Crazy multimeter, need help!

[tehfunk]

So, I was using my multimeter yesterday I first was testing the voltage of a 9v battery, and then I was gonna test the current across a resistor, and the multimeter starts giving readings that I know are way off and it won't hold either, the reading keeps changing. So, I think maybe I'm doing this wrong, so I switch back to dc voltage mode to just make sure it's working and it starts off at like 200 something millivolts, and I put it on the battery and it starts increasing gets to like 300 and goes back to 200, when a few minutes before it read 9.12 volts. it seems that it wont read anything by the probes (i think thats what theyre called) because it wont even do the continuity or capacitance, but then when i insert a resistor or capacitor into the little tester slot it gave me a correct reading. Does anything know whats wrong? I'm confused, please help. Thanks!

-Sam

[Minion]

Have you tried changeing the battery in the DMM???

[tehfunk]

yes i did  .

[Roobin]

You might want to check where you placed your DMM in your circuit. Positioning depends on what you're measuring. If you're measuring current, you will want to put your DMM in series with the resistor and battery. (This makes sense - you want to measure the current in the wire, thus the ammeter (which we assume to be near perfect thus no loss) has to be in series, otherwise we would not be measuring the 'whole' current). If you're measuring voltage, you want to measure the potential difference. It's the 'difference' that is important here - voltage is a measure of potential of one point relative to another. Thus, we place the DMM in parallel to the component we are measuring. So, one DMM lead goes to ones end of the resistor, one goes to the other end.

Also, I'm not sure what circuit you've set up, but if it is just a resistor and 9V battery, you need to be careful. The resistor basically determines the current that flows in the circuit, and in this case through the battery. Too high, and it'll cause your battery to heat up and do weird stuff. So, you want to choose a resistor that will limit current. So you might pick a large resistor. However, there's a caveat here - too high a resistance, and it'll mess up the input impedance of the DMM. If it's too high, your reading will look weird. (Imagine reading voltage. DMM in parallel with a resistor. If the DMM Resistance is 10MOhms and the resistor 1KOhm, there will be very little current through the 10MR, so you're ok (current is inversely proportional to resistance). However, imagine 2 10MOhm resistors in parallel. The DMM is now indistinguishable from the resistor, and an equal current will flow through both.) I would imagine a resistance in the order of KOhms to be ok (however I'm just hypothesising).

As far as the continuity/capacitor testing goes, the continuity tester is useful for debugging circuits. If you turn to the continuity setting, and touch the tips of the probes together, you should hear a beep, indicating continuity. Try it with a piece of wire. The capacitance setting measure capacitance. (The slots do the same thing).

[tehfunk]

You might want to check where you placed your DMM in your circuit. Positioning depends on what you're measuring. If you're measuring current, you will want to put your DMM in series with the resistor and battery. (This makes sense - you want to measure the current in the wire, thus the ammeter (which we assume to be near perfect thus no loss) has to be in series, otherwise we would not be measuring the 'whole' current). If you're measuring voltage, you want to measure the potential difference. It's the 'difference' that is important here - voltage is a measure of potential of one point relative to another. Thus, we place the DMM in parallel to the component we are measuring. So, one DMM lead goes to ones end of the resistor, one goes to the other end.

Also, I'm not sure what circuit you've set up, but if it is just a resistor and 9V battery, you need to be careful. The resistor basically determines the current that flows in the circuit, and in this case through the battery. Too high, and it'll cause your battery to heat up and do weird stuff. So, you want to choose a resistor that will limit current. So you might pick a large resistor. However, there's a caveat here - too high a resistance, and it'll mess up the input impedance of the DMM. If it's too high, your reading will look weird. (Imagine reading voltage. DMM in parallel with a resistor. If the DMM Resistance is 10MOhms and the resistor 1KOhm, there will be very little current through the 10MR, so you're ok (current is inversely proportional to resistance). However, imagine 2 10MOhm resistors in parallel. The DMM is now indistinguishable from the resistor, and an equal current will flow through both.) I would imagine a resistance in the order of KOhms to be ok (however I'm just hypothesising).

As far as the continuity/capacitor testing goes, the continuity tester is useful for debugging circuits. If you turn to the continuity setting, and touch the tips of the probes together, you should hear a beep, indicating continuity. Try it with a piece of wire. The capacitance setting measure capacitance. (The slots do the same thing).

