yuo don;t need a constant current soucre ius part of the probe.
That's there's not a DIY solution using something from a disc drive you can cobbled together by a DIYer.
then what;s asked can't be done then can it, you'd be wasting uor time mess ing about taking disc drives apart in the hope of finding something you can use to teast the current flow in a PCB track, which is what the OP was hop ing to do.
if he;'s prepared to break a track and use an ordinary DMM then that's fine but that set-up IS NOT the same as the £500 probe.
Of course it can be done, just because you don't have the skills doesn't mean someone else doesn't have the skills. The fact the current probe exists is evidence that it can be done.
That's counter to requirements, notably the LOW impedance. What you
*don't* want is a meter path impedance that becomes comparable to the contact resistance where you're trying to pick a voltage reading from.
In this case, what you need is a meter impedance that's 3 or 4 orders of magnitude greater than any contact resistance you might expect to see (including the meter lead resistance). In this case, half an ohm contact resistance plus another half ohm test lead resistance suggests that a voltmeter impedance of 10,000 ohms should be sufficiently high enough to make any such contact and test lead resistance insignificant factors in measurement errors.
Compared to the 10 or 11 Meg Ohm DMM voltmeter impedance, this *is* a very low impedance voltage measuring instrument indeed but that doesn't make LOW impedance a desirable feature of such a voltmeter method of using a short section of circuit trace as a current shunt where you need to measure a volt drop virtually independent of the effects of contact resistance and measuring instrument impedance.
Making a voltmeter sensitive enough to detect microvolts might require a lower input impedance circuit than the standard 10/11 M Ohms 200mV FSD DMM circuit but this is only an acceptable compromise when we're talking about a 10 to 100 K ohm range of input impedance (about 2 or 3 orders of magnitude smaller than the standard DMM voltmeter impedance) for measuring microvolts in an extremely low impedance circuit.
If using contact spikes to pierce the solder resist mask is acceptable (you can always apply a blob of varnish over the holes afterwards), then a galvanic connection such as this represents the the cheapest and most accurate alternative to a 500 quid current probe.
However, the effectiveness does depend on what frequencies may be included in the DC component you're trying to measure and whether or not the trace in question is also carrying high frequency voltage transients which would require that the probe leads be effectively isolated so as not to introduce additional loading.
The classic solution to such galvanic connected probing is to use suitable stopper resistors at the probe tip to isolate the rest of the probe circuit and its leads from the circuit trace under test. You might need to use an op-amp in differential mode, one with a very high common mode rejection ratio where you tie it to a reference ground rail of the circuit under test, if necessary via an offset voltage to centre the working voltage range of the circuit to the mid point voltage of the differential amp's input voltage range.
You might even need to use a couple of DC to HF RF amps to create a differential probe amp with very high CMRR, again tied to the ground reference via a suitable offset voltage when you need to include DC measurements with very high frequency transients as per the op-amp example.
The complexity of your probing circuit rather depends on what sort of signals you're expecting to measure. As always, you're using a short section of circuit trace to act as an ammeter shunt where you're measuring the volt drop between the contact points, requiring that the voltage measuring circuit be relatively high impedance in order to diminish errors due to contact resistance variations as I'm sure you're only too aware. In this case, a measuring impedance of 10000 ohms or higher should more than suffice (no need to go to the 10/11 M ohm standard of a DMM).
If you really need a non-invasive means of probing, I doubt the idea of repurposing a GMRR read/write head is a viable proposition since I'm almost certain the heads include signal conditioning amplification that has no interest whatsoever in handling DC signals. It's either a galvanic probe or that 500 quid contactless probe with, afaics, nothing in between (BICBW - you might be able to google up a cost effective contactless probe solution if you search hard enough).
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