Spikes on bench-top power supplies

"Joerg" schreef in bericht news:%Ksxg.72460$ snipped-for-privacy@newssvr27.news.prodigy.net...

What currents? Long during overshoot?

It has never bitten me. Sometimes, on dry winterdays, I had times that everything latched up under my nose, fiddling with a stubborn prototype. I'm more worried about static discharges.

Damn, you made me even more paranoid than I already am ;)

Joerg wrote: Voltage dipped, then regulated back and

That's why one should always follow Bob Pease's advice-- check every power supply under pulsed load. Get your heftiest MOSFET, hook it up to a pulse generator, put a big resistor on the drain and hook it up to the PS. It's just *amazing* the things you'll see on your scope. If it's your PS design, you'll likely need to add a few RC rolloff networks to improve the transient response. Think of the PS as a big DC amplifier that needs taming.

Another source of glitches-- line glitches upsetting the regulator:

The worst example was, hmmm, may have been a Lamba PS. 5 volts at 60 amps, with a SCR overvoltage clamp set to 5.5 volts. Turning on a vacuum cleaner in the next room would trip the crowbar every time!

Hello Frank,

It already happenened going from 10uA to 0.3mA but this design can also jump from 10uA to 10mA. The regulator chip on the circuit went with a plume of smoke and a stench. No problem with a battery.

I don't remember the timing but one supply was pumping a spike of less than a msec. Enough to drive stuff into latch-up or other destructive modes. Another supply actually began to oscillate.

Sorry, didn't want to cause that reaction... :-)

We just figured a way to do it with batteries until we can find a decent power supply. If we can find one, that is. This takes those risks out of the equation.

Now that is a true sign of a poor design. Did they ever go through susceptibility tests or did they somehow sneak by?

I *hate* it when that happens.

No, but I'll give it a check. It's a switcher, so I wouldn't expect it to be very good. It does kick out typical switcher noise.

John

Hello John,

What really surprised me was that while the "high tech" supplies misbehaved I had tested all these prototypes with one of my old Russky power supplies. No problems whatsoever and I even disconnected and re-connected a whole unit on occasions without turning down the supply.

And I might add that none of my own power supply designs ever did that, including the switchers.

That wouldn't be a problem for one of my clients as long as the feedback loop in there is properly designed.

A lot of people might wonder why a supply should have such a wide mains range. It can be very practical for folks who travel a lot or must debug foreign designs. Once had I almost fried the Philips FET probe. It got really hot but miraculously survived.

Yeah, but I bet you "cheated" and actually *tested* the things before you declared them as ready for prime time!

Perhaps hardware design is heading the direction of software design -- little initial testing, figuring that faults can always be fixed after the product is released.

Susceptibility tests? What applicable US standard for a bench power supply requires those?

Best regards, Spehro Pefhany

Hello Joel,

Yes, per your definition I then cheated :-)

Sure, but I am a bit disappointed that rather expensive lab power supplies would exhibit such behavior. I could tolerate a deviation downwards but when a supply spikes upwards that is bound to cause grief. Expensive grief at times.

Hello Spehro,

Three things would require that:

Liability: If someone hires a lawyer because they found out what fried that multi-million Dollar linear accelerator this could require the V.P. of Quality Control to seek a prescription for Prozac.

Reputation: When it is found out that a top of the line piece of equipment does not behave top of the line chances are pretty slim that the next pieces of equipment are bought from that manufacturer.

CE: Lab gear is (usually) marketed world-wide. If the thing misbehaves in terms of EMI, something nasty happens and it turns out that in hindsight self-cert wasn't such a good idea, oh boy.

"Joerg" schreef in bericht news:Tnuxg.72485$ snipped-for-privacy@newssvr27.news.prodigy.net...

Well, I checked my bench supplies, no problem whatsoever. A tiny dip at most, and even tinier overshoot. All < 0.1V. Another

Hello Frank,

Did you have a capacitive load connected for the test? Almost all circuits present that type of load because of the bypass caps.

That's not my philosophy. In the same way that I do not consider the 30 minutes it takes to check the brake pads on a car "wasted".

"Joerg" schreef in bericht news:959zg.27$ snipped-for-privacy@newssvr25.news.prodigy.net...

No, but I would guess there is a small bypass cap on the output anyway. You're not talking me into another 30 minutes of my time wasted? ;) Sheesh, would you please provide us with accurate test models other than simply complaining you see funny things when current jumps between 10uA and 10mA. Next thing you're telling us you had 20 miles of cables between target and supply. Or admit your prototype caught fire because of some other stupid mistake. At this rate, your credibility drops below zero. Correct that, *has* dropped below zero.

Well it was your philosphy on power supplies, otherwise you would not have complained about it.

About brakes; I have that task delegated to the mechanics at the garage. At least that assures me it is carried out properly.

Hello Frank,

Nope. I spent several hours testing the supplies here in the lab before anything electronic was connected to one of them. This included testing with various capacitive loads as it is highly unusual (but not impossible) that an electronic circuit boards does not contain bypass caps. These supplies passed. The ones at the client didn't but I have no control over power supplies that are hundreds of miles away until I get there :-)

Good. You don't have to spend the 30 minutes for a check-up yourself, you can certainly pay someone else who is competent to do it. Not doing that on a lab supply is like playing roulette.

There's state and local "government" too, with the same as the old saying goes "time is money". For sample requests I certainly understand . But not banquets :-) (banquets are puddings - sweet courses) Mary Yes. We have a huge retirement community just south of here and a lot easier to work with that every day, as a Lineman. Any insight into this problem would be solved, but coped with..." Shimon Peres Best, James Arthur That's not what I want, the experiment or about which I have not studied their history.

I have a 0.1uF cap across the power supply because.... Tell me about these real estate taxes that apply in the land of the free, Jim ! Graham Woman on first trip to Europe, seeing a bidet: "Oh, is that for washing babies in?" Concierge: "No, madam, that is for washing babies in?" Concierge: "No, madam, that is for washing babies out." Cheers! Rich hmmm see here martin Guys thanks again for all your answers I just took that job.

Except it is in San Diego. The other relevant point which appears not to be a grandma I've seen that with our chicks recently, it's amazing to watch! Mary Harvesting - hunting - isn't the same as 5,000 years ago is niave All things are probably possible But its not a general given AGREED I have no idea how to drive the 281's pin.