Probing SMPS with oscilloscope

[bdunham7]

The problem is not the enamel isolation from a winding but leakage between the windings! Just look at a teardown of a differential probe. There is not 1 resistor but a whole bunch of them in series. IOW you need a catastrophic failure in multiple parts before there is a dangerous situation. Also when a differential probe starts to fail it won't work right so you are aware of problems immediately.

In any proper isolation transformer there isn't any 'leakage' (in the DC or resistive sense) between windings, just capacitive coupling on the order of 1nF or less.  Nowadays they typically are on separate plastic bobbins. They are tested with thousands of volts for DC leakage.  I don't think there's much of a case to argue that properly designed diff probes, opto-isolators or isolation xformers are unsafe if properly designed, tested and used.  I didn't raise the issue of the diff probe impedance as a safety issue, it's a circuit loading issue IMO.  2M to ground may be too much in some cases, although the nice low ~2pf typical capacitance may be optimal in others.

[PKTKS]

In any proper isolation transformer there isn't any 'leakage' (in the DC or resistive sense) between windings, just capacitive coupling on the order of 1nF or less.  Nowadays they typically are on separate plastic bobbins.
(..)

Exactly. There is no such thing as leakage 'current' on
healthy transformers - you do have inherent loss but
this is not "leakage" in the sense we are talking.

The more recent higher voltage are indeed separated
plastic bobbins - some of higher frequency built with
proper high freq. wiring.

For the record. I never said HV differential probes are
unsafe or unrated. Thing is: their size by using exclusive
SMD parts are indeed small (compared to old school long ones)
and they do not allow for proper arc safety when the
layout of the ladder is cracked by a strong surge.

I like them very much. Very handy. Although I would never
consider them isolated and rather prefer to use battery
operated scopes in critical cases.

For half the price of a differential here locally a 1000x old school probe
https://www.aliexpress.com/item/4001233837188.html

Paul

[bdunham7]

For the record. I never said HV differential probes are
unsafe or unrated. Thing is: their size by using exclusive
SMD parts are indeed small (compared to old school long ones)
and they do not allow for proper arc safety when the
layout of the ladder is cracked by a strong surge.

I like them very much. Very handy. Although I would never
consider them isolated and rather prefer to use battery
operated scopes in critical cases.

For half the price of a differential here locally a 1000x old school probe
https://www.aliexpress.com/item/4001233837188.html

Paul

The MicSig appears to be SMD.  You can look at the Sapphire versions (details at about 7:00 in video) and see what you think of the 'HV ladder'.  Both are purportedly CAT rated at some level.





The HV probe you mentioned, besides probably not working properly on anything but DC due to compensation issues, doesn't work for non-ground referenced measurements.  One side is grounded.  And if you use a floating scope, you need to be prepared for it to float up to whatever you are measuring--and that system is not designed for that since the ground side is not going to be well insulated, if at all. 

[tggzzz]

For half the price of a differential here locally a 1000x old school probe
https://www.aliexpress.com/item/4001233837188.html

I wouldn't touch that except with a (fibreglass) bargepole.

The lack of a specification might lead you to think it is 20kV at 10MHz. Even decent Tek probes (e.g. P6015) don't get anywhere near that - and state the frequency derating.

[tautech]

For half the price of a differential here locally a 1000x old school probe
https://www.aliexpress.com/item/4001233837188.html

I wouldn't touch that except with a (fibreglass) bargepole.

The lack of a specification might lead you to think it is 20kV at 10MHz. Even decent Tek probes (e.g. P6015) don't get anywhere near that - and state the frequency derating.

It's a DMM HV probe and that's all !
AC rating is there but garbage for anything serious:
AC : 1 ~ 28KV ( 50 / 60 Hz )

[tggzzz]

For half the price of a differential here locally a 1000x old school probe
https://www.aliexpress.com/item/4001233837188.html

I wouldn't touch that except with a (fibreglass) bargepole.

The lack of a specification might lead you to think it is 20kV at 10MHz. Even decent Tek probes (e.g. P6015) don't get anywhere near that - and state the frequency derating.

It's a DMM HV probe and that's all !
AC rating is there but garbage for anything serious:
AC : 1 ~ 28KV ( 50 / 60 Hz )

Well, that's exactly what it looks like! But it was posted in relation to scope probes and mentions "Bandwidth 10MH   z".

