DIY hw probe

[Boschi]

before starting, im trying to measure the output waveform of an inverter. we are near the distribution transformer of our electricity provider, so our ground resistance to earth its just few ohms (ergo im not connecting the ground clip).

i have already done some measurement but with a normal 10x probe, and after a while the trimmer cap happily released the blue smoke but im threathing everything as directly connected to the mains and im also following the rule of the "pocket", so i was away and nothing dangerous happened.

now i want to do some more measurements, but a bit safer, and because its a one-off and i need it quickly i wonder how safe is to make a hw probe by myself.

even better, because its just 50Hz how bad would it be to add 9 10M resistor in series with a 10X probe? (so making a 100x probe whitout compensation cap)

i dont need absolute precision, i just want to see what appens at the waveform while i switch off the grid and the inverter kicks in.

thanks for any advice 

[Ian.M]

If I had to hack that without a Cat II rated isolated differential probe, I'd put a chain of ten 10K 1/4W metal film  resistors between Live and Neutral, then take two X10 probes, remove the ground clips and put them across the SECOND resistor from Neutral in the chain (so there is a resistor between the probe and neutral to act as a fuse if anything goes wrong) and use waveform maths to subtract the two signals.

YMMV and *PLEASE* try not to kill yourself.

[Boschi]

thanks for the reply.

i dont need to use two probe to create a poorman differential probe here. im near the power distribution transformer, so the ground pin on the scope is just few volt away from neutral. what i want to do is to put nine 10M resistor in series between the hot of the line and the positive of the probe. its safe enough for a one-off measurement, and also im too young to kill myself now 

anyway, im going to set up everything and then switch on everything from few meter away, so i think im not going to kill myself.

my main concern is the fidelty of the sinevawe, or at least of the fuzzy thing that i see when i switch from grid to inverter.

because i have tons of 10M resistor but i dont have any HW 10-15pf cap 

[Ian.M]

my main concern is the fidelty of the sinevawe, or at least of the fuzzy thing that i see when i switch from grid to inverter.

because i have tons of 10M resistor but i dont have any HW 10-15pf cap 

That's why I suggested a *MUCH* lower impedance divider chain.   A chain of 100K resistors would also work or in fact any value between 10K and 100K.   The loading of a typical x10 probe (10Meg||15pF) on such a divider chain is negligible.  If you are happy to ignore the ground to neutral voltage, probe with a single ungrounded probe ONE resistor from the Neutral end of the divider chain. 

However the fact your probe's trimcap let the smoke out is worrying, as it implies there may be HF high voltages present, probably from the inverter chopping its output to PWM an approximation to a sinewave.  If you want to avoid further smoke incidents its *ESSENTIAL* that you verify there are no high amplitude fast edges on the Neutral line so start with the divider chain between Neutral and Ground, probing one resistor from the Ground end.

[Boschi]

i dont think that the blown capacitor was fault of some HF. i attached a screen of the sinewave (before the probe blew...), and i think that even at 50Ms/s i should probably be able to see any HF / HV ripple.
also, the sucker has a big bulky transformer inside him, so even that should block the HF component.

i dont remember the probe that i used, but probably was too close to his voltage rating 

but let me understand, i cant just increase the impedance of the probe because whitout proper trimming cap the vawe would be horribly distorted right?

sorry but i never studied transmission line theory 

[Ian.M]

Its not transmission line theory as at well under 10% of the bandwidth of the x10 probe you can treat it as if its a simple 10Meg || 15pF load.    However that 15pF is problematic - Your divider using the 10Meg of the probe itself + 90Meg of external resistors has an equivalent resistance at the probe tip of 9Meg.  That gives an uncompensated time constant of 1.35ms which gives you a bandwidth of under 1.2KHz.  In practice there will be a lot of stray capacitance so that's no better than an order of magnitude estimate, but its fairly certain it will smear any sharp switching edges

To fix it, its essential to reduce the divider impedance by a couple of orders of magnitude to push the -3dB bandwidth up high enough.  The limit in that direction is the power dissipation in the divider chain and 10x 10K resistors keeps it under 25% of the rating of 1/4W resistors.