Thank you for that help I read it a couple of times to get it down, I have one question, since I was trying to measure current with just a resistor and a battery how would i put the dmm's probes in series with the resistor and battery? I guess I was putting the probes in parallel to the resistor because I had the probes on either end of the resistor. I am a little confused about the impedance and all, I was using a 1 M resistor. About the continuity and capacitance, I understand their function, I was just trying to point out that after I couldn't get proper readings of voltage and current, I went to try the continuity setting to make sure everything else was working and it wasn't working either and same with the capacitance from the probes. But, when I put a capacitor into the little slots in my dmm, it gave proper readings. So, somethings wrong with the typical probing testing. Is it because of the resistor I used or something I did?

[Roobin]

With the DMM probes, you literally make a circuit with the probes acting a component 'leads'. So put one on one terminal of the 9v, another touching one leg of the the resistor. The touch the other end of the resistor to the other battery terminal. Hey presto! The 1M might be ok - I don't know the specs of your DMM. You could try experimenting with the values of the resistor to see when it does become large enough to affect the reading on the DMM. If you measured the resistance of the resistor using the DMM, you could then calculate the theoretical current, and see how it compared to the actual current.

[frank_p]

Voltage is a difference of energy potential.  So, it's like using a ruler: a reference point and at the other end, a value in relation to that reference.  The ruler is in parallel to the real distance.  The voltmeter is in parallel to the difference in potential energy.  It can be there but not doing any work (a battery that is not connected.

So if you were to measure the height of a fall it would be a potential for the water to acquire speed (cinetic energy).

If you wanted to make electricity from the falling water, you would have to make it pass in a turbine.  So the amount of water per unit time would be like current.  Instead you have electrons per unit time.

So for measuring current you have to visualise that the flow of electrons, ALL HAVE TO PASS trough your measuring apparatus, the ampmeter.  So it has to be "in line" (or series) with the potential source (voltage) and the thing that will do some work (a heating resistance).

The more you have potential (voltage) the more you have "pressure" on the restrictive object (resistance) to make electron flow trough that resistance.  Thus, the more you have potential to do some work (more heat in the resistance and more electrons per unit time trough your resistance and your ampmeter).  To measure all the electrons that are passing in the resistance the ampmeter have to be on the same path as the resistance in question.

So:
Ampmeter is like a tube that measure flow.
Voltmeter like a ruler that measure height.

[tehfunk]

thank you for all that help, I seriously think it's broken now, I mean I've used it before, I know how to use it, at least I thought I did  . So, now the question is, is it possible that I broke it simply by taking a 1 m resistor and a 9v battery and measuring current incorrectly, is that possible?? I mean, the thing won't even register continuity or measure resistance or capacitance anymore. Any ideas on whats up? Thanks.

[frank_p]

No I don't think that a 9V battery would do any harm to a multimeter.  It would be really weird.

[tehfunk]

wow, that is really weird. I noticed these two replacement fuses in the battery compartment so I said wth why not, I did it and as far as I know it's working again. I guess I must have blown a fuse. Can someone elaborate on how you blow a fuse. By the way the little red (+) probe, at the slot says, 1000v DC, 750V AC, 400mA max, and the black probe (-) says 500v max, I could not imagine that I exceeded any of those. So what happened?

[frank_p]

Too much current.  Not too much voltage.
You meaured your battery voltage with current setting proabably.
Since the intern resistance of the ampmeter is small you had a short (current going very high).

V/R=I
9V/O =  infinity

R = intern R of ampmeter  : you used it as the load

[Paul Perry (Frostwave)]

The other source of multimeter problems, is a broken cable.
You would be surprised how long it took me to realise my cable had an intermittent break 
But I'm sure frank_p is correct about what has happened here..

[tehfunk]

ok thanks for the help guys, one last question to close this up, say I wanted to measure the current coming through a certain resistor from a 9v battery would it go like this?

9v (+) - resistor - (+) probe - (-) probe - 9v (-)

I know you can use ohm's rule, I just want to know this.

EDIT: By the way, I must say, I'm really surprised how easy it was to break my multimeter.

[PerroGrande]

Your multimeter may contain internal fuses designed to protect the meter circuitry in case of too much current.

[frank_p]

9v (+) - resistor - (+) probe - (-) probe - 9v (-)
EDIT: By the way, I must say, I'm really surprised how easy it was to break my multimeter.

Yes, you just forgot your measuring equipment:
9v (+) - resistor - (+) probe - ampmeter - (-) probe - 9v (-)

Note: you can put the ampmeter anywhere in the loop since the current is the same everywhere.  Test it you'll see.

PS: you didn't break your DMM.  It's a common error to do so when you are learning.  Only, when you blow the fuse three times in a week, then you have to revise the concepts.

[oldschoolanalog]

I have one question, since I was trying to measure current with just a resistor and a battery...

Since the resistor is a known value, and the the battery is a known value...
You don't need a meter. You need a calculator.
Ohms Law.