Specification of current reading on multimeter

I am in the UK. I have a £20 digital multimeter and would like some advice. The meter is a Maplin model PG10B (order code GW18U)

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(Q1) Under "DC CURRENT" my user guide says, "voltage drop: 200 mV". Is this the disturbance in a circuit when measuring DC current?

(Q2) If so, then does that 200 mV figure mean that the potential difference for the rest of the circuit will be reduced by 200 mV once I have interrupted the circuit and inserted my meter for DC Current measurement?

(Q3) Isn't that figure of 200 mV rather high? I mean, if I use the 2 mA or 20 mA scale in an electronics circuit powered by a 1.5 volt cell then a 200 mV drop in the circuit is very significant.

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My real need is to measure current while charging a AAA NiMH cell.

When I set my multimeter to measure DC Current and insert it in the circuit, the charger's light fails to come on!

It's as if the prescence of the meter is disturbing the circuit too much.

The voltage of my uncharged cell might typically be 1.1 volts and the charger says it delivers up to about 120 mA.

(Q4) Is my multimeter simply too low-spec to measure current in this application?

nikk

Reply to
nicola
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Yes, at full scale. (You didn't say what the range was.)

The current ranges are probably implemented by switching the meter to its

200mV range and shunting the meter with a resistor that will produce a 200mV drop for that current range.

So, for example, the 10A range uses a 20 milliohm shunt, while the 2mA range uses a 100 ohm shunt. The actual drop varies linearly with the current, of course.

How much this disturbs the circuit depends on the circuit's impedance. If the charging light comes on when you're not using the meter, but doesn't come on when you are, then, by definition, you're "disturbing" the circuit.

If you charge a lot of nicad and NiMH cells, get a MAHA (Powerex) MH-C9000 charger. Not cheap, but very convenient.

Reply to
William Sommerwerck

Buy a precision (0.1%) 0.1 ohm resistor, place it in the line, and read the voltage across it and extrapolate what the current is.

With that figure, you should be able to calculate charging voltage, battery internal resistance, and charge state.

Perhaps you were testing on a fully charged battery. OR you did not place the current meter in the circuit properly.

The Phil retard's response can (should) be ignored by you, since he did not actually provide any real assistance. Typical of him.

Reply to
Archimedes' Lever

It is worthless if it will not accept C cells, which are quite readily available now.

Pretty sad that I did not see any features that would make it worth 3 times what others go for either. No thanks.

Reply to
Archimedes' Lever

It is the volt drop across the meter at "full scale deflection".

If you measure 2mA on the 2mA range it will cause a 200mV drop. If you measure 2mA on the 20mA range it will cause 20mV drop.

Depends on the current you are measuring and the range you have selected - see above

200mV is a common figure for the DVM chips used in such meters. The chips measure voltage. A current is measured by passing it through a resistor (shunt) within the meter and measuring the volt drop across it
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Reply to
Stuart

Yes.

Only at full scale reading. Adjust pro-rata.

Pretty average.

Yes.

No, but you could measure the current further 'upstream' if possible to avoid the problem.

Graham

Reply to
Eeyore

MH-C9000

to you...

My electronic flashes and portable CD players, etc, do not accept C cells.

You weren't looking. You might not need the features, but it has features galore, including (but not limited to) the ability to select charging current, to measure cell capacity, to "refresh" cells, etc.

Thomas Distributing currently (ar, ar) has it on sale for $40. I'm going to get a second one.

Reply to
William Sommerwerck

My search found it at $37, but it still has problems. Most today auto select the charge rate based on the battery being charged, and yes, they can tell.

NiCads and nmih batteries to not get "refresh" hits, and the cell capacity is usually declared on the cell package or the cell itself.

I did not see a display on it, so I do not know how your claim of "measuring cell capacity" works.

Anyway, I am not convinced.

Reply to
Archimedes' Lever

The MAHA lets you choose the capacity. You don't have to settle for what the charger thinks is best.

A charger has no way of knowing a cell's capacity until it's measured it. Most chargers have separate "tiered" contacts, so that AA cells are charged at a higher rate than AAA. But that's the limit of their "knowledge".

