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Electronics => Beginners => Topic started by: danmcb on January 12, 2017, 05:52:03 pm

Title: HF current pulse measurements
Post by: danmcb on January 12, 2017, 05:52:03 pm
hey guys,

I've been at this game a few years, but every now and again we hit a head scratcher that makes us *feel* like a beginner ;-)

I have worked mostly on audio in the past, but recently I needed to look at some HF stuff. What I am doing is looking at the transients omn a 24V DC plug when the user hot plugs it. I wondered how big the transients really are and was looking for an excuse to buy a DSO (i use an old  analogue Tek scope for this, it's what I am used to - but obviously that has its limits sometimes).

So today my nice new Siglent 1102 arrived, and I jigged up a little circuit to capture some transients. (sketch is attached.) The 22u cap obviously kills most of the ringing dead, but then you have a big current pules, so here I am measuring it.

here's the odd thing. I see a big measurement difference depending on whether I use a 10x or 1x probe. Like a factor of 10. And we are not at *that* high frequency here.

What I don't get is - the cct has a low  output resistance (.22R). The frequencies involved here are not that high - based on the 25uS timebase in the scan attached, I'd say in the 50-100kHz range. Capacitances are 10's of pF, not nearly enough to become significant with a .22R resistor. The probe input resistance (1M/10M) cannot be at play here.

In the end I figured lead inductance has to be the deal, and did the trick of using a very *short* piece of TCW wrapped around the probe and measured a peak voltage across the resistor of about 5V - around 20 - 25A peak current, which is believable. But I still see a big difference between x1 and x10.

(By the way - I tried this with various probes - the cheap 100MHz switchable ones that came with the scope, some decent Tek 50MHz ones I have, and got the same-ish results. Voltage recorded with x10 were always much larger. In the attachment, CH1 is x10, CH2 is x1).



 

Title: Re: HF current pulse measurements
Post by: danadak on January 12, 2017, 09:59:45 pm
The two wavefoms, one taken at X1 and the other at X10 ?
I cannot see the scale settings for the two channels, and does
the scope require you tell it what X factor the probe is on each
channel ?


Regards, Dana.
Title: Re: HF current pulse measurements
Post by: w2aew on January 12, 2017, 10:01:01 pm
There are a few things you should consider:

1) A 1X probe will have something on the order of 100-120pF or so of probe capacitance, while the 10X probe is likely something more like 10pF or so - an order of magnitude different.

2) Make sure that you have properly compensated the 10X probe.  If it is not properly compensated, HF signals can ready lower or higher than they really are.

3) There are frequency components present that are much higher than what is indicated by just the pulse width.  The 10X probe trace shows a much steeper (faster) risetime, as expected due to the lower capacitance / higher BW.  The 1X probe is filtering this rising edge, so it never reaches as high in amplitude.

4) Also consider the length of the scope ground lead - ground lead inductance can also alter the transient results.

5) and of course, make sure that you've properly adjusted the vertical scale of the scope to account for the 10X attenuation of the 10X probe.
Title: Re: HF current pulse measurements
Post by: tautech on January 12, 2017, 10:19:25 pm
All good advice so far.  :-+

The point raised by Dana and Alan re channel input attenuation: the UI to change this is within the menu for each channel and yes for the V/div shown and any OSD measurements to be correct it need match the probe switch setting.

For danmcb:
To capture screenshots to USB the most convenient is to use the blue Print button on the front panel which then also captures the active on-screen menu on the RH side which can help convey more information to us.  ;)
Title: Re: HF current pulse measurements
Post by: danmcb on January 13, 2017, 08:18:59 am
thanks for the replies.

Both probes are compensated. I don't bother telling the scope the probe attenuation as it's easy enough to multiply by 10. (I did try setting it in the scope as well, just to check whether the scope might be somehow altering the input cct based on probe setting, the results are just the same.)

PRINT button is handy - I re-did the measurements, also took a snap of my test setup. As you can see - x1 setting gives a peak voltage of around 760mV, on x10 it is about 4.2V. That's a big discrepancy.

As for capacitance - the probe spec sheet says 14pF - it doesn't say that it is different on x1. I suppose  on x1 you also have the input C of the scope, which is 18pF, while for x10 the attenuator stops the circuit seeing that. So, say 32pF for x1,  14pF for x10, no?

