gloves
    b) a non conductive work bench  (kitchen work top or wood, no metal!)
    c) an appreciation that touching something might kill you

3) you only need to measure a relatively low frequency AC component (you say transformer, so i assume 50 or 60 hz fundamental)


Then you can make a simple, cheap, passive restive divider out of some suitable resistors (daisy chain lots of resistors together to get the overall impedance and ideally pot or seal those resistor chains to avoid arcing or touching. a 100:1 divider is what you're after.  The impedance value will depend upon the input impedance of your scope (see the datasheet for it) but use an external low side resistor, don't rely on just the scopes impedance (but make sure to include it in your resistive divider calcs.  You can "calibrate" the divider with a simple, safe  low voltage dc source, like a 9v battery before you use it on HV stuff

[daedalux]

What I usually do to measure line voltages is using some kind of battery powered oscilloscope and FIXED attenuation 100X probes that I put the ground insulation tip on. If the oscilloscope must be mains connected you should remove the grounding clip at the probe also and see if you can cope with that.
The reason to use FIXED 100x probe is to prevent a 40Vpp max oscilloscope blowing up when you inadvertely move the attenuation switch.

I also built a very simple differential probe out of some cherry picked divider resistors and centering potentiometers, unity gain op amp  and two 9V batteries to make +-9V rail. It works fine but it will always have less bandwidth that a unbalanced one and make a less convenient probe even in commercial versions. It will frequently do stuff you'll want the multimeter better for, at least in my case.