Specification of current reading on multimeter

[nicola]

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)

http://www.maplin.co.uk/Module.aspx?ModuleNo=4402

(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.

-----

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

 

[Phil Allison]

"nicola"

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)

http://www.maplin.co.uk/Module.aspx?ModuleNo=4402

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

** Yep.

(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?

** Yep.

(Q3) Isn't that figure of 200 mV rather high?

** Nope.

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.

** Could be.

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.

** Could very well be.

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?

** Yep.

Maybe try using a Hall effect sensor - no voltage drop involved at all
there.

Or just be a tad smarter bear and measure the current at a point in the
circuit that is NOT so damn sensitive to small voltage drops.



..... Phil

 

[William Sommerwerck]

(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?

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.

 

[Archimedes' Lever]

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

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.

 

[Archimedes' Lever]

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.

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.

 

[Eeyore]

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)

http://www.maplin.co.uk/Module.aspx?ModuleNo=4402

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


(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?

Only at full scale reading. Adjust pro-rata.

(Q3) Isn't that figure of 200 mV rather high?

Pretty average.

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.
Yes.


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?

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

Graham

 

[William Sommerwerck]

If you charge a lot of nicad and NiMH cells, get a MAHA (Powerex)
MH-C9000

It is worthless

to you...

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

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

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

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.

 

[Salmon Egg]

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)

http://www.maplin.co.uk/Module.aspx?ModuleNo=4402

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

This probably means that that the 200 mV corresponds to the drop across
the meter required to give a full reading. If you do not need accurate
current measurement, increasing the current range setting will reduce
the voltage drop for the same 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?

Find the impedance of the meter by dividing the 200 mV drop by the full
scale current. Use that to cvalculate the drop for measured current.
Subtract that off of the applied voltage. That is then what is left for
the rest of the circuit.

(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.

Not necessarily. Higher resistance may be a way to increase sensitivity
for a low cost,

-----

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?

Could be.

nikk

Because electronic meters can vary all over the place, there can be all
kinds of peculiar nonlinear behavior that MAY kick in. That is not
likely, but possible.

Bill

 

[Archimedes' Lever]

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.

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.

 

[William Sommerwerck]

On Sat, 3 Jan 2009 07:17:07 -0800, "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.

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".

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

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.

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

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.

 

[Archimedes' Lever]

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

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

A charger has no way of knowing a cell's capacity until it's measured it.

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.

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".

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.

 

[Archimedes' Lever]

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.

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.

 

[Archimedes' Lever]

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

What parameters does it provide?

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

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.

 

[Fred McKenzie]

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!

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

 

[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.

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.

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.

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

If the fuse is only in the current
circuit, the meter may function normally for all other functions.

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.

 

[William Sommerwerck]

A charger has no way of knowing a cell's capacity until it's measured it.

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.

HOW? Explain.

"knowledge".

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

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

The internal resistance on the two sizes is the same...

Yeah. Uh-huh.

 

[William Sommerwerck]

The ability to measure capacity makes it possible to sort out your cells
so

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.

Your answer has nothing whatever to do with my remark.

 

[Archimedes' Lever]

HOW? Explain.

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.

 

[Archimedes' Lever]

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

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?

 

[Archimedes' Lever]

Yeah. Uh-huh.

Maybe one day, you'll get it.