(How do I show images inline here - do you have to have them hosted somewhere?)
Title: Re: HF current pulse measurements
Post by: bktemp on January 13, 2017, 08:24:47 am
Why goes the voltage negative after the pulse when using the 10x probe? Looks like some AC coupling issue or incorrectly compensated probe.
How does a square wave look like using both probes (signal from the probe compensation output)?
Title: Re: HF current pulse measurements
Post by: danmcb on January 13, 2017, 08:58:11 am
the probe has been compensated. the cal o/p  of the scope is fine via the probe, on x1 and x10.

The undershoot you see is ringing in the cct due to lead inductances. Without the 22u cap it is much more  pronounced. It is both over/under voltage, and the current pulse, that I am am investigating.

Also bear in mind that the two traces are not the SAME waveform - I have to hot-plug the cct each time I make a reading, and it is not possible to have the probe switched to x1 and x10 at once! But EVERY time I do it on x10, I get a max voltage around about 4V, and on x1, around 0.7V. That's too much of a discrepancy.
Title: Re: HF current pulse measurements
Post by: capt bullshot on January 13, 2017, 09:14:58 am
Check your +24V DC power supply -- there are two kinds of them:
- SELV: isolated output
- PELV: typically the minus pole is connected to Ground, same is the BNC shell of your oscilloscope

If you happen to use a PELV supply, you simply cannot measure the current this way as there will be a ground loop diverting parts of the inrush current through the probes GND to the supply return.
Even if you use an isolated output power supply, there might be some current diversion through the EMC and parasitic capacitors inside the power supply

So check your GND loops (DC and AC through hidden capacitors) and solder some BNC to pigtail coax cable directly to your shunt resistor, don't use a probe at all. This may lead to useful results.

For best results, I'd recommend a proper current probe like the Tektronix A6302 / A6303 (or a modern variant) -- beware these things are really expensive, even in used condition.
Title: Re: HF current pulse measurements
Post by: danmcb on January 13, 2017, 09:20:49 am
Great point! thank you!

I had assumed that it was isolated - it isn't. Good to know!

I guess the other way to measure this (without a current probe) would be to use a decent differential amp  (that has high input impedance on both legs).
Title: Re: HF current pulse measurements
Post by: tautech on January 13, 2017, 09:21:42 am
thanks for the replies.

Both probes are compensated. I don't bother telling the scope the probe attenuation as it's easy enough to multiply by 10.
Yep, the usual ex CRO users, DSO newbie mistake.
Set the probes and the input attenuation to 10x and leave them there.
For a few reasons:
Better HV protection for the scope.
Much lower capacitance loading of the DUT.
OSD measurements will be consistent with actual DUT values.

Quote
PRINT button is handy - I re-did the measurements, also took a snap of my test setup. As you can see - x1 setting gives a peak voltage of around 760mV, on x10 it is about 4.2V. That's a big discrepancy.
Something is wrong with your DSO setup or measurement technique then.
Sanity check time....use the probe cal outputs and you should see a little over 3V @ 1 KHz.
Tip:
For us to have as much info as possible have the menu up showing the some relevant menu selections.

Quote
As for capacitance - the probe spec sheet says 14pF - it doesn't say that it is different on x1. I suppose  on x1 you also have the input C of the scope, which is 18pF, while for x10 the attenuator stops the circuit seeing that. So, say 32pF for x1,  14pF for x10, no?
Correct- NO.
Just grabbed the first probe spec sheet (not a Siglent one) and this 60 MHz probe is spec'ed @ 18 pF 10x and 47 pF plus scope input @ 1x.
60 pF + is not unusual for a 1x probe.

Quote
(How do I show images inline here - do you have to have them hosted somewhere?)
To embed them within the body of your post from the thumbnails, copy the thumbnail URL and within Modify post paste the URL then highlight it and click in Insert Image.
 I'll edit this post to demonstrate and you can examine the syntax if you quote this post.

(https://www.eevblog.com/forum/beginners/hf-current-pulse-measurements/?action=dlattach;attach=284882)

Check your +24V DC power supply -- there are two kinds of them:
- SELV: isolated output
- PELV: typically the minus pole is connected to Ground, same is the BNC shell of your oscilloscope

If you happen to use a PELV supply, you simply cannot measure the current this way as there will be a ground loop diverting parts of the inrush current through the probes GND to the supply return.
Even if you use an isolated output power supply, there might be some current diversion through the EMC and parasitic capacitors inside the power supply

So check your GND loops (DC and AC through hidden capacitors) and solder some BNC to pigtail coax cable directly to your shunt resistor, don't use a probe at all. This may lead to useful results.

