Questions about Using My Multimeter

[Alex Hughes]

So I'm fairly new to electronics. I took physics 2 in college and thought I'd give the hobby a try. I started with a very basic circuit. Just a power supply connected to a light-bulb. However, as simple as it was, I had several questions when taking the reading of my circuit with a multi-meter.

Question 1:

When I hook up my multi-meter as shown (I'm aware this isn't a good way to do it I'm just experimenting), I get a reading of 0.64A when I set it to measure current but the light-bulb goes out. Now I'm aware this is probably because the circuit is skipping over my light-bulb and just going through my multi-meter, but aren't the terminals to the light-bulb still in contact with the wire? Why would the current choose to go through the path of the meter and not through the light-bulb?

Question 2:

Now this time, I set my multi-meter to read voltage and it gave me a reading of around 3V which is what my power supply is set at. However, the light-bulb does not come on. Now I don't see how there is a voltage drop in the areas I connected my meter to. Does the meter somehow drop the voltage down when the current goes into it and that's why my light-bulb doesn't come on, yet I get a voltage reading? I'm so confused.

Question 3: (Last Question)

Ok, so in this circuit I hooked up my ammeter two different ways. The first reading gave me a current of 0.64A at a constant voltage from my power supply of around 3V. The second circuit also gave me a current reading but at 0.74A. The second way also caused my power supply to switch from C.V mode to C.C mode instantly. I understand that it probably reads more amps because the light-bulb is not drawing power from it, but why is my power supply making a slight hissing noise and switching to CC mode when I do this?

I think a lot of this would make more sense if somebody could describe to me exactly how a multi-meter works, and how it changes how it reads things when it goes from measuring voltage, to current, to resistance. Sorry, I'm a perfectionist and these aren't making much sense to me. Would love some detailed feedback. Thanks!

 

[lekh2003]

When I hook up my multi-meter as shown (I'm aware this isn't a good way to do it I'm just experimenting), I get a reading of 0.64A when I set it to measure current but the light-bulb goes out. Now I'm aware this is probably because the circuit is skipping over my light-bulb and just going through my multi-meter, but aren't the terminals to the light-bulb still in contact with the wire? Why would the current choose to go through the path of the meter and not through the light-bulb?

Now this time, I set my multi-meter to read voltage and it gave me a reading of around 3V which is what my power supply is set at. However, the light-bulb does not come on. Now I don't see how there is a voltage drop in the areas I connected my meter to. Does the meter somehow drop the voltage down when the current goes into it and that's why my light-bulb doesn't come on, yet I get a voltage reading? I'm so confused.

Ok this is weird.

Ok, so in this circuit I hooked up my ammeter two different ways. The first reading gave me a current of 0.64A at a constant voltage from my power supply of around 3V. The second circuit also gave me a current reading but at 0.74A. The second way also caused my power supply to switch from C.V mode to C.C mode instantly. I understand that it probably reads more amps because the light-bulb is not drawing power from it, but why is my power supply making a slight hissing noise and switching to CC mode when I do this?

Oh, this is very dangerous. You should not be plugging an ammeter in series in a circuit. The ammeter has (or should, and in this case obviously does) very little resistance and the current will short circuit everything in the circuit. Your power supply, gone, ammeter, gone, wires, on fire.

I seriously don't know what is happening with your multimeter. It is obviously malfunctioning. Furthermore, the ammeter doesn't even have a fuse in case you plug it in series. Please show us what ammeter you are using and what setting it is on.

Also, please do not continue experimenting without consulting with us or at least revising a little bit more about electricity.

 

[the_emi_guy]

What you need to understand is that an ideal ammeter shorts its two terminals, then displays how much current flows through that short.
Ideal voltmeter is an open circuit between its terminals, and displays how much voltage is across those open terminals.

Thus, you can't force current through an ideal voltmeter, and you can't force voltage across an ideal ammeter

 

[Borek]

Ok this is weird.

Nothing weird here, everything works as expected.

During measurements ammeter should be connected in series with the load, otherwise (if connected in parallel) it will "steal" the current, as it has a very low internal resistance. Voltmeter should be connected parallel to the load, as it has a very high internal resistance and - if put in series - will effectively block the current. Details can be easily explained using Kirchoffs laws and should be a part of every discussion of how to correctly use meters. Khan academy has a video on the subject (although he doesn't go into math).

 

[lekh2003]

Nothing weird here, everything works as expected.

During measurements ammeter should be connected in series with the load, otherwise (if connected in parallel) it will "steal" the current, as it has a very low internal resistance. Voltmeter should be connected parallel to the load, as it has a very high internal resistance and - if put in series - will effectively block the current. Details can be easily explained using Kirchoffs laws and should be a part of every discussion of how to correctly use meters. Khan academy has a video on the subject (although he doesn't go into math).

Ahhh, yes. I have forgotten my electricity just like the OP. I recommend that the OP follow Borek's post instead of mine.

 

[Alex Hughes]

Nothing weird here, everything works as expected.

During measurements ammeter should be connected in series with the load, otherwise (if connected in parallel) it will "steal" the current, as it has a very low internal resistance. Voltmeter should be connected parallel to the load, as it has a very high internal resistance and - if put in series - will effectively block the current. Details can be easily explained using Kirchoffs laws and should be a part of every discussion of how to correctly use meters. Khan academy has a video on the subject (although he doesn't go into math).

