How did I blow the fuse in my multimeter?

[Cyde Weys]

I blew the fuse on my Radio Shack multimeter recently. I was testing
the current across a 9V battery. I verified that it is just the fuse
that was blown by removing the fuse, shorting across the gap, and
verifying that the needle on the meter did indeed move on resistance
tests (no way am I gonna test anything other than ohms without a fuse,
hehehe).

Anyway, my multimeter has the following options on it: 15, 150, 1000
DCV, RX 1K(Ohms), OFF, 150mA DC, 1000, 150, 15 ACV. I believe I left
the battery attached to the leads when switching modes (is this bad?).
I did not go into any of the ACV modes.

So, can someone tell me how, with a 9V battery, I managed to blow a
315mA, 250V fuse? Thanks.

 

[ric]

So, can someone tell me how, with a 9V battery, I managed to blow a
315mA, 250V fuse? Thanks.

A 9v battery will easily put out more than 315mA.

 

[Cyde Weys]

Cyde Weys wrote:




A 9v battery will easily put out more than 315mA.

So this multimeter can't even measure the current of a 9V battery?
Laaaame ... what about a AA battery?

 

[Ken]

So this multimeter can't even measure the current of a 9V battery?
Laaaame ... what about a AA battery?

Did you put the meter in "ampere mode" across the battery?
Never do that! That's a big beginner misstake.

 

[Christiaan van Bladel]

Cyde Weys schreef:

I blew the fuse on my Radio Shack multimeter recently. I was
testing the current across a 9V battery.

When you say "across" , do you mean that you connected the black lead to
the minus of the battery and the red lead to the plus of the battery? If
so, this is not the correct way te messure the current, how ever it is
the correct way to messure the voltage. Mistakes are easely made.

Anyway, my multimeter has the following options on it: 15, 150, 1000
DCV, RX 1K(Ohms), OFF, 150mA DC, 1000, 150, 15 ACV.

The way i read this, your meter only supports current messuring
up to 150mA DC. So 315mA from the battery is enough to blow the fuse.
The rest of the ranges are used for resistor messuring and voltage AC
and DC voltages

Succes
Chris

 

[Cyde Weys]

Cyde Weys schreef:



When you say "across" , do you mean that you connected the black lead to
the minus of the battery and the red lead to the plus of the battery? If
so, this is not the correct way te messure the current, how ever it is
the correct way to messure the voltage. Mistakes are easely made.

So what is the correct way to measure current?

The way i read this, your meter only supports current messuring
up to 150mA DC. So 315mA from the battery is enough to blow the fuse.
The rest of the ranges are used for resistor messuring and voltage AC
and DC voltages

Yeah, that must be what I did. I must've had it in current measuring
and put it across the 9V battery and fried the fuse. Oh well. What is
the mA option for? Obviously it doesn't work directly across battery
leads ... is it for putting into a circuit that has resistance? I.e.
you could measure the current being drawn by an LED, but it won't work
if you just put it across the battery?

 

[Ken]

So what is the correct way to measure current?

You can't measure current in a battery.
You can measure current a load takes from a battery.

 

[S.Heenan]

So what is the correct way to measure current?


Yeah, that must be what I did. I must've had it in current measuring
and put it across the 9V battery and fried the fuse. Oh well. What
is the mA option for? Obviously it doesn't work directly across
battery leads ... is it for putting into a circuit that has
resistance? I.e. you could measure the current being drawn by an
LED, but it won't work if you just put it across the battery?

Imagine a simple circuit with a battery source and a resistor. Each leg of
the resistor is connected to the battery; one to the +ve side, one to
the -ve side. To measure voltage across (in parallel) the resistor aka load,
place the meter in DC voltage mode. Place the black probe to the the -ve
side of the load, red probe to the +ve side. Take a reading. That's the
procedure for voltage.
A voltmeter can be thought of as a resistor with an infinitely high internal
resistance.

