Dave Plowman (News) wrote:
In article ,
newshound wrote:
Managed to resist the (naked) battery SDS at Lidl for £60. They had some
batteries and single and double chargers. Bought a digital clamp meter
for £13. Only AC amps (to 600) but has separate leads for AC and DC
volts, continuity/diode, ohms and (unusually) capacitance and Hz. Nice
fast, stable 4 digit display.

DC clamp meters tend to be rather expensive and none too accurate, sadly.
At lowish currents, anyway. Would be very useful on a car to get an easy
accurate measurement of a small(ish) current.


They can use Hall Probes.

https://www.electronics-tutorials.ws...ll-effect.html

There is no particular reason for them to be
expensive. It's a heavily doped piece of
semiconductor material.

Magnetic field goes in one axis.

Constant current is pumped along a second axis.

The third axis develops a voltage, which can be
buffered with an opamp before being measured with
a dual slope converter (for cheapness). It follows
the Right Hand Rule you're taught in physics. On a
clamp on ammeter, they've already arranged the jaws
to feed the Z-axis for you. Magic.

Hall Probes aren't particularly sensitive. A lab-grade
electrometer can measure currents below a picoampere.
The clamp-on DC ammeter is probably 12 orders of magnitude
less sensitive.

Where the DC ammeters excel, is measuring heavy currents without
heating effects. When I wanted to check the starter on my car,
the peak hold on the DC clamp meter could tell me the max
current was 150 amps. And nothing in the measurement kit
gets warm while doing so. A conventional multimeter, has in
small print to "not be measuring 20 amp loads without allowing
sufficient cooling time (duty cycle)". A DC clamp meter has
no such limit. It can measure a heavy current, for as long
as the nine volt battery in the meter lasts.

There is some drift in the offset voltage of the Hall Probe
and buffer amp. That's why typical DC clamp meters have
a "Zero button" on them. Part of the zeroing problem could
be attributed to the magnitude of current run through the
semiconductor detector. The one I set up for physics lab,
we were using 250mA for that, which is a lot more than a
hand-held clamp meter uses. And that's going to cause some
die heating (resistive loss). But since the sensitivity, it's
a multiplier, and multiplies the constant current times
the mag field, the higher the current, the more output
voltage. Then, if your stupid move caused a big-assed
drift, that's your fault :-) The clamp-on ammeter has
some different tradeoffs to consider. And battery life
precludes using large constant current values. The battery
budget might only have 1mA for the sensor to use, and
6mA for the meter chip.

Paul