For best results, I'd recommend a proper current probe like the Tektronix A6302 / A6303 (or a modern variant) -- beware these things are really expensive, even in used condition.
Yep, in this case that's a strong possibility.

OP, search for Daves vid on "how not to blow up your scope"
Title: Re: HF current pulse measurements
Post by: capt bullshot on January 13, 2017, 09:32:46 am
I guess the other way to measure this (without a current probe) would be to use a decent differential amp  (that has high input impedance on both legs).

Yes, if you happen to have one ...

Another way would be using two identical probes, connect their GND to the supply "-" and the tips to the ends of your shunt resistor. Then set the scope to identical V/div on both channels, then use Math "-" mode to calculate the difference. Last: triple check your results, as there are many sources of possible errors
Title: Re: HF current pulse measurements
Post by: danmcb on January 13, 2017, 01:10:15 pm
Thanks Captain Bullshot - once I realised that the supply is grounded (silly to assume it was floating) the problem is obvious. I had the wrong side of the sense resistor connected to ground. So the loop resistance via scope/probe and mains wiring is in parallel with the sense resistor. I guess switching the probe from x1 to x10 changes that impedance somehow, so inconsistent readings.

Anyway, with the connection reversed, I get much more consistent (and higher) readings,  regardless of probe setting, which is what bothered me. Typically 10-20V. With x10, I can also see an occasional nasty HF pulse that gets to about 25V - across 0.22R! Glad that I found this. (It wasn't visible at all, before my silly error.)

I disagree however, Capt B, that you cannot measure the current like this because of the ground. Whatever current goes through into the input cct flows in the sense resistor - period. Kirchoff's Law says that. You just have to connect it correctly. (Of course the potential across that resistor gets subtracted from i/p  voltage, so things look a little on the rosy side. But that is just the nature of using a sense resistor - it does a bit of current limiting itself. The .05R 1% that I ordered yesterday just arrived, I'll repeat with that - after coffee).

Tautech - thanks for your response. But this has nothing to do with DSO's  versus CRO's. A CRO also has the sleeve of the BNC connected to mains earth (unless of course you've been naughty and disconnected it), and you can blow it or the UUT just the same way - as I found out about 30 years back. Also - there are plenty of places where a x1 probe is needed - for instance, looking at output noise on a linear PSU designed for low noise stuff (typically a very small number of mV). These things disappear into noise on x10.

The DSO is cool for non-repetitive signals, or things where you want to see a 1uS pulse that only happens  every second, that kind of thing. But for most bench work, I'll probably stick with my trusty Tek 2225, for its vertical sensitivity (5mV with a x10 amp gives 500uV, although it is a bit noisy down there), its immediacy - and the fact that there is no fan.

As for a differential probe - well, I wouldn't buy one, but it would be a cool thing to build. There is a nice LTC app note, also some circuits online. But probably not needed for this.


Title: Re: HF current pulse measurements
Post by: danmcb on January 13, 2017, 01:44:59 pm
OK, this is the correct setup (there is also a series limit resistor of 0.47R in the photo). Sense resistor is .05R. This time I wrapped the leads right onto the probe to get a good HF measurement. The spike is close almost 500mV - probe is x10, so 5V => 100A!

(https://www.eevblog.com/forum/beginners/hf-current-pulse-measurements/?action=dlattach;attach=284960;image)

(https://www.eevblog.com/forum/beginners/hf-current-pulse-measurements/?action=dlattach;attach=284958;image)

Nest is how to limit it. Tried a 0.47R resistor, which I thought would at least help - gets it down to about 60A, but that is still  a lot. Smaller capacitor, and maybe a NTC thermistor. Experiment time.

Thanks for helping guys!
Title: Re: HF current pulse measurements
Post by: bktemp on January 13, 2017, 01:47:10 pm
Anyway, with the connection reversed, I get much more consistent (and higher) readings,  regardless of probe setting, which is what bothered me. Typically 10-20V. With x10, I can also see an occasional nasty HF pulse that gets to about 25V - across 0.22R! Glad that I found this. (It wasn't visible at all, before my silly error.)
By putting a resistor in series with your inrush current, you affect the current. Therefore you have a totally different circuite than without the current shunt resistor. If you see 10-20V across the resistor, that is 42-83% if your input voltage!
Also your resistor looks like a wirewound resistor. It has quite some inductance, therefore it is probably useless above maybe 100kHz or so.
You could redo the measurements using a purely resistive load. If you still get ringing/overshoot it is caused by the inductive component of your shunt resistor.