As I said, get out your bargepole

[tautech]

For half the price of a differential here locally a 1000x old school probe
https://www.aliexpress.com/item/4001233837188.html

I wouldn't touch that except with a (fibreglass) bargepole.

The lack of a specification might lead you to think it is 20kV at 10MHz. Even decent Tek probes (e.g. P6015) don't get anywhere near that - and state the frequency derating.

It's a DMM HV probe and that's all !
AC rating is there but garbage for anything serious:
AC : 1 ~ 28KV ( 50 / 60 Hz )

Well, that's exactly what it looks like! But it was posted in relation to scope probes and mentions "Bandwidth 10MH   z".

As I said, get out your bargepole

Manufacturers webpage clearly lists it's only a 50/60 Hz DMM probe:
http://www.pintek.com.tw/product_detail/landersound/index.php?Product_SN=19259&Company_SN=6002&Product_Site_Classify_SN=17071

Some of their other products are the the real thing for scopes and quite like their compact HVP-08:
http://www.pintek.com.tw/product_detail/landersound/index.php?Product_SN=127265&Company_SN=6002&Product_Site_Classify_SN=17071

[tggzzz]

For half the price of a differential here locally a 1000x old school probe
https://www.aliexpress.com/item/4001233837188.html

I wouldn't touch that except with a (fibreglass) bargepole.

The lack of a specification might lead you to think it is 20kV at 10MHz. Even decent Tek probes (e.g. P6015) don't get anywhere near that - and state the frequency derating.

It's a DMM HV probe and that's all !
AC rating is there but garbage for anything serious:
AC : 1 ~ 28KV ( 50 / 60 Hz )

Well, that's exactly what it looks like! But it was posted in relation to scope probes and mentions "Bandwidth 10MH   z".

As I said, get out your bargepole

Manufacturers webpage clearly lists it's only a 50/60 Hz DMM probe:
http://www.pintek.com.tw/product_detail/landersound/index.php?Product_SN=19259&Company_SN=6002&Product_Site_Classify_SN=17071

I have Testec probe similar to that. Remarkably it has an MoD test sticker indicating the next service/cal date was as late as last September

Some of their other products are the the real thing for scopes and quite like their compact HVP-08:
http://www.pintek.com.tw/product_detail/landersound/index.php?Product_SN=127265&Company_SN=6002&Product_Site_Classify_SN=17071

The spec says  3pF, 40MHz, <6kVrms => 4Arms through the probe. Should be entertaining - from a suitable distance.

I'll stick with my well-specified Tek P6015 (or even my P6013).

[bdunham7]

Well, that's exactly what it looks like! But it was posted in relation to scope probes and mentions "Bandwidth 10MH   z".
As I said, get out your bargepole

Assuming you could find a low-impedance source for 28kV @ 10MHz, your bargepole might not keep you far enough away. 

[tggzzz]

Assuming you could find a low-impedance source for 28kV @ 10MHz, your bargepole might not keep you far enough away.

It's not about toggle rate, but rather edge rate.

10MHz has a rise time of 35ns, and it's common for pulse applications to go that far.

Pulsed accelerator, EM propulsion, nuclear weapon detonator, wide band jammer, and I can list more.

Normally I'm the one to make that point, but in this case 10MHz can be assumed to be a sinewave. Not that it makes any practical difference, since the spec is clearly rubbish, possibly dangerous rubbish.

[tggzzz]

Well, that's exactly what it looks like! But it was posted in relation to scope probes and mentions "Bandwidth 10MH   z".
As I said, get out your bargepole

Assuming you could find a low-impedance source for 28kV @ 10MHz, your bargepole might not keep you far enough away.

Well, bargepole isn't a well-defined standard. Personally I regard it as a punt pole as used in Cambridge or Oxford. That's 4.9m or 16ft, which ought to be enough

[PKTKS]

try to keep as short as possible.

I have dealt with a large number of SMPS for CRTs
PCs and mostly kind of consumer gear for repair
and for my own projects.

I am not sure what kind of SMPS you folks are
dealing with.. but 10MHz and other numbers
placed here are really way too far to me.

I have a pretty large baggage of SMPS from 70s and 80s
were I stopped servicing CRTs (by late 80s) definitely

All of my Service Manuals are pretty clear in details for
alignment and service those SMPS - for those not already
familiar they were 15KHz or in some rare cases some multiple
in 3x order. Tube  based  SMPS (as in the figures) were always tricky
a typical figure of flyback input is 1.2KV input  25kV output

Methods to attach the probe are very specific to avoid sparking
in the "AQUADAG" tint and safety net. Also a common source os issues
was polarized dust around the tubes which could detonate
sparks rather randomly and very dangerously.

