DIY Current Probe

[Bob Minchin]

Why on earth does that method need a high impedance input????

[whisky-dave]

Getting a good connection is the first problem, then thre's the small curre nts involved and hence the small voltage drop, so you'll end up measuring m icrovolts and microamps.

[tabbypurr]

due to an excess of drugs maybe. Track drop measurement can be out by an order of magnitude, it has its uses but is largely too inaccurate.

NT

[The Natural Philosopher]

Needs a very LOW impedance very low voltage meter, though

[dennis]

That's a lot easier with a high impedance, even a few hundred ohms isn't going to have much effect on a 10 Mohm input. You could experiment with some conductive plastic rather than metal probes.

[dennis]

Don't be silly he is measuring voltage not current.

[whisky-dave]

the impedence needs to be very low and good connection to the surface of the copper without damaging it. Sure stick a pin in it why not, or course usually the track is covered in an insulative materail to protect it from oxidisation.

irrelivent to the probe as it has very low impedence.

Well when I see this sort of device in lidle we'll both understand. I've never seen a plastic probe for measuring low voltages/current I have a 20KV one for high voltages.

[whisky-dave]

from the manual.

Unlike a conventional current probe, the I-prober 520 has an extr emely low insertion impedance and negligible stray capacitance. This enables current observations and measurements to be made with out disturbance to the circuit.

[dennis]

Why does it need to be low impedance? Te current flow into the meter needs to be as low as possible, its the voltage you are measuring not the current.

See above.

Well you won't see one for current.

Just think about it..

you want to measure the voltage so you want as little current flow in your measuring device as possible ie. you want high impedance.

So if you start with a 10 Mohm probe and you add a few hundred ohms in series using conductive plastic its going to make sod all difference. However it will prevent damage.

If you still don't believe me then look at a DVM... low impedance shunt and high impedance meter to measure current and high impedance meter to measure voltage.

[whisky-dave]

Not sure but small impedences will affect the current as we are NOT measuirng voltage or current in the normal sense.

We were going to buy one here so I read the manual first, and decided it'd be too cpomplex for what we were going to use it for, so decided it wasnt worth the teaching budget money. But said it could be good for research so if they wanted to buy it on their research grant that would be OK, but I'm not going to push for us to get one on the teaching grant.

from the manual availble on-line at rapid E.

Unlike a conventional current probe, the I-prober 520 has an extremely low insertion impedance and negligible stray capacitance.

The I-prober 520 in use The magnitude of the signal from a positional current probe is critically related to its position relative to the conductor. The size of the conductor (e.g. the width of a PCB track) also has a significant effect. This means that the sensitivity of the I-prober has to be adjusted to match the track width when quantitative measurements are required. A calibrator within the control box enables sensitivity adjustment in conjunction with a calibration graph. The measurement result will also include other field effects present at the tip of the probe and not just that coming from the current through the conductor. This may include DC effects from adjacent magnetised components and from the earths magnetic field, plus AC effects from transformers and other field radiating sources. Current in adjacent tracks, or tracks on the opposite side of the PCB will also affect the measurement. There are solutions to these potential problems. The unwanted DC can be nulled out by observing the measurement without power to the circuit, whilst AC interference can be attenuated using bandwidth filters. The I-prober control box includes a wide range DC offset control and switchable filters. Nevertheless, the use of the I-prober

520 requires interpretation based upon a proper understanding of circuits and systems. It is a tool for the professional engineer.

doesn't that depend on how it works.

I did and that's why we don;t have one.

Only when using old style meters. Thsi uses a difernt methode and that is one iof teh reasons is expensive and overly complex to use for us.

I have over 200 DVMs well DMMs really. have 1/2 dozen LCRs too.

[The Natural Philosopher]

Which is why you will always be a technician.

[dennis]

Don't start being an idiot again if you know any good reason for us to believe what you said then spit it out.

Otherwise I am correct.

[dennis]

That might be why I said use high impedance then.

We are talking about a DIY solution to measuring the current. Ie. the track is a low impedance shunt across a high impedance volt meter. Its not rocket science.

The only variable is the impedance of the track. With the current probe there are dozens of variables that have to be taken into account. I would use a constant current source to inject a reference current down the track to calibrate whichever method I was using if it had to be accurate.

Having them and understanding them is different AFAICS.

[tabbypurr]

have you ever been correct?

[dennis]

I have always been correct when TNP disagrees.

[The Natural Philosopher]

Which is why you will never be a logician.

[The Natural Philosopher]

I think he's about random - one in around 20

[The Natural Philosopher]

And he will never be a philosopher either.

[dennis]

Up to your usual wriggles I see. Say I am wrong and when I ask you to show where you can't and just spout cr@p to hide your stupidity. Others have spotted that you keep doing this so you can give up now.

[Cursitor Doom]

I like the idea (in principle) of measuring the volts drop to derive the current. But the impedance of the meter is of secondary importance to the meter's ability to *differentiate* between two very, very, very similar voltages. So.... how about using a top-notch precision op-amp to vastly multiply this voltage differential, then calibrate with a precision current source? Might need a bit of tinkering but better for a DIY level job than lashing out 500 quid or more (in some cases a *lot* more).