Wideband ammeter

Why bother? Measuring the voltage drop across the same device is easier, and just as fast, if not faster.

That depends on what you actually want to measure. And "wideband" makes it even more difficult.

Wideband current shunts made for AC-DC transfer are all of very special costruction and cost arm and leg. If you want to measure the voltage drop over those resistors, without making AC-DC transfer, you're up to another challenge, measuring the AC voltage. Should start from the definition, what IS the AC voltage? What the actual number your measurement shows means and so on.

Look at e.g. not all that precise but much better than most LT1088 chip, long obsolete. There is another one, proprietary and much better precision inside e.g. Fluke 5790A Standard (which is a misnomer -- it is actually an AC and DC voltmeter, 10x more precise that the venerable HP/Agilent/Keysight

But John hasn't spelled out any advantage that thermal imaging might offer.

Mainly because it's a small and specialised market. You need a resistive material whose resistance doesn't change much as it warms up and wideband means that you need something flat and compact.

Measuring AC isn't - in principle - different from measuring DC. You just have to do it more frequently.

Nothing to do with thermal imaging.

Many otherwise great DVMs have an AC bandwidth that doesn't even handle the audio range.

The resistor has great common-mode rejection too.

Fluke 5790A is good to 1MHz on itself, without the wideband option. The option makes it to 30 MHz but with limited voltage. The 5790A measures up to

However, with its accuracy and resolution it is not all that trivial to do precise measurements at higher frequencies. You wouldn't notice it at all on

The N-type side is connected to 5790A, the banana part goes into my Wavetek/Datron 4808 calibrator. 4808 stands right on the 5790A and the shortest cable that I can use is a custom made 8" thick LMR400 with right-angle N-type connector on the 5790A side. This cable is not noticeable.

Hoping to measure wideband AC current by measuring a voltage drop over regular resistor is simply insane.

This is what's used to measure AC current ACCURATELY:

That is if one wants good precise measurement. If it is to, say, plus/minus couple per cents, all that complication is not needed.

And that is up to something like 50kHz, maybe up to 100kHz tops. Higher frequencies require more sophisticated and significantly more expensive measures.

There is a skin effect in a surface-mount resistor (see

What's the big deal here? Just use a oscilloscope with a current probe.

Some so called true RMS dvms on AC ranges do not respond to DC.

Thermal measurement is inherently true RMS

Most such probes are AC-only, and none can clamp onto a PCB trace.

My current (in both senses) issue is to measure true RMS currents in a high-frequency isolated power supply. The idea is to have one STSPIN958 full-bridge make anti-phase 48-volt 500 KHz square waves that drive some number N of DRQ127 isolation transformers and then schottky bridge rectifiers, for some unknown N. Four would be nice.

I have an engineer, a kid right out of school, working on this. It's a great educational project. He'd never heard of core satutation, shoot-through current, diode reverse recovery, skin effect, The Devil's Staircase, the quirks of STspice, any of that practical stuff. And I'm teaching him how to Dremel and solder and shear and drill holes. This ain't bad for a first try:

My Tek one goes dc-20 MHz. It uses a Hall sensor and a regular current transformer.

Cheers

Phil Hobbs (In Austin visiting #1 daughter)

What model is that? Maybe we could put a little loop thingie on a board and clip onto that.

Check out the bats at sunset.

I lied—it’s 50 MHz.

Tektronix P6042 50MHz Current Probe

Cheers

Phil Hobbs

Rats, Phil. You have a better one than my P6016! It only goes to 17Mhz (at the 3db down point) and also requires the Type 131 current amplifier which I do have (although it seems quite happy to work without it for higher currents (will take up to 15A). I very rarely use it but it's really handy for those odd times when it is needed. Does the 6042 need a separate current amp?

It has all the signal conditioning built in.

Cheers

Phil Hobbs

Yeah, I kind of thought so. Much more recent than my ancient old thing. Mind you, it still works as advertised even after 60 odd years. Old Tek gear is hard to beat and I very much doubt anything come out of China today will still be around in 60 years from now.