[Boschi]

...
To fix it, its essential to reduce the divider impedance by a couple of orders of magnitude to push the -3dB bandwidth up high enough.  The limit in that direction is the power dissipation in the divider chain and 10x 10K resistors keeps it under 25% of the rating of 1/4W resistors.

but whith just 100K in series with the probe im still too near the max probe voltage 

i'm resigned, ill order some HV caps.
its something like the drawing attached acceptable?
the cap are 3kv rated, and also im buying new resistor 350v rated.
should it work good enough?

[Ian.M]

You don't put the 100K in series with the probe - you put it as a divider across the supply so you only get 10% of the peak supply voltage at the probe tip.  Even if you had a 415V supply, 10% of its peak is under 60V.  If you don't understand this you've got absolutely no business probing mains circuits so stop before you DIE!    The only major hazard is the risk of loss of Neutral between the point you are probing and the supply which could cause the low end of the divider to go up to full phase voltage.

Don't expect your improvised x50 probe circuit to be properly compensated . . .

[Boschi]

You don't put the 100K in series with the probe - you put it as a divider across the supply so you only get 10% of the peak supply voltage at the probe tip.  Even if you had a 415V supply, 10% of its peak is under 60V.

this make sense. anyway, i know how a resistor divider works

The only major hazard is the risk of loss of Neutral between the point you are probing and the supply which could cause the low end of the divider to go up to full phase voltage.

this is going to happen. as soon as i switch off the main the inverter will start to output two sinewave from its line and neutral, one out of phase of 180° from the other, in order to create the full 230Vrms voltage, both centered around ground. thanks to this i should be able to use ground as my "virtual" ground, so connecting one end of the divider to hot and the other one to ground, right?

[Ian.M]

What makes you think the inverter will drive Neutral?  To comply with the electrical wiring and supply regulations in most 1st world countries, as its the source of the supply it provides, its Neutral output should be grounded (but it must not ground the incoming supply neutral).

I suppose if its US market 115V-0-115V kit that's been grey-imported into the EU, it could be as you state, but it shouldn't have been placed into service configured to provide a split phase supply.

You are rapidly getting out of the 'safety zone' for DIY HV probes, and IMHO should be looking at a CAT III or better isolated differential probe if you want to investigate the switchover waveform.

[StillTrying]

i dont need absolute precision, i just want to see what appens at the waveform while i switch off the grid and the inverter kicks in.

In that case can't you view the inverter's output mains waveform through a low voltage isolated transformer.
I have a small 12Vac 20W halogen desk lamp with very usefully 2 metal bars that take the 12Vac up to the bulb and I've checked that the isolated 12Vac waveform is near enough exactly the same shape as the 230Vac mains.

[Cliff Matthews]

The thread's interesting Ian.. I like the 12v transfo idea, but a 2nd probe and the math function sounds much better.

*edit "..you've got absolutely no business probing mains circuits so stop before you DIE!" - Is it just me, or do others wonder what happens during an ops silence?  hopefully no more blue smoke. (or flames, etc..)

[Boschi]

im still alive here 

maybe i can avoid to do the measurements, im not sure yet.

anyway, the inverter is "legal", i checked the setup, and i was disconnecting both line and neutrum while switching off the line, this mean that the inverter was floating, hence the neutrum driving.

[Ian.M]

Yuck.   The inverter *SHOULD* have a grounded Neutral output unless it falls under some special regulations.  However as it does not, I'd go with two x10 scope probes (in Y1-Y2 mode) and divider chains of 10x 100K 1/4W metal film resistors between L and E and N and E respectively for an overall ratio of x100.   The 10Meg||15pF scope probe loading will only be a 1% amplitude error.

The worst case earth leakage current will be under 1/4mA so it wont trip any RCDs or ELCBs.  With 1Meg between the Live and the common Earth point, it isn't a significant hazard to personnel even if the Earth gets disconnected, but as that would risk blowing more scope probes, you should make the Earth connection with screw terminals, not just clip the probes ground clips on that point.

[TheDane]

before starting, im trying to measure the output waveform of an inverter. we are near the distribution transformer of our electricity provider, so our ground resistance to earth its just few ohms (ergo im not connecting the ground clip).

(cut....)

i dont need absolute precision, i just want to see what appens at the waveform while i switch off the grid and the inverter kicks in.

thanks for any advice 

I wonder why you're disconnecting ground??