Figures of my bench stuff attached with rich schemas and
detailed Service Manuals mostly 70s/80s era. I got spoiled
 by computers and Internet on the 90s  and stop servicing
SMPS CRTs since them

BUT NEVER FOUND A 10MHz SMPS with 25KV so far...

I had one os those HV DMM probes on th 80s  which I sold
for a fellow technician by early 90s.  This probe allowed my
 to service a great number of  SMPS mostly on that range
of 30KV up to 70kHz

The form factor of these probes are carefully crafted to service
those rather tricky spots on furniture like CRTs almost 50Kg each

I miss that probe I even consider getting another one today.
They were very expensive at that time.

All those comments I just can not fit with those decades
of real life service...

Paul

[Fungus]

Am I right in thinking that with a 10:1 probe the actual voltage reaching the scope is 42V and hence I am perfectly safe to use it that way?

Yes.

But:
* I personally would get a fixed x10 probe if I was going to do it on a regular basis.
* There's safer ways to do it.

[tony359]

Thank you all for the contributions.

I was thinking of adding a 100x probe to my toolkit. I appreciate a differential probe would be ideal but it's beyond my budget. I understand that as long as I do not attach the ground clip to anything but actual ground, I should be safe even though I imagine that measurements may be somehow inaccurate.

I was thinking of this one: https://uk.farnell.com/multicomp-pro/mp770213/test-probe-100mhz-oscilloscope/dp/3265030?st=oscilloscope%20probe

I see the max voltage allowed lowers with frequency but SMPS shouldn't really go up to Megahertz, am I mistaken?

Thanks!

[bdunham7]

I was thinking of adding a 100x probe to my toolkit. I appreciate a differential probe would be ideal but it's beyond my budget. I understand that as long as I do not attach the ground clip to anything but actual ground, I should be safe even though I imagine that measurements may be somehow inaccurate.

I was thinking of this one: https://uk.farnell.com/multicomp-pro/mp770213/test-probe-100mhz-oscilloscope/dp/3265030?st=oscilloscope%20probe

I see the max voltage allowed lowers with frequency but SMPS shouldn't really go up to Megahertz, am I mistaken?

Your scope should already be grounded and the safe thing to do with the ground clip is to remove it from the probe entirely and put it in a drawer.

The probe you posted would be fine, I use this one and I think it is better:

https://www.ebay.co.uk/itm/P2301B-P2301C-Oscilloscope-Clip-Probe-300MHz-5000V-High-Voltage-For-Tektronix-HP/163762675918?hash=item26210420ce:g:UP8AAOSwUTddHFJS

The concept of derating voltage by frequency or dV/dt is important and unfortunately, both the probe you posted and the one I have come with datasheets that are a bit suspect and should not be relied upon too literally.  However, I can tell you that the one I have works well in reasonable circumstances and I have no desire to test it's limits.  Or, I do, but don't have the equipment and a supply of probes!    Anyway, it works on a Tek 22XX scope HV supply that is about 2kV RMS @ 20kHz.  You aren't likely to find 1MHz on a high-power SMPS, although I'd recommend RTFM before probing where possible and hands-off-only probing where there is any doubt.

[CDaniel]

If you work in the primary side of a SMPS you must use an isolation transformer ... without it you can't measure anything right with the ground clip not used  And you can't touch the SMPS primary ground , as the scope ground is tied to earth.
If the farnell probe is not made by a crappy chinese company like the one on e-bay , for sure I would chose it .

[bdunham7]

If you work in the primary side of a SMPS you must use an isolation transformer ... without it you can't measure anything right with the ground clip not used  And you can't touch the SMPS primary ground , as the scope ground is tied to earth.
If the farnell probe is not made by a crappy chinese company like the one on e-bay , for sure I would chose it .

Yes, you typically need isolation to make any sense out of it, but a better practice is to isolate and then deliberately ground one point, typically starting with the negative side of the bridge, with a separate, secure ground connection and not rely on the scope ground clip for your ground reference path.  And still leave the scope ground clip off unless the readings are too noisy.

I assure you both probes are Chinese made.  The one I have is made by YPioneer, not a crappy company at all. 

