I wanted to measure to consumption of a 12V pump. It was obviously going
to be more than 10A when working hard, but I only had 10A multimeters. I
used two meters in parallel, and added the readings. The two meters gave
different readings, so obviously there was some imbalance. But will the
summed readings I got be valid?
5.15A and 10.30A
3.32A and 7.31A
I have read some excellent analyses of the situation but nobody is stating the final answer to the problem clearly. Of course the total current will be the sum of the two measured currents but that will be very slightly smaller than the current without the meters. That is always the case.
Can I also take this opportunity to criticise the quoting of Ohm's Law. It is not R=V/I. R=V/I is always true for any circumstance, it is the definition of resistance. Ohm's Law is only true for metallic conductors at constant temperature.
A similar approach was taken for weighing clinically obese people where
the readings of a set of scales beneath each foot were added together
when the estimated weight exceeded the capacity of one set.
Cheers Alan. I didn't bother reading the rest as it was not my concern.
My concern was with the idea that 2 resistors in parallel can be put
into a single path and display 2 different voltage readings across them.
If both shunts are tied to the source and the ground they share the same
On Sun, 6 Sep 2020 00:30:45 +0100, RayL12 firstname.lastname@example.org>
But ITRW, don't we effectively have 3 resistors in series (lead, meter
lead) in parallel with the same?
So the shunts aren't actually directly in parallel?
Cheers, T i m
Yes if you add the resistance of the leads, connector, fuse etc. then
the voltage across each shunt may be very slightly different, assuming
the series resistance of the leads, connector, fuse etc. is different
for the two meters.
It doesn't alter the validity of adding the two meter readings together.
Irrespective of the extra series resistance of the items external to the
shunt the total current will still be shared between the two meter shunts.
What is possibly more important is the stability of the components in
series. Can the leads actually handle, say, 10A without rapidly heating
up (or burning up) and changing resistance as the reading is being
taken? I suspect the leads supplied with a no-name £5 DMM made in china
cannot. I suspect that the shunts in many cheap meters would also heat
up and change value thus allowing a sensible accurate reading only in
the first few seconds of operation.
There are other sources of measurement inaccuracy such as the
meters/shunts themselves reducing the voltage seen by the load and thus
reducing the current it takes. A quick voltage measurement across the
meters and a simple calculation would compensate for this but I suspect
in many real world cases this compensation would be quite small and be
less than the equipment's measurement accuracy.
There are some benefits from a dynamo too.
I have an old car with a dynamo. A couple of winters ago a neighbour who
was going away for Christmas knocked on my door and asked if I had any
jump leads (and I had). Apparently her diesel car had failed to start
and she had run the battery down trying.
I put my old car next to hers and linked the batteries, and as a
precaution I left my car idling. I tried to start hers and despite the
light coming on showing the glow plug circuit was operating and then
going out to show that it was ready for starting, the engine showed no
sign of starting when I turned the key. It was obvious that the glow
plugs weren't heating.
An alternator is current limited, and there is no way it can deliver a
continuous 100+ amps to turn a starter motor, but a dynamo wired through
an original control box has no such limitation. So I propped my car's
throttle giving somewhere around 2500 revs and then went to the
neighbours car, gave it a quarter throttle and turned the key and kept
it turned and I watched the rev counter as it started at just under 500
revs and as the compression warmed things up it gradually climbed the
scale until just about a minute and a half later it got to 1500 revs and
the engine suddenly fired up.
By this time I had smoke coming out of my starter motor, but it
obviously wasn't badly damaged because it has started my car ever since.
I told my neighbour that she wouldn't have any trouble restarting once
the engine had warmed up, but she needed the glow plugs repaired before
it would start from cold again.
In article <n625H.302661$ email@example.com, Indy Jess John <jimwarren
@OMITblueyonder.co.uk> scribeth thus
Errmm.. Why did you have smoke coming from your starter motor don't you
mean your dynamo?...
On Sun, 6 Sep 2020 at 13:36:20, williamwright firstname.lastname@example.org
(I'd always do that, I think.)
Yes, the going out is often just a timer, rather than detecting warming.
(I don't know anything about that.)
They'd probably failed gradually over time; depending on weather etc., a
diesel engine may start from cold with only three, two, or possibly even
only one glowplug still operating (I think in particularly warm weather,
it can with none). What probably happened here was they'd been
deteriorating, and the failure (to start) happened on the first really
cold morning. (The owner would perhaps have been experiencing needing
more and more cranking to start, and would be pleased at how well it
started after she'd had them all changed.)