Imagine the same circuit, a battery and a resistor. To measure current, the
ammeter must be in _series_ with the load. Set the meter to measure DC
current. Current will flow _through_ the meter, as if it is another resistor
in the circuit. In fact, it will be just that, a very low resistance. Open
the circuit by lifting the -ve leg. Connect the red probe to the now open
leg of the resistor. Connect the black probe to the -ve terminal of the
battery. The circuit is now closed. Current will flow. Take the current
measurement.

Voltage|in parallel|voltmeter like a 10G Ohm resistor.

Current|in series|ammeter is like a .1 Ohm resistor.

 

[kony]

So this multimeter can't even measure the current of a 9V battery?
Laaaame ... what about a AA battery?

What purpose is there in measuring current of a 9V battery? You want to
see if it's adequate for jump-starting a car?

Current consumed by a device is limited by that device, while current from
a shorted battery is limited only by the battey's internal resistance, so
from a small battery you may be looking at dozens of amps, and potentially
a vented battery (some call that "exploding") if left shorted.

No, you can't measure current of a AA battery either, except in circuit,
device it's to be powering.

 

[half_pint]

I blew the fuse on my Radio Shack multimeter recently. I was testing
the current across a 9V battery. I verified that it is just the fuse
that was blown by removing the fuse, shorting across the gap, and
verifying that the needle on the meter did indeed move on resistance
tests (no way am I gonna test anything other than ohms without a fuse,
hehehe).

Anyway, my multimeter has the following options on it: 15, 150, 1000
DCV, RX 1K(Ohms), OFF, 150mA DC, 1000, 150, 15 ACV. I believe I left
the battery attached to the leads when switching modes (is this bad?).
I did not go into any of the ACV modes.

So, can someone tell me how, with a 9V battery, I managed to blow a
315mA, 250V fuse? Thanks.

Sounds to me like you tried to test a huge current.
Current=voltage/resistance so the only resistamce in the circuit
is the interlnal resistance of the battery (very small) and the resistance
of
the multimrter (neglible) so you qould have say 9/0.01 ie 90 Amps,
probably a lot more in reality.

 

[half_pint]

So this multimeter can't even measure the current of a 9V battery?
Laaaame ... what about a AA battery?

The current from a 9V battery is a meaningless statement.
A 9V battery could supply a current on many thousands of amps.
same for an AA battery, dependant on the internal resistance of the battery.

 

[half_pint]

So what is the correct way to measure current?


Yeah, that must be what I did. I must've had it in current measuring
and put it across the 9V battery and fried the fuse. Oh well. What is
the mA option for?

mA is milli amps for measuring pretty small currents.
315 mA is less than 1/2 of an amp.

Obviously it doesn't work directly across battery
leads ... is it for putting into a circuit that has resistance? I.e.
you could measure the current being drawn by an LED, but it won't work
if you just put it across the battery?

I think LED's (like bulbs) have fairly low resistance too so
they might blow the fuse (or LED) if there is not a resistor in the circuit
to
limit the current.

http://www.makegizmos.com/ledlight.htm

 

[ric]

So this multimeter can't even measure the current of a 9V battery?
Laaaame ... what about a AA battery?

You never measure the current of a battery with a DVM. This only hastens
the discharge of the battery. To test a battery, measure the VOLTAGE
across it.

Why are you attempting to measure a battery's current?

 

[John McGaw]

I blew the fuse on my Radio Shack multimeter recently. I was testing
the current across a 9V battery. I verified that it is just the fuse
that was blown by removing the fuse, shorting across the gap, and
verifying that the needle on the meter did indeed move on resistance
tests (no way am I gonna test anything other than ohms without a fuse,
hehehe).

Anyway, my multimeter has the following options on it: 15, 150, 1000
DCV, RX 1K(Ohms), OFF, 150mA DC, 1000, 150, 15 ACV. I believe I left
the battery attached to the leads when switching modes (is this bad?).
I did not go into any of the ACV modes.

So, can someone tell me how, with a 9V battery, I managed to blow a
315mA, 250V fuse? Thanks.