The HF pulses you see is contact bounce when you connect the input voltage.

That's why your measurement is wrong:
24V/(0.47R+0.05R)=46A
So how can there be 60A?
A 22uF cap has a typical ESR of >1ohms, so the actual current is probably much lower.
Title: Re: HF current pulse measurements
Post by: danmcb on January 13, 2017, 03:30:11 pm
read my last post again, bktemp ;-)
Title: Re: HF current pulse measurements
Post by: bktemp on January 13, 2017, 03:56:59 pm
read my last post again, bktemp ;-)
Sorry, but I don't get it.
Without the 0.47R resistor you measure 100A, with the 0.47R resistor you get 60A.
Both values look wrong too me.
Title: Re: HF current pulse measurements
Post by: danmcb on January 13, 2017, 04:20:52 pm
The 0.47 is a series *limit* resistor, to try to reduce the current inrush. The *sense* resistor is 0.05R (that's the big black one that is wrapped around the scope probe). Sorry if that wasn't clear.

I get up to 100A without the 0.47, that drops to about 60-70A with it.

I agree that a sense resistor does affect the measurement - that's why I changed to 0.05, from 0.22R. (a few posts further up). In fact, the lower the current, the less of a problem that is - and all I am doing here is trying to get the inrush acceptably low - I don't need to know the *exact* value when it's up at 80A.

Now tweaking values to get the best compromise of voltage ringing at the input and inrush.  I must say the DSO is great for this kind of work.
Title: Re: HF current pulse measurements
Post by: bktemp on January 13, 2017, 04:52:56 pm
Going from 0.22R to 0.05R reduces the influence of the resistor to the inrush current, but also makes the effect of the parasitic inductance of the resistor much larger (because the voltage drop on the resistor is now very small). That's why your measurement is still wrong.
How can you get 60-70A @24V if you have at least 0.52 ohms in your circuit?
That is impossible.

You should use a low inductance resistor or a current transformer.
Title: Re: HF current pulse measurements
Post by: w2aew on January 13, 2017, 05:22:46 pm
FYI - for the 1X probe, you also have to add the capacitance of the coax between the probe and the scope.  The Ctip+Ccoax+Cscope will often give you 100pF or more.
Title: Re: HF current pulse measurements
Post by: danmcb on January 13, 2017, 05:23:33 pm
you're right. I was also wondering about that also. I expected 0.47 to keep it less than 50A, which it should.

Vishay say "low inductance" but don't say how low. I can't measure it with my old Beckman LCR tester, but anyway, I need to order a thick film one instead.

Anyway, the solution seems to be a classic RC snubber circuit across the input - 3R3 and 22u seems OK - but I'll wait until I get a better current measurement to be sure.

Anyway,  that's likely good news - if I think I am seeing 20A now, the real number is probably only about 2 :-)

Title: Re: HF current pulse measurements
Post by: tautech on January 13, 2017, 06:44:57 pm
Tautech - thanks for your response. But this has nothing to do with DSO's  versus CRO's. A CRO also has the sleeve of the BNC connected to mains earth (unless of course you've been naughty and disconnected it), and you can blow it or the UUT just the same way - as I found out about 30 years back. Also - there are plenty of places where a x1 probe is needed - for instance, looking at output noise on a linear PSU designed for low noise stuff (typically a very small number of mV). These things disappear into noise on x10.

The DSO is cool for non-repetitive signals, or things where you want to see a 1uS pulse that only happens  every second, that kind of thing. But for most bench work, I'll probably stick with my trusty Tek 2225, for its vertical sensitivity (5mV with a x10 amp gives 500uV, although it is a bit noisy down there), its immediacy - and the fact that there is no fan.
Great.  :-+
For us it's not always easy to distinguish the users experience and I take no issues with your reply.
They're a good little basic DSO that's a cheap jump into the DSO world from where you'll probably start looking for something to replace your Tek with for when it starts to get unreliable. The next step up from your CML is the 1000X series with a 500 uV range as standard.  ;)
Title: Re: HF current pulse measurements
Post by: T3sl4co1l on January 13, 2017, 09:08:23 pm
FYI, it's not true that HF stuff is completely out of the problem, here -- the action of a mechanical contact, or spark, is down in the fractional nanoseconds!

