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Use of electronic meters

by eightsquare
Tags: electronic, meters
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eightsquare
#1
Sep10-13, 08:13 AM
P: 96
I bought some equipment for carrying out some electronics experiments- a cell, wires, an ammeter and a voltmeter. However the ammeter and voltmeter are AC meters, while the cell will provide DC current. Will this damage the meters if I connect them in the circuit?
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eigenperson
#2
Sep10-13, 09:52 AM
P: 160
I doubt that you will damage the meters, though of course there is no way to know for sure what components are in there.

However, you will not get a meaningful reading. AC meters are for measuring AC, not DC.

You need to get an AC/DC portable multimeter. If you have any interest in electronics, it's an instrument you'll use for the rest of your (or at least its) life.
Drakkith
#3
Sep10-13, 02:48 PM
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You should be able to find a multimeter for a VERY low price nearly anywhere. Look online and get a used one if you're seriously lacking money.

Bobbywhy
#4
Sep10-13, 10:02 PM
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Use of electronic meters

eightsquare, Welcome to the world of electricity/electronics! Drakkith is exactly correct: Every person who intends to test, maintain, or repair anything electrical or electronic NEEDS a multimeter! I suggest you begin your journey by reading up on these essential instruments at this Wiki page:

“Multimeters are available in a wide range of features and prices. Cheap multimeters can cost less than US$10, while the top of the line multimeters can cost more than US$5,000.”
http://en.wikipedia.org/wiki/Multime...DMM_or_DVOM.29

Next, you need to get one. Just for one example, here’s one multimeter that sells for less than twenty bucks:
Equus 3320 Auto-Ranging Digital Multimeter
http://www.amazon.com/Equus-3320-Aut.../dp/B000EVYGZA

I myself, would invest more than that to get a higher quality, higher reliability, and higher accuracy. Many consider the “Fluke” to be the best brand name, but others are nearly equal. You might check “Consumer’s Reports” to read reviews before you purchase one. Also, you might talk with an experienced technician or engineer who uses multimeters for their suggestions.

If you have any more doubts or questions, return here to Physics Forums and post them. Members here are always willing to assist someone who is a true searcher.

Cheers, Bobbywhy
meBigGuy
#5
Sep11-13, 01:47 AM
P: 1,084
You can get a multimeter with leads that works "fine" for $5.00 at harbor freight.
http://www.harborfreight.com/catalog...t?q=multimeter
eightsquare
#6
Sep11-13, 06:12 AM
P: 96
Thanks to everyone for the replies! Okay so I think I should get a multimeter. Turns out the ammeter I bought can read BOTH AC and DC, so that's good. The voltmeter says only AC, but its super cheap, so I guess I should just try it and see what happens? Also, when I connect these instruments in the circuit, what kind of resistor should I connect in series with the circuit? The ammeter reads up to 5A, and I suppose the cell will be 1.5V, so should I match the resistance so as to get around 2-3A?
meBigGuy
#7
Sep11-13, 06:47 PM
P: 1,084
You connect the ammeter in series with your load. It measures the load current. You do not add resistors for the instrument. In fact, there will be a small voltage drop across your ammeter since they are not 0 ohms.

If you put the ammeter across a battery it will draw a huge current. It is essentially a short circuit. Again, you connect the ammeter in series with the battery to your load. One wire (+) to the battery, one (-) to the load. It becomes the lead between the battery and the load (there, I said it three ways). Ammeters measure current flow from positive to negative.

You can connect a small motor as the load and watch how the current changes as you load the motor.

If you have a 10 ohm load and 1.5v the meter will read ..... you calculate it. How much power will be dissapated in the resistor?
eigenperson
#8
Sep11-13, 07:02 PM
P: 160
Quote Quote by meBigGuy View Post
You connect the ammeter in series with your load. It measures the load current. You do not add resistors for the instrument. In fact, there will be a small voltage drop across your ammeter since they are not 0 ohms.
However, you MUST add a resistor if the current through the ammeter would otherwise be greater than the maximum current it can handle. (Otherwise you could blow the fuse, if there is a fuse, or damage the meter, if there isn't one.) I think this is what he is asking.
davenn
#9
Sep11-13, 09:46 PM
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Quote Quote by eigenperson View Post
However, you MUST add a Shunt resistor if the current through the ammeter would otherwise be greater than the maximum current it can handle. (Otherwise you could blow the fuse, if there is a fuse, or damage the meter, if there isn't one.) I think this is what he is asking.
My Bold addition


Agree totally, and since he hasn't told us what current capability the meter has
we cant advise him of what shunt resistance would be appropriate
meBigGuy
#10
Sep11-13, 10:03 PM
P: 1,084
He said it is a 5A meter.

Basically you just connect the ammeter in series with your load and it measures the current. If the current it too large for your meter, get another meter or measure it a different way.

A shunt resistor across an ammeter? NO WAY. A series resistor you measure the voltage across? Yes.
eightsquare
#11
Sep12-13, 07:52 AM
P: 96
What if I use an electrolyte with a small cell? Will the electrolyte have enough resistance to prevent the ammeter from getting damaged?
ehild
#12
Sep12-13, 09:58 AM
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Never connect an ammeter directly across a voltage source. Ammeters are designed to measure current flowing through an element, resistor for example. For that, you have to connect it in series with the element.
Voltmeters measure voltage across an element, for that you need to connect it parallel with the element.

ehild
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meBigGuy
#13
Sep13-13, 05:19 PM
P: 1,084
An ammeter appears as a short circuit from its input lead to its output lead. It's purpose is to measure the current passing through it as if it was not there. All ammeters have some very small resistance, and so cause some voltage drop. (That is a basic "instrumentation problem").

A voltmeter appears as a open circuit between its leads. It's purpose is to tell you the voltage without drawing current. All voltameters draw some small amout of current. (That is another "instrumentation problem").

So, if you connect an ammeter across a battery, you have essentially shorted the battery and a huge current will flow.
davenn
#14
Sep13-13, 08:52 PM
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Quote Quote by meBigGuy View Post
..............
A shunt resistor across an ammeter? NO WAY. A series resistor you measure the voltage across? Yes.
Obviously you have never learned in electronics courses how to change the shunt across an ammeter to increase its current reading capacity!!

Dave
meBigGuy
#15
Sep13-13, 09:17 PM
P: 1,084
Obviously you have played in school with an ammeter with calibrated shunts and think such an approach is applicable to random meters. In order for that to work one needs to properly match the shunts to the meter. If you are going to do that, it's much easier to just measure the voltage across a resistor of your choosing.

BTW, I like the anti-gravity joke.
eightsquare
#16
Sep13-13, 10:08 PM
P: 96
Thanks for all the replies. I've got the necessary answers.
davenn
#17
Sep14-13, 03:34 AM
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BTW, I like the anti-gravity joke.
LOL yeah
I have had 3 people PM me over the last 4 months asking me where they can get a copy haha


Dave


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