Two ammeters in parallel

Kirchhoff is your friend.

I think it's a bit more complicated than that.

Moving-coil ammeters are actually ammeters - the movement is proportional to the current. The current is split, so each will show directly the current passing through the coil and you can sum them.

However digital ammeters aren't really ammeters, they're voltmeters. They measure the voltage across a fixed resistance and, by ohm's law, determine the current. The problem comes when you put two in parallel, becuase the two resistances are effectively in parallel too. So you change the effective resistance the voltmeter sees and thus the ohm's law calculation is wrong. Effectively both voltmeters are across the same pair of parallel resistors, but each meter doesn't know the resistance has changed.

Now a decent digital ammeter will compensate for this effect (eg add components in series to isolate the measurement resistance), but that would depend on how they were constructed.

I don't know if this is a real problem, or one that they all compensate for. I think I'd want to do some calibration runs to find out.

Theo

You're thinking too hard and getting it wrong.

Of course adding the two results gives the correct answer.

While correct, you're a bit late to the party.

I can assure you that in fact everybody else was early.

I checked my invitation.

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

I agree that when the Sergeant-Major asked for volunteers, the rest of us all took a step back.

But there it is, that's the army for you.

Into a non-inductive load.

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 snipped-for-privacy@btinternet.com wrote: []

Fascinating, thanks.

Indeed. A development (to alternators) for which I'm very thankful!

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.

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

Errmm.. Why did you have smoke coming from your starter motor don't you mean your dynamo?...

(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.

Paul

Yes I did mean dynamo.

Brain fade!

Jim