For events that fast, the measurement will depend critically on the resistor's construction, how you're probing it, and oscilloscope bandwidth (i.e., you'd need a 1GHz+ scope to fully resolve all frequency content of the event).

Very little current or energy is delivered in that time, of course; so we can quite safely ignore the highest frequencies.  After the initial contact event, not much happens.  The next interesting thing happens at a time scale a thousand times longer (microseconds).

The RLC equivalent circuit shows the behavior here.  As long as ESR >> sqrt(Lstray / C), the pulse is overdamped, and the peak current will be around Vin / ESR.  (If not, the waveform will have significant ringing, and the peaks will be at around Vin / sqrt(Lstray / C).)
http://www.cvel.clemson.edu/emc/calculators/RLC_Calculator/index.html (http://www.cvel.clemson.edu/emc/calculators/RLC_Calculator/index.html)

Tim
Title: Re: HF current pulse measurements
Post by: David Hess on January 14, 2017, 02:26:10 pm
The DSO is cool for non-repetitive signals, or things where you want to see a 1uS pulse that only happens  every second, that kind of thing. But for most bench work, I'll probably stick with my trusty Tek 2225, for its vertical sensitivity (5mV with a x10 amp gives 500uV, although it is a bit noisy down there), its immediacy - and the fact that there is no fan.

I went the Tektronix 2230/2232 route for the reason you identify, low repetition rate signals.  Except for lack of isolation, they work great for switching power supply startup problems.

Tektronix also made the 2210/2211 which is a 2225 with added digital storage capability.  I would not normally recommend them because you have a DSO now and they lack peak detection but they would work for something like this and at least in the US can be had for less than $100.
Title: Re: HF current pulse measurements
Post by: danmcb on January 14, 2017, 03:46:03 pm
Great.  :-+
For us it's not always easy to distinguish the users experience and I take no issues with your reply.
They're a good little basic DSO that's a cheap jump into the DSO world from where you'll probably start looking for something to replace your Tek with for when it starts to get unreliable. The next step up from your CML is the 1000X series with a 500 uV range as standard.  ;)

No  worries! and I appreciate well-intentioned replies anyway. Nobody can work with electronics for any length of time without making mistakes at times - and that definitely includes me ;-)

It's a great little scope, absolutely fine for what I want. I have two Teks, one is >50 years old, the main one about 35 I guess.  I doubt either will start playing up anytime soon! But the Siglent is a different tool  for different situiations (like this one) and has already proved its worth. I'm sure I will  not regret the purchase at all.

Title: Re: HF current pulse measurements
Post by: danmcb on January 14, 2017, 03:51:25 pm
FYI, it's not true that HF stuff is completely out of the problem, here -- the action of a mechanical contact, or spark, is down in the fractional nanoseconds!

For events that fast, the measurement will depend critically on the resistor's construction, how you're probing it, and oscilloscope bandwidth (i.e., you'd need a 1GHz+ scope to fully resolve all frequency content of the event).

Very little current or energy is delivered in that time, of course; so we can quite safely ignore the highest frequencies.  After the initial contact event, not much happens.  The next interesting thing happens at a time scale a thousand times longer (microseconds).

The RLC equivalent circuit shows the behavior here.  As long as ESR >> sqrt(Lstray / C), the pulse is overdamped, and the peak current will be around Vin / ESR.  (If not, the waveform will have significant ringing, and the peaks will be at around Vin / sqrt(Lstray / C).)
http://www.cvel.clemson.edu/emc/calculators/RLC_Calculator/index.html (http://www.cvel.clemson.edu/emc/calculators/RLC_Calculator/index.html)

Tim

Indeed. The maths is not really my strongest suit,  but I take the point.

Well, because the C is directly across the voltage source, the instantaneous current is theoretically infinite! In fact, source impedance, wire impedance, and so on, give the limit, but it is going to be pretty high. And of course, the finer the time resolution of the measurement, the higher the current you are likely to observe.

