Measurement Techniques without Earth Grounding an Offline smps

You have a dual trace scope?

Can it display an "A+B" trace?

Can it invert one trace?

If so then you can display the voltage difference between *any* two points anywhere in your SMPS

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Gibbo

This email address isn\'t real.

It will not if you use an isolation transformer!

Otherwise you are stuck with differential measurements, which suck because you use two channels on the scope for each trace.

I only have a single channel scope.

The challenge is making measurements and test techniques without a isolation transformer nor an isolated scope to detect RF bursts "birdies", ringing, spikes and overshoots.

I suspect it's a little bit like making radio tuner/demodulator ccts and measuring a DC out with a DMM.

D from BC

Single trace. :( It's the stone age over here :) D from BC

you

pointless ... like deciding to run the 100 m sprint with your left hand tied to your right foot behind your back?

That's how I do it. But not at the end of a scope probe. Take a coax but make sure it's 50ohms and not 75ohms, they can look deceptively similar. Then switch you scope to 50ohms input or terminate it via a T-connector. Now wind a coil of 1/2" diameter or less. About 3-4 turns, nice and tidy, and use wire that has an insulation that isn't easily worn or scraped off. Solder to shield and center at the end of the coax. Now you have an H-field sniffer. If ye olde Hickock scope has enough bandwidth, that is.

The other trick is the E-field smeller: Take the same kind of coax, strip the shield about 1/4" but do not strip the center conductor. Insulate the tip well. I use Kapton tape. Anything that won't scrape off. Make sure the center doesn't poke through (which it easily does when bending the coax back and forth). Now you have an E-field probe. Same here, if the scope doesn't have enough BW you may not see a thing even when the birdie cometh along.

Well, those are the tricks, pretty much. Since I do this stuff for a living I abandoned the poor man's method a long time ago and use an EMCO probe kit. But that would set you back by about $1000. Believe it or not but I managed to wear out one of its probes already.

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Regards, Joerg

http://www.analogconsultants.com

If you're crazy enough, you could float the scope, but of course, then the case could be at line voltage - use one of these or equiv:

I do not advise this, it is dangerous and can be lethal - it's up to you to decide if you want to take the chance. In any case, do NOT touch any metal part of the scope.

Good Luck! Rich

Definately read AN70 from Linear.com, nice bit about sniffers

martin

[snip]

Cool... I'm hoping for low freq. Big Bird size birdies to see on that 40yr old

25Mhz BW Hickock scope. I'm guessing my scope has a 6dB/oct attenuation. So....mmm.. down 24dB's@ at 400Mhz....4th harmonic. Those birdies have a f limit somewhere... Maybe I can alter my PCB construction and make some "cats" :)

I might try too..

I didn't know it was an industrial product... So I did some googling.. and found some reading material on:

About near field probes use.

Thanks D from BC

With a 25MHz bandwidth I am afraid you may not see them. Unless there is some serious "slow" ringing going on. Think of humming birds, very small, very fast and cats usually can't catch them ;-)

Yep, even came in a little black suitcase, with calibrations sheets and all.

Can't see it. Seems to be one of those members-only sites. You'll be surprised what you see on the scope when you hold that probe near one of those laptops power supplies. Makes the toe nails curl.

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Regards, Joerg

http://www.analogconsultants.com

I've been tempted.. The scope I'm using has an all metal grounded face plate so turning knobs will be dangerous.. I could do a back away, power up, power off, adj and repeat routine. Or maybe poke at the knobs with a stick :) With no earth ground connection to the scope there's no line power

60Hz loop.. No fuses will blow. However, should I be concerned with my scopes high frequency isolation from line and neutral? Just wondering if it will throw off the measurements when testing the offline smps. D from BC

Found it...Page 55 on

That's cool... Sleek and compact... Looks like I could cruise over 30mil traces and not pickup too much crosstalk from other mag sources.. Not only that...this app note has lots of other goodies too.

In my OP, I called it a "sniffer coil". It's something I probably heard of some 15years ago. Then forgot all the details but the sniffy name..

Thanks.. D from BC

Sounds like an excellent idea. :-)

If your probe has one of those little ground clippies at the end, that should provide a reference - the freq. response might be affected, and there could be some 60Hz pickup because the scope case will be capacitive to whatever's around it, kinda like an antenna, but if it's a decent scope, it should ignore that and just show what's between the probe tip and its ground clip.

I've been known to do this, and I've seen others do it, but never while alone in the room. :-)

And put your other hand in your pocket, seriously. In the USAF, they _ordered_ us to keep our other hand in our pocket, at least if we were probing anything over, say, 28V.

Good Luck! Rich

What do you think about catching birdies with a tuner (bandpass) and diode detector cct.? In other words, this is the poor mans spectrum analyzer. Is there a module or inexpensive unit that can tune 100khz to say

1Ghz? I'm doing a google on that now.. [snip] D from BC

Won't work very well. Birdies are short bursts of oscillations that happen during the phase where the FET goes through its linear region. IOW the duty cycle is extremely low. Even expensive spectrum analyzers have a hard time diagnosing that. I had to do that a lot in pulsed ultrasound and had to build myself some fancy limiters to get any kind of information out out the analyzer. So yes, it can work but it's a whole lot of effort.

Probably. People have used TV tuners for that. But I never bothered after finding out the dynamic range of the average TV set. Sometimes I wonder whether their designers actually knew what a dynamic range is.

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Regards, Joerg

http://www.analogconsultants.com

pick up "sniffer coil" on a scope probe.

Think you will need some serious insulation on yours. Remember, always keep one hand your pocket playing with this stuff

martin