# Two ammeters in parallel

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?
Examples:
5.15A and 10.30A
3.32A and 7.31A
Bill
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. Lawrence
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.
Jim
On Sat, 5 Sep 2020 at 12:44:28, John Williamson []
Fascinating, thanks.
Indeed. A development (to alternators) for which I'm very thankful!
<snip>
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 voltage.
On Sun, 6 Sep 2020 00:30:45 +0100, RayL12 snipped-for-privacy@tiscali.co.uk> wrote:
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.
Jim
I don't understand what you did. 1. I thought dynamo control boxes limited the output to 22A 2. Why had all four glow plugs failed on the same occasion? 3. Why did your starter motor smoke?
Bill
In article <n625H.302661\$ snipped-for-privacy@fx01.am, 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 snipped-for-privacy@f2s.com wrote:
(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?
John
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).
Paul
Yes I did mean dynamo.
Jim
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.
Jim
Instead we get days of discussion in a group that mostly doesn't understand the basics of electronics, yet insists it does.
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 less.)
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 ?
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.
Paul
So re-reading your text, you use the dynamo to start the engine?
Bill
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.
NT

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