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How to use a multimeter Options
darbyvet
#1 Posted : 23 November 2020 03:16:41

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A multimeter is a very versatile tool.There are tons of different ones to choose from.You dont need to spend a lot of money.Mine was $15. They all have the same basic set up. A display screen, a selector for the different functions, and some sockets for the probes you attach to your circuits.

Here is my multimeter. The selector has a bunch of options.You can measure voltage,resistance,current,continuity and some other less commonly used functions. You use this selector to tell the multimeter what you want to measure.

The bottom of the meter has 4 sockets to plug in probes.
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multi3.jpg
darbyvet
#2 Posted : 23 November 2020 03:23:36

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My multimeter has 4 probe sockets. The black probe is the ground (or common-COM) and the red probe is the positive.

If you are measuring voltage or resistance or frequency you plug the black probe into the COM socket (white arrow) and the red probe into the far right socket (red arrow) .

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multi4.jpg
darbyvet
#3 Posted : 23 November 2020 03:30:55

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If you are measuring current the black probe plugs into the COM socket (white arrow).

The red probe gets plugged into one of the 2 left sockets marked A and uAmA.
IF you are expecting current of less than 400ma you would use the right of the 2 red sockets (red arrow). This socket has a maximum of 400ma current based on the writing under the socket (blue arrow). you meter may have a different maximum current. THis socket has a fuse.If you attach this to a current load higher than 400ma it will likely blow the fuse and damage the meter.
If you are expecting current over 400ma you plug the red probe into the leftmosts socket (labelled A). For my meter there is a maximum of 20A current for no more than 10 seconds.If you leave the meter plugged in for longer than that the fuse will blow and the meter will be damaged.
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multi5.jpg
darbyvet
#4 Posted : 23 November 2020 03:52:03

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OK so now you know your way around a multimeter lets look at some practical uses.


There are 3 main reasons to use a multimeter;

1.Measuring Voltage

2.Measuring Resistance

3.Measuring current


Voltage requirements vary from circuit to circuit and LED to LED. If you are powering some LEDs you can use anything from 3-12 V if you attach resistors to the LEDs.If you are powering a microcontroller like an arduino you need 5V. You may find that you can find a battery or power supply that provides the voltage or you may need to use a voltage regulator to change the input voltage to match the voltage.The multimeter can check to make sure the voltage you are supplying is what you need.if apply too much voltage to you may damage the circuit or LEDs.


Measuring resistance is really useful if you are using a bunch of resistors in a project.Mark showed you how to read the bands on a resistor to determine the resistance.You can also use a multimeter to check the value of the resistor.I always do this before soldering a resistor onto an LED or circuit because it is a pain de solder and remove a resistor is you mess up.


Measuring current is extremely useful to determine what kind of power supply you need. If you have a circuit that draws 800ma of current and you power it with a battery rated at 800maH the circuit will drain the batter in less than 1 hour. if you buy a DC power supply rated at 1 amp and your circuit needs 2 amps of current the circuit wont work.SO if you can measure thr current draw of your circuit you can pick the best power source.A good rule of thumb is to use a power supply that is rated for at least double the current draw of your circuit because pwoer supplies are rated for maximum current draw and if you are close to that with your circuit the power supply may not be able to consistently provide enough current and weird things may happen to your circuit.
So say your circuit draws 500ma of current I would go for a 1 amp power supply.

darbyvet
#5 Posted : 23 November 2020 03:58:29

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Measuring voltage


Here I have a battery pack with 2 AA batteries in it.Now I am guessing this will give me 3v of voltage output.How do I check?

I use the multimeter with the probes plugged into COM and the V sockets and then attach the V probe to the positive output of the battery pack and the COM probe to the ground of the battery pack.I move the selector to V.On some meters you have to select a range of voltages you expect and some meters auto detect the voltage range. I get a reading of 3.2V.

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darbyvet
#6 Posted : 23 November 2020 04:05:05

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Measuring resistance


I have a resistor on my bench .I think it is 470 ohm, but I am too lazy to go online and check the color band coding.So I hook up the probes in the same way as measuring voltage and then switch the selector to resistance.I then connect one probe to one end of the resistor and the second probe to the other end (doesnt matter which way round) and I get 470 ohms. I always check my resistors especially if I am soldering them onto a circuit board because desoldering is a pain.
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resistance2.jpg
resistance21.jpg
resitance3.jpg
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#7 Posted : 23 November 2020 04:19:07

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Measuring current.

I made a small LCD screen that plays video clips.I want to use it inside a model.But I dont know how much current it draws so I am not sure how to power it.I know it needs 12v, but since this is a custom designed circuit there is no posted current draw.


Measuring current is a bit more complicated.Basically you have to make the multimeter part of the circuit for it to measure the current draw.

I am using a benchtop power to test the current draw. (we will ignore the fact that the benchtop power supply actually measure the current).

Here is the basic circuit
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multi6.jpg
darbyvet
#8 Posted : 23 November 2020 04:21:25

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To use the multimeter it has to be connected in series to the circuit.Here is the modified circuit with the multimeter incorporated.

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multi7.jpg
darbyvet
#9 Posted : 23 November 2020 04:23:02

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So here is the set up.The screen is playing video with the benchtop pwoer supply providing 12V DC.
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current1.jpg
current2.jpg
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#10 Posted : 23 November 2020 04:35:40

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The positive wire from the power supply is attached to the black probe plugged into the COM socket.The red probe is now plugged into the A socket (far left). When you are not sure how much current will be drawn it is safer to use the higher rated plug because remember the uAmA socket has a 400ma limit. The red probe is then connected to the positive input to the lcd screen.Current passes from the power supply through the multimeter and into the screen. The ground from the pwoer supply still goes to the ground of the screen.

So the power supply is saying 103ma and the multimeter is saying 101ma.Pretty close.

Now I know the screen draws about 100ma of current.So if I want to put it in a hand held prop like a Star Trek Tricorder or Space 1999 communicator I need a battery that can provide 12v with at least 200ma current (remember double the current draw).If I use a boost power converter and a lipoly battery rated at 800mah I could power the screen for several hours.

darbyvet attached the following image(s):
current3.jpg
current4.jpg
current2.jpg
birdaj2
#11 Posted : 23 November 2020 22:34:05

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Carl

That is a very useful and well written post.

Odd you should post about multi meters as i need mine and it seems i have “put it in a safe place” and i cannot find it for looking :-)

Tony
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#12 Posted : 24 November 2020 00:26:30

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Great addition to the topic Carl.... Appreciate the effort you've gone to in putting this together and I know it'll be very helpful to members, especially me....LOL LOL .. Thanks for sharing....

Regards

Alan
Markwarren
#13 Posted : 24 November 2020 09:04:23

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Great topic Carl. Most people know what a multimeter is, but very few really know how to use one.

Mark
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