Just because you're close to the grounding point, and the wiring is short, does not (always) allow you to cut traces and/or corners.
Electrical resistance is still a governing factor, and the few ohms in a mains/measuring wire is going to 'develop an induced voltage across it' when you draw current through it.


I suggest when the inverter kicks in that the broken mains current flow amounts to a very high voltage, and your measuring device is picking up coupled energy (Think EMP - ElectroMagneticPulse - it is basically a one-shot switch mode transformer, without a controlled energy draw/load you're trying to measure. And just as ESD destroyes electronic, so does EMP's) <-- a nuke emoticon

Most  transformers have capacitive/resistive couplings between primary/secondary - to lower emission and make them safer.

Electricity isn't fun to touch... those ions and electrons 
- take great care


 
Read more about wire resistance at: http://www.epanorama.net/documents/wiring/wire_resistance.html
A straight wire is also an inductor, read more about voltage, changing currents and time: https://www.allaboutcircuits.com/textbook/direct-current/chpt-15/inductors-and-calculus/

[Cliff Matthews]

He's likely now down to one probe, FWIW, it'd be nice to know what's wrong with StillTrying's 12v transformer idea..

[Ian.M]

Good point.  Trying to measure this single ended, with the probability of an unknown amplitude spike at the moment of switching and unknown amplitude ground bounce between the inverter and the scope, and without a probe with an adequate CAT rating, when the O.P's already admitted that the inverter Neutral may be floating is *STUPID*!    If he's down to one probe its time to *STOP* and get probes that can do the job. 

A NE-2 Neon bulb across the 100K resistor nearest to Earth in each divider chain would be a sensible addition to my proposed 2x x10 probes + 2 divider chains solution.  It will strike at under 100V and clamp the input to the x10 probe if any HV spikes occur, but should otherwise have negligible effect on the observed waveform.

[Boschi]

Keep calm 

i have more than 2 probe left, actually 5.

anyway, the floating neutrum was my fault.

i dont need to take the measurements anymore, but i still would like to know how to safely measure mains voltage, everyone sometimes need to.

so a 1/10 divider referenced to earth with a gas discharge lamp across the low side resistor would be adequate to protect user AND the equipment?

anything that could be done better?

[Ian.M]

i dont need to take the measurements anymore, but i still would like to know how to safely measure mains voltage, everyone sometimes need to.

so a 1/10 divider referenced to earth with a gas discharge lamp across the low side resistor would be adequate to protect user AND the equipment?

That depends on your definition of adequate.  Building your own divider chains is a safety compromise unless a qualified engineer with experience of mains measurement instrument design has signed off on your divider's design and construction.   What may be acceptable risk for your own personal use may *NOT* be acceptable if its for your employee, customer or other persons that you have a duty of care towards.

Note that I increased my recommended total divider resistance to 1Meg because its going from Line to Earth.  That was to get the earth leakage current well under the 0.75mA regarded as safe for EARTHED portable appliances, and down to the 0.25mA permitted for class II (unearthed) appliances.

anything that could be done better?

To safely measure mains voltages you use an appropriately rated multimeter, preferably CAT III or better with fused test leads.   If you need  to see the waveform you should use either a battery powered  scopemeter with an appropriate CAT rating for it and its probes or a bench oscilloscope + an isolated differential probe with appropriate ratings.

[TheDane]

Keep calm 

anything that could be done better?

Using an isolation transformer, isolating 'real live' mains, could technically/legally allow you to use 100:1 probes, CAT I, to even measure phase-phase mains safely on your scope. 10:1 probes tend to have a shorter neck, posing a higher risk of 'issues'.

Having 'real power' probes, and 'toy' probes also serves as a great reminder, and keeps you safe (hopefully!!!) - though quality probes aren't cheap. Hameg HZ-53 is a nice and somewhat cheap probe, one I like to use - I like my scope too much to blow it up, so never 50 impedance settings for me.

3 phase isolation transformers at higher power levels are costly though - but don't go down on lack of equipment - that's better, for a word 

What are Measurement Categories (CAT I, CAT II, etc...)? - Please consider any de-ratings/safety margins that might apply to your work place/setup.
http://www.ni.com/white-paper/5019/en/