I wondered that, then thought: is it possible that Indy's car uses the
starter motor _as_ the dynamo?
This is why we have Hall Probes and DC current flow measurement.
With Hall based solutions, the faffing about would have stopped
two days ago.
If you want to do it the old way with shunts, that's fine.
Nothing prevents you from making the shunts
"really large and adequate". Not like modern multimeters
today, which have the duty cycle printed right on the
casing of the instrument, as "proof of failure in design".
"Thirty seconds ON, Fifteen minutes OFF". My best multimeter
is 20A full scale, but it has the same duty cycle as the
other multimeters. It doesn't say, at what current flow
level it could operate continuously. (And shockingly, I just
noticed my cheapest meter only has a 200mA scale, which is
no fun at all.)
If you want to make a better front end, you can use a large
shunt (with low developed shunt voltage), then use a
chopper stabilized opamp for amplifying the voltage enough
so the multimeter volts scale can read it out. The chopper
stabilized opamp, flips between nulling out the offset, then
flipping back to mission mode and amplifying the signal.
Many times a second.
"Chopper stabilization enables the TS94033 amplifier to
provide high gain bandwidth, while reducing input offset
and 1/f noise."
And that's a high-side monitor, with a ground referenced output.
The chip has a nominal gain of 50 (fixed gain). Unidirectional
current sense (the output doesn't go below ground).
Go back to the original post. Only two 10A multimeters were available
and the OP asked if adding the two readings together was valid. What
answer would you have given? Yes? No? I wouldn't have done it that way?
Which would have been the most helpful answer?
Maybe it is supposed to. Mine clearly didn't.
The workshop manual describes the operation of the control box as a
voltage regulator "incorporating constant voltage control for the
charging circuit", with the contacts opening to prevent over-voltage
being output. The implication is that if I am taking a lot of amps out
of the dynamo, the voltage is unlikely to exceed the contact opening
voltage and everything the dynamo produces goes down my jump leads.
I assumed it was a circuit board failure or a blown fuse that stopped
them all working at the same time, but I have never looked at the way
they work to see the possibilities. I have seen a diesel tractor started
from cold using smouldering blotting paper as a glow plug, so it
obviously doesn't need anything complicated in a car to provide heat,
but it probably has associated computer linkage for the fault diagnostic
system to report faults, and circuit boards do fail.
Elsewhere in this thread someone has suggested that perhaps the glow
plugs failed progressively and when the last one failed the engine
wouldn't start. That is a possible alternative.
I have no idea which is correct.
It was the dynamo not the starter motor. I typed the wrong thing.
It was driving another car's starter motor (in parallel with my own
battery) for a lot longer than would normally be the case, and the
windings will have heated from the number of watts being generated.
Dynamos don't have much of an integral fan for cooling.
But you don't have to do it that way.
You can buy a 50A four point (kelvin connection) shunt,
flip your multimeter to volts, and measure up to a
50A load. Cost to you is the cost of a shunt. You can get
a panel meter that measures volts and connect that to the
shunt. In one case, there was a panel meter for amps, where
they said you could put a shunt in parallel with it, of
a certain type, to extend the range. (That was a panel
meter with a "200A" readout, for which it would almost
certainly never read 200.00. The max would be quite a bit
One of the panel meters for sale, is designed that
way and includes an external shunt. But being Amazon,
none of the units I reviewed have specs, and it's like
buying a pig in a poke. You can't tell if it's 10%
accurate or 1% accurate. There was one panel meter
with 4.5 digits and *no* accuracy spec. What's the point ?
Why have 4.5 digits unless you can brag about it ?
Maybe two of the digits read "00" all the time ?
Yes, you can add two meter readings together.
For the ops purpose, motor current, you don't need
1% accuracy. If it's off a bit, who cares.
But there are many ways to do this properly, they're
sitting on Amazon right now, and they have no specs...
You have to switch over to a real supplier, Digikey,
Element 14 or whatever, and find a supplier with specs.
You should be able to buy metrology with specifications
just like the specs that come with a multimeter. There
is usually a page or two with "+/-1% and 3 digits"
type specifications. You want to buy a product with
specs. Some day, you may need the accuracy.
These days a lot of consumer products have >1 sets of specs that contradict each other :)
But the ones to beware of are when they agree - then they're lies.
I can only hope the move away from China might solve this.
PS no need to buy a shunt, cut a food can. Metals change R as they get hot but are good enough for many jobs. Dunk em for better accuracy. And don't use ali for current shunts, the oxide is a problem.