Some people believe it's a good idea to periodically run down the cells.

Cells vary in capacity, despite their markings, and age differently, even when those from the same production run. One of several I bought from Thomas did not meet its spec'd capacity, even after several break-in cycles, and MAHA replaced it.

The ability to measure capacity makes it possible to sort out your cells so that every cell in a device "drops out" at about the same time. This reduces the likelihood of cell reversal.

It has an LCD that shows what's going on.

If _you_ don't need it, fine. But it's a terrific product. I'm glad I bought mine.

Reply to
William Sommerwerck

The watchdog chip in most all chargers is pretty damned good at deciding on the proper charging current.

Not true. It "measures it" as soon as it gets turned on (plugged in with batteries installed). Itdetermines the battery type, and begins its charging session based on its decision.

Nope. The battery type, not form factor determine the charge rate.

The internal resistance on the two sizes is the same, as is the final charged voltage. The only thing that would change is the charge period, and that is determined by the watchdog chip, monitoring the current and voltage involved.

Reply to
Archimedes' Lever

Sorry, but your charger does NOT send different voltages to each set of cell contacts.

The thing energizes ALL the contacts at once, and the sensing takes place on the bank as a group. As each battery is added, the only thing that changes is the time period before the "charged" indicator lamp lights.

Reply to
Archimedes' Lever

What parameters does it provide?

Well, I don't know if a simple LCD display warrants a triple price on the product though.

The damned things are not really worth much more than $12. Yours... maybe $16.

They really should be no more than a dongle supply would be.

Reply to
Archimedes' Lever

Nikk-

As Bill mentioned, switching to a higher current range may lower the meter's resistance to the point where the charger's light comes on.

Even though the charger's light fails to come on, do you read any current at all on the meter? If not, you may have blown a fuse that protects the meter's current ranges. If the fuse is only in the current circuit, the meter may function normally for all other functions.

Fred

Reply to
Fred McKenzie

Usually not True. Handheld meters current circuit usually uses a single shunt. That is the cheap ones. It won't matter what range he uses.

The more expensive, better made meters usually use two, and in that case a different range will place a different value shunt in the circuit.

I can't help thinking that he hooked it up wrong, because there is nothing about such a small resistance that should stop the charger's circuitry from sensing a battery in need of charge in place.

That is more likely. If it even has one. Again, the cheaper meters do not, and the better ones do.

Current metering that incorporates a fuse does so IN series with the shunt, and it is the shunt, and other meter circuitry, including the probe leads that the fuse 'protects' from over-current conditions.

I can still remember a co-student back in '76, placing a meter on the AC line with it set to current. It fried the Heathkit totally. It was only $36, but back then that was a lot for a student. Now, the fuse, which is nearly always incorporated in all but the cheapest meters, would open, protecting the meter guts.

Still, either shunt resistor should still be low enough in value to have no effect on the charger's sense circuitry. I am leaning toward there being a blown fuse. On the lowest setting, and ammeter can even pick up "current" from local induction, so even with the fuse blown, he

*may* see some numbers flickering across the display.
Reply to
Archimedes' Lever

HOW? Explain.

"knowledge".

You don't know what you're talking about.

Yeah. Uh-huh.

Reply to
William Sommerwerck

so

reduces

Your answer has nothing whatever to do with my remark.

Reply to
William Sommerwerck

The internal resistance of the battery, of course.

The different types of batteries have different internal resistances.

They vary due to charge level, but they are also different enough that the charger can tell the type of battery as well, and then uses the charging profile for that battery type.

You should never try to charge Nmih batteries and NiCad on the same charger at the same time. You should always charge each battery or each set, according to their type, separately.

Reply to
Archimedes' Lever

Bullcrap. I have both AAA, AA, and C size cells here, and the C size cells have the same ampere hour rating that the AA batteries do. Guess what that means to the charger, dingledorf?

Reply to
Archimedes' Lever

Maybe one day, you'll get it.

Reply to
Archimedes' Lever

On Sat 03 Jan 20:54, Archimedes' Lever wrote

I will try again to get a reading and see if I did not do it correctly the first time.

Reply to
nicola

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