If you are using the meter to measure the current flowing from a battery YOU
DO NOT set the meter to a current range and then connect the meter's leads
to the battery. This is effectively a short circuit if you discount the
small resistance of the leads and meter's circuitry and a fresh alkaline 9V
battery could probably put out over an Ampere in that mode causing the fuse
to do exactly what it was designed to do.

 

[nascartrekkie3]

Be thankful! The fuse did it's job. Without it, you'd be replacing the
meter.

I blew the fuse on my Radio Shack multimeter recently. I was testing
the current across a 9V battery. I verified that it is just the fuse
that was blown by removing the fuse, shorting across the gap, and
verifying that the needle on the meter did indeed move on resistance
tests (no way am I gonna test anything other than ohms without a fuse,
hehehe).

Anyway, my multimeter has the following options on it: 15, 150, 1000
DCV, RX 1K(Ohms), OFF, 150mA DC, 1000, 150, 15 ACV. I believe I left
the battery attached to the leads when switching modes (is this bad?).
I did not go into any of the ACV modes.

So, can someone tell me how, with a 9V battery, I managed to blow a
315mA, 250V fuse? Thanks.

 

[ChrisJ9876]

From: ric (e-mail address removed)

Date: 05/21/2004 2:47 PM Eastern Daylight Time
Message-id: <[email protected]>



You never measure the current of a battery with a DVM. This only hastens
the discharge of the battery. To test a battery, measure the VOLTAGE
across it.

Why are you attempting to measure a battery's current?

My first thought was that the OP doesn't know the difference between voltage &
current, and therefore shouldn't be messing with a multimeter.

 

[kony]

My first thought was that the OP doesn't know the difference between voltage &
current, and therefore shouldn't be messing with a multimeter.

... but at least the learning is being done with a 9V battery... not
impossible to kill oneself with a 9V batt but not very easy either.

 

[George]

On 21 May 2004 21:45:58 GMT, (e-mail address removed) (ChrisJ9876) wrote:

.. but at least the learning is being done with a 9V battery... not
impossible to kill oneself with a 9V batt but not very easy either.

You have no idea how many times I saw, in my college engineering labs,
students trace all their wiring, call over the professor (who, then, traces
all the wiring too), have everyone ponder it for about 5 minutes until
SOMEBODY finally realizes that either the power supply isn't plugged in or
isn't turned on.

 

[Raymond Sirois]

I blew the fuse on my Radio Shack multimeter recently. I was testing
the current across a 9V battery. I verified that it is just the fuse
that was blown by removing the fuse, shorting across the gap, and
verifying that the needle on the meter did indeed move on resistance
tests (no way am I gonna test anything other than ohms without a fuse,
hehehe).

Anyway, my multimeter has the following options on it: 15, 150, 1000
DCV, RX 1K(Ohms), OFF, 150mA DC, 1000, 150, 15 ACV. I believe I left
the battery attached to the leads when switching modes (is this bad?).
I did not go into any of the ACV modes.

So, can someone tell me how, with a 9V battery, I managed to blow a
315mA, 250V fuse? Thanks.

Testing current is done in series with a load, to determine how much
current a load is drawing from the source. If the ammeter is placed
in parallel with the load, there is a low resistance shunt resistor
inside the ammeter which allows ALL the current to flow through the
meter. That is what blew your fuse. You can test voltage across a
battery, but current flow is never tested across the terminals of a
battery. You must always have a load in the circuit when using an
ammeter.
Raymond Sirois
SysOp: The Lost Chord BBS
607-733-5745
telnet://thelostchord.dns2go.com:6000

 

[Raymond Sirois]

So what is the correct way to measure current?


Yeah, that must be what I did. I must've had it in current measuring
and put it across the 9V battery and fried the fuse. Oh well. What is
the mA option for? Obviously it doesn't work directly across battery
leads ... is it for putting into a circuit that has resistance? I.e.
you could measure the current being drawn by an LED, but it won't work
if you just put it across the battery?

LEDs draw an insignificant amount of current.
Raymond Sirois
SysOp: The Lost Chord BBS
607-733-5745
telnet://thelostchord.dns2go.com:6000