[tony359]

TESTEC is not exactly a well known brand but I hope Farnell sell stuff of reasonable quality - as a minimum they must meet all EU regulations which I'm not sure about the one on ebay. 
That said, thank you bdunham7 for the link, appreciated!

[2N3055]

Testec is a known OEM wirh decent quality

[nctnico]

If you work in the primary side of a SMPS you must use an isolation transformer ... without it you can't measure anything right with the ground clip not used  And you can't touch the SMPS primary ground , as the scope ground is tied to earth.
If the farnell probe is not made by a crappy chinese company like the one on e-bay , for sure I would chose it .

Wrong on both accounts.

1) Never ever use an isolation transformer. Only use differential probes. Isolation transformers are a death trap in untrained hands c.q. in an  unsuitable measurement setup. 

2) Without an isolation transformer the primary side of the power supply is ground referenced just like the oscilloscope. You can still make useful measurements but there will be some offset. I have done this myself many times.

[tony359]

Testec is a known OEM wirh decent quality

That's great to know, thanks! I buy some stuff from China but I didn't feel like buying a 100:1 probe on Ebay. Even if it's of good brand, it's quite easy to get a copycat.

I believe I now fully understand what to do and not to do when playing with an SMPS, thanks you all for the great advice!

[bdunham7]

Wrong on both accounts.

1) Never ever use an isolation transformer. Only use differential probes. Isolation transformers are a death trap in untrained hands c.q. in an  unsuitable measurement setup. 

2) Without an isolation transformer the primary side of the power supply is ground referenced just like the oscilloscope. You can still make useful measurements but there will be some offset. I have done this myself many times.

Everyone has their opinions and this subject goes round and round endlessly.  There's nothing wrong with the way I mentioned and it is often a lot more convenient and less noisy.  You can even do it with isolated or differential probes if you like.  I fail to see how the isolation transformer itself is unsafe--the unsafe situation always involves some mistake or another and you could just as easily make the same--or different--mistake without one.  At least a proper isolation transformer will limit current to a few amps and then stop (fuse or breaker) if you do short something.

[nctnico]

Wrong on both accounts.

1) Never ever use an isolation transformer. Only use differential probes. Isolation transformers are a death trap in untrained hands c.q. in an  unsuitable measurement setup. 

2) Without an isolation transformer the primary side of the power supply is ground referenced just like the oscilloscope. You can still make useful measurements but there will be some offset. I have done this myself many times.

Everyone has their opinions and this subject goes round and round endlessly.  There's nothing wrong with the way I mentioned and it is often a lot more convenient and less noisy.  You can even do it with isolated or differential probes if you like.  I fail to see how the isolation transformer itself is unsafe--the unsafe situation always involves some mistake or another and you could just as easily make the same--or different--mistake without one.  At least a proper isolation transformer will limit current to a few amps and then stop (fuse or breaker) if you do short something.

Isolation transformers are bad in the hands of novices and people who think they understand the risks (and I hate to say it but you seem to fall in this category). Think about the situation where you clip the ground of your grounded oscilloscope to a random point in the primary side of a PSU. You'll ground the PSU AND bypass the GFI (you should have) making the isolation the isolation transformer provides go down the toilet. On top of that: If things go wrong you can fry your probe (and scope if you are unlucky). It is very easy to clip the ground of the second probe to a different point and creating a short between those points. Working safely should not depend on not making mistakes; people do make mistakes.

Besides all that a differential probe costs the same as a good (=with safety rated isolation between primary and secondary) isolation transformer. The whole point of trying to prove an isolation transformer is a good solution is totally moot. There are better ways to limit the current through a circuit as well. For example by putting a lamp in series or use a quick acting low amp breaker. But even then there are much better ways to test SMPS safely. A method I use when the control circuitry is acting weird is to use a low voltage PSU to power the controller chip and provide the DC bus voltage (say 30V to 40V); that way you can usually test everything without needing to apply mains at all.

[tony359]

A method I use when the control circuitry is acting weird is to use a low voltage PSU to power the controller chip and provide the DC bus voltage (say 30V to 40V); that way you can usually test everything without needing to apply mains at all.

Uhm... I like that! I think I'll give it a go, thanks!

[tautech]

A method I use when the control circuitry is acting weird is to use a low voltage PSU to power the controller chip and provide the DC bus voltage (say 30V to 40V); that way you can usually test everything without needing to apply mains at all.

Uhm... I like that! I think I'll give it a go, thanks!

Always check SMPS IC datasheets for VCC max beforehand.