But this has been a really instructive exercise, as it led me to the (now obvious) idea of putting an RC snubber right on the input, to limit voltage (damping oscillations) without too much of a current pulse. And the DSO is invaluable as it allows me actually find good values of R and C by observation and measurement, rather than guesstimating, which is where I'd be at without it. Great tool!
Title: Re: HF current pulse measurements
Post by: danmcb on January 14, 2017, 03:54:45 pm

I went the Tektronix 2230/2232 route for the reason you identify, low repetition rate signals.  Except for lack of isolation, they work great for switching power supply startup problems.

Tektronix also made the 2210/2211 which is a 2225 with added digital storage capability.  I would not normally recommend them because you have a DSO now and they lack peak detection but they would work for something like this and at least in the US can be had for less than $100.

I think the CRO/DSO combination is absolutely ideal for what I do. I do mostly audio or audio related, but sometimes there is microcontroller stuff and so on involved. But when there is, it is usually at lowish frequency (this project has a PIC, I clock it at 500kHz because it just doesn't need more). The only reason I might upgrade from here is if I start working on high speed digital at some point.

Title: Re: HF current pulse measurements
Post by: David Hess on January 14, 2017, 04:52:06 pm
I think the CRO/DSO combination is absolutely ideal for what I do. I do mostly audio or audio related, but sometimes there is microcontroller stuff and so on involved. But when there is, it is usually at lowish frequency (this project has a PIC, I clock it at 500kHz because it just doesn't need more). The only reason I might upgrade from here is if I start working on high speed digital at some point.

As a low cost option for someone who can maintain an old instrument, they are acceptable but the same applies for any test instrument of a similar age.  Otherwise if you can live within their limitations, a modern cheap DSO is usually a better option unless the old instrument provides a unique capability like high bandwidth, high sensitivity, or accurate RMS noise measurement.
Title: Re: HF current pulse measurements
Post by: T3sl4co1l on January 14, 2017, 05:47:36 pm
Well, because the C is directly across the voltage source, the instantaneous current is theoretically infinite!

One would be a pretty poor theorist to ignore an entire fundamental property of the universe -- magnetism! ;)

Quote
In fact, source impedance, wire impedance, and so on, give the limit, but it is going to be pretty high. And of course, the finer the time resolution of the measurement, the higher the current you are likely to observe.

The current peak is given, to second order accuracy (hard to beat that! :D ), by adding the source resistance, and wiring inductance.

In practice, the source won't be an ideal voltage source, but the lowest impedance it has will be, guess what -- another capacitor!  So to be accurate, one must have the C and ESR of that, and connect it in series with the load side ESR and C.  (If Cload >> Csource, then it could take so long to charge that the transformer, rectifier, and maybe AC mains source, have to be taken into account.  But the current will be much lower.)

ESL is the tricky one, though.  You can guesstimate stray inductance based on wiring length: the inductance of free space (that is, the inductance due to a current simply flowing through a distance) is 1.257uH/m (more correctly, mu_0 == 4*pi x 10^-7 H/m).  The inductance of most cables is around 1/2 to 1/4 of this (because the magnetic field inside a cable isn't in free space, but it's trapped between conductors that oppose!), so you can guesstimate 0.3 to 0.6 uH/m.

So, a typical 2m cable is on the order of 1uH, and paired with 47uF, the sqrt(1uH/47uF) ~= 0.14 ohms.  (So, ESR > 0.14 ohms gives good damping, and the peak current from 24V will be under 160A.)

Quote
But this has been a really instructive exercise, as it led me to the (now obvious) idea of putting an RC snubber right on the input, to limit voltage (damping oscillations) without too much of a current pulse. And the DSO is invaluable as it allows me actually find good values of R and C by observation and measurement, rather than guesstimating, which is where I'd be at without it. Great tool!

Yup.  And, you also know the value of RC required: the C needs to be more than double the non-resistor C value, and R needs to be equal to sqrt(L/C).

sqrt(L/C) shows up so often, it has a name: characteristic impedance, Zo.  Note that it does indeed have units of ohms (Google Calculator will show this works).  Which should tell you something interesting will happen when you add a resistor of similar value to the circuit! :)

In practice, electrolytic capacitors have enough ESR to avoid ringing except for very long cables (but then, the cable resistance itself may be significant, too).  The inrush current can be quite large, so it needs to be directed away from sensitive circuits.  Tantalum capacitors have a range of ESR available (so can be used for damping, or can be prone to more ringing), but more importantly, shouldn't be exposed to surge currents and voltage spikes, which can ignite them!  Polymer caps usually have very low ESR (like a tiny little 25V 47uF aluminum polymer having an ESR of 20mohms!), so they can cause big problems with ringing in power supplies and have to be used appropriately.

