Bode plot measurement using only an oscilloscope

Look up Middlebrook's lab method. ...Jim Thompson

I usually do this with a network analyzer. Ok, not everybody has one but many PC sound cards reach past 20kHz which is sometimes good enough.

You absolutely have to do a pulse load test. I still remember when I did that on a "vetted" design last year because it didn't look quite kosher to me. The results provoked words like "Oh s..t!".

Time domain is a lot easier. Squirt a little bit of low-frequency square wave into the sense node and scope the resulting output voltage transient. As you note, you can get about the same information by applying a square-wave load current, with is less invasive. All you really need to know is response time and damping.

We have a Kikusui power supply load box, which can combine a static load with a pulsed load. That's great for exploring the "load space" dynamics. You can make the equivalent with a function generator, a power fet, and a source resistor.

John

There are people who think you need FRA's to build reliable PSU's. My thoughts were a pulsed load test should reveal any stability issues. This is how I've always tested my PSU's.

For anyone else interested here is how.

"Overshoot as a Function of Phase Margin"

That and I leave them running all day at rated power in an elevated ambient temp. I have no desire to burn my house down or anybody else's for that matter.

The article was interesting and adds another inexpensive tool the box.

I still dont understand why they are so pricey? The HF VNA's for RF frequencies yea but the ones that only go to a couple of MHz?

The cheapest one I could find is the Circuit Sleuth. Base model $5k.

Even that is far more then what you need for testing PSU's.

There's free software around that turns a sound card into a VNA.

John

Those folks probably can't change a tire on their car and most likely can't decently barbecue :-)

Yup. I cobble together the needed power resistors to create a max load situation, a big FET and its gate goes to a Philips pulse generator.

That almost happened at a client. Someone else's design, and they left it running overnight. Suddenly the Shepherd of the night guard began to bark rather ferociously. Turned out the carpet underneath the power supply had begun so smolder.

The only mishap I ever had was a tiny pinhole leak that sprung up it the oil cooler pot for my load resistors. Well, it used to be a metal bucket for honey and I guess it was too old. It made a mess, took a while to clean that up, SAE10 everywhere.

I used to smoke Douwe Egbert's Amphora...

About a pound a week, until about 1993 ;-)

So I had an ample supply of tins for "load oil", and other bread-board needs. ...Jim Thompson

A pound a week? Yikes! As a student I had the occasional pipe with "Scandinavic Mixture" in there but a small can lasted many months.

Way too small. Anything under half a gallon would have fried up in most of my cases. I found that the honey bucket could take a kilowatt for about 10-12 minutes before the metal handle would become uncomfortably toasty and the whole thing began to smell. Sadly, recently I had to throw it out because it was too far gone. To me that was like when a little kid gives up his woobie ...

Back in those days I rarely dealt in anything exceeding 100 Watts. ...Jim Thompson

I use a five gallon pail of water for my induction heater tests. At 1kW power output (which might be 300W dissipated), it hardly gets warm, with only a gallon of water inside. As my 10kW tests progress, I suppose I'll want to fill it up to two gallons or so. And as the duration increases, I can patch in an oil cooler sized radiator. No biggie.

Water isn't handy if you've got live voltage on hand, but it's not completely useless. If you've got aluminum body resistors, you can water cool them quite nicely. I suppose you could seal a water jacket onto a vitreous enamel resistor, that'd be pretty sweet (100W resistor will probably handle over 1kW continuous?).

Tim

Water heater elements. Series/parallel them to make a useful resistance. If the energy dissipated gets pretty high, just run a garden hose in and siphon the hot water out.

Maybe do a load of laundry with it.

In coordinated development of source and platform, there are more tales of the platform self-destructing, with the power supply surviving, in my book. But then, power supplies are simple.....

RL

I did think about that but didn't want to risk it with a pump because all that could leak some day.

Problem with any sort of seal is that it inhibits a temperature exchange to the outside. Meaning like my honey pail it heats up over xx minutes and then needs forever to cool off.

Talking about platform self-destruction do you remember the *KABLAM* ad from an EE magazine in the late 80's? Rocket test, turned sideways, hit a building and exploded in a fireball. Li'l circle in the picture around a speck of all the flying debris with the words "That's our power supply". Further down they wrote that it fired right up on the bench.

:-)

"Dear, can I use the washing machine for a dummy load?"

"No! And who you calling 'dummy'?"

You should put your dummy load inside another bucket. Ask me how I know. :-(

Ed

Sno-o-o-ort ;-) ...Jim Thompson

By comparision to today. Drop one two inches onto a rug and it rattles like Santas bells, plug it in and phoof smoke!

Gotta love mass produced Chinglish shit.

You can always tell on forums who is from China.

Heres an example subject.

" Give me schematic art work bom for 5kw UPS NOW PLEASE URGENT"

else's

left=20

to=20

=20

Sure do. That was Abbot IIRC.