That makes so much sense, thank you very much. I'm still curious though on my third question onto why it switches to CC. Is this because I'm increasing the current since there isn't a load in my circuit other than the meter and it passes the max setting I allowed for my power supply? If this was the case, If I set the max allowed current of my power supply all the way up, would it possibly cause a short circuit and blow a fuse?

 

[Guineafowl]

Question 3: In the right-hand diagram, the ammeter is shorting out the power supply leads because it has a very low resistance. The power supply is limiting the current to whatever you set it to. The power supply current setting is a maximum level - in fact, the way to set this is to short the leads and turn the knob until the current is at your desired maximum. The supply will then limit any current to this level, even in the event of a dead short, by dropping the voltage. You should see this on the readouts.

 

[Guineafowl]

The ammeter measures current by being placed in series in the circuit. It passes current through a very low, but known resistance, say 0.01 ohm. It measures the voltage drop across this resistance (called a shunt). This means, because R=V/I, that the meter will measure 0.01 volts per amp flowing through the shunt. It converts this voltage reading to amps and displays the reading.

Because the ammeter is a shunt, or short, you should never place it directly across a voltage source, especially a non-regulated one. A car battery would make short work of your meter, and a mains socket is potentially lethal. Never leave the meter set in amps mode for this reason - get into good habits early and set it to volts, pulling the lead out of the amps socket, before you put it away.

The voltmeter measures by passing a very small current through a high resistance. If you place it in series with the circuit, it will block almost all the current and just measure the source voltage.

May I gently suggest you stick to your regulated power supply and simple bulbs till you have a better understanding of meters, supplies, and electricity in general? Questions 1 and 2 could be placed in a textbook as examples of exactly how NOT to use a meter. Experimenting and questioning are good things, though. I wish you all the best, and stay safe.

 

[Alex Hughes]

The ammeter measures current by being placed in series in the circuit. It passes current through a very low, but known resistance, say 0.01 ohm. It measures the voltage drop across this resistance (called a shunt). This means, because R=V/I, that the meter will measure 0.01 volts per amp flowing through the shunt. It converts this voltage reading to amps and displays the reading.

Because the ammeter is a shunt, or short, you should never place it directly across a voltage source, especially a non-regulated one. A car battery would make short work of your meter, and a mains socket is potentially lethal. Never leave the meter set in amps mode for this reason - get into good habits early and set it to volts, pulling the lead out of the amps socket, before you put it away.

The voltmeter measures by passing a very small current through a high resistance. If you place it in series with the circuit, it will block almost all the current and just measure the source voltage.

May I gently suggest you stick to your regulated power supply and simple bulbs till you have a better understanding of meters, supplies, and electricity in general? Questions 1 and 2 could be placed in a textbook as examples of exactly how NOT to use a meter. Experimenting and questioning are good things, though. I wish you all the best, and stay safe.

Thank you for your help, and I was aware that what I was doing was not what I was supposed to do, but figured since the voltage was low enough I wouldn't cause any harm. I was just curious on what the meter would say. I understand that I have to be safe and will only be sticking to low voltage circuits for now.

 

[jim hardy]

This is how we learn. Keep up the good work.

 

[jrmichler]

I was aware that what I was doing was not what I was supposed to do, but figured since the voltage was low enough I wouldn't cause any harm. I was just curious on what the meter would say.

Worst case is that you would have let the magic smoke out of your multimeter, in which case you would have performed a smoke test. Google the terms, all EE's know them.

 

[CWatters]

Tip: When you finish with a meter always leave it on a high volt setting. If you leave it on a high current setting then next time you go to use it (perhaps to check the voltage of a car battery!) you might forget to check and...

 

[davenn]

Tip: When you finish with a meter always leave it on a high volt setting. If you leave it on a high current setting then next time you go to use it (perhaps to check the voltage of a car battery!) you might forget to check and...

NO, turn it to the off position ... else next time you go to use it, the internal battery is likely to be flatD

 

[Borek]

NO, turn it to the off position ... else next time you go to use it, the internal battery is likely to be flat

Actually - do both I have an older multimeter with a separate on/off switch and dial for function selecting.

But thing that I really, really have to remember is to move the red cable from the A to V socket, and never to leave them in COM/A combination.

 

[lekh2003]

Actually - do both I have an older multimeter with a separate on/off switch and dial for function selecting.

Picture? I have only ever seen the new age easy to use ones. What does vintage feel like?

 

[Borek]

It is actually not THAT old, I got it second hand somewhere in nineties but I don't think it was older than 2-3 years at the time.

(Image stolen, no time for taking pictures ATM)

 

[davenn]

all my digital ones (4 or 5 different models) all have an off position on the rotary switch

eg my Fluke meters. My Fluke 87 is more than 25 yrs old
this is the model ( but not my well worn one haha)

Picture? I have only ever seen the new age easy to use ones. What does vintage feel like?

many of the modern ones have a separate power switch

modern one with a separate power switch ...

Dave

 

[lekh2003]

many of the modern ones have a separate power switch

modern one with a separate power switch ...

Well then I haven't been exposed to the entirety of the world of multimeters yet. Still a lot of years left for me to see all kinds of things.

 

[davenn]

It is actually not THAT old, I got it second hand somewhere in nineties but I don't think it was older than 2-3 years at the time.

(Image stolen, no time for taking pictures ATM)

ohhh I have one of them too ... different brand name on it

 

[CWatters]

Yup both of mine have a separate power switch.