Nothing at all more complicated than algebra-level arithmetic! :D

Tim
Title: Re: HF current pulse measurements
Post by: danmcb on January 15, 2017, 05:15:35 pm
Thank you Tim!

well, as is often the case, there turned out to be more to this than meets the eye. The "more" in this case being the earth loop from the PSU being hot-plugged to the UUT via the scope.

To summarise - the sense resistor being wirewound was not the issue. I still got too high current pulse readings using an 0.1R carbon resistor instead of the 0.05R wirewound. Readings up in the 20-40A range. With a 3R3 resistor in the RC snubber. Something is not right.

The issue turned out to be the earth loop.  Although it measures 0R, it is also quite inductive. When I used a short length of thick wire to short out the earth loop (tricky, holding it on the barrel of the DC jack plug while plugging), the readings became honest. Now I got readings with my sense resistor that tally with the expected 8A or so of initial charge current.

I was also curious about the lead inductance, and decided to try to measure it. What I did was,  instead of the RC snubber, connect a 470pF 1% cap. Big enough to swamp the 14pF or so of scope probe capacitance, not big enough to kill the ringing when hot plugging.

Then catch the voltage waveform with ringing, and measure the frequency. Rearranging the well known tuned cct formula, we get:

L = 1 / (C * (2 * pi * f)^2 )

With the UUT ground NOT directly connected to the PSU ground (i.e. with the earth loop inductance in place) I got about 2.6MHz, which equates to 8uH.

With the shorting link in place, the frequency goes up to about 10MHz => 540nH! OK, the ground  loop as an RL cct may be oversimplified, but it suggests that there can be quite some reactance, which can resonate and create a lot of extra current.

In fact, I think there is maybe even more to it than that - I noted that it was hard to get ringing AT ALL if I didn't leave the whole circuit to settle down for a while before hot plugging (even if I took care to discharge the cap). I don't know why that should be.

But all of this is enough to persuade me that HF measurements are very tricky with the scope earth loop.

This is why I like bench work - you don't always get all your questions answered, but you find out a lot about what does and doesn't work, and things to look out for. In this case, it is pretty useful, as the charger might be hotplugged into a unit which is, or is not, grounded already. So good to know that this can have a big effect on the current pulse magnitude. Never would have picked that from a simulation, I think.

Thanks all!



Title: Re: HF current pulse measurements
Post by: T3sl4co1l on January 15, 2017, 06:48:21 pm
Moral of the story:
There is no such thing as absolute "ground", at AC.

Everything has a nonzero impedance.

The only point where "ground" is true, is at the point it's defined.  The scope defines its input connector as ground, so that's also what you're measuring with respect to.

Any voltage drop in the probe cable, ground clip, and other outside wiring, is part of the circuit you're measuring: it ain't ground! ;)

To avoid the ground loop, simply short it out better.  Instead of hot-plugging the barrel jack, leave it connected, with the ground connected straight in.  Switch only the "hot" wire.

0.54uH sounds perfectly plausible.  Like I said, about that per meter, give or take whatever the exact cable length is.

8uH also sounds reasonable for an about room-sized ground loop.  The inconsistency you saw may have to do with the two contacts of the barrel jack touching at different times, or voltage between the ground points (ground loop voltage, which is induced in the ground wire due to mains current flow).

Tim
Title: Re: HF current pulse measurements
Post by: danmcb on January 15, 2017, 07:25:14 pm
indeed! You always have to think about what you are measuring, and with respect to what. If I was really bothered about refining this, I'd have cut the connector off the charger, soldered a new one on with an extra short wire ... etc ... but really I just wanted to figure out what was going on. Measuring 25A via a 3R3 resistor  and 25V just ain't right! The extra voltage of course being across the "ground" connection.

It kind of sucks that most differential probes are silly money, because they are designed to also provide isolation. What is needed is a diff input that has reasonable protection against overvoltage, a sensible stepped attenuator (pretty much like a standard scope - and really good CMR. 50MHz BW would be fine for many apps, but it would get rid of the troublesome groundloop issue. Things are the way they are because they've always been done like that, but they are not really as good as they should be.