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Old 12-24-2012, 01:40 AM   #1
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Basic Multi-meter Questions


Testing out my new Craftsman 82141.


I decided to test voltage of an AC wall power adapter (for my dustbuster)
The little plug block says "Output 15V DC 200mA"

I set my Vdc to 20 and got a reading of 1.
I set my Vdc to 200 and got a reading of 24.2

If this AC adapter is supposed to be giving out 15V, how can the multimeter reading be so far off??


Last edited by Mathlete; 12-24-2012 at 09:46 AM.
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Old 12-24-2012, 06:42 AM   #2
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Quote:
Originally Posted by Mathlete View Post
Testing out my new Craftsman 82141.


I decided to test voltage of an AC wall power adapter (for my dustbuster)
The little plug block says "Output 15V DC 200mA"

I set my Vdc to 20 and got a reading of 1.
I set my Vdc to 200 and got a reading of 24.2

If this AC adapter is supposed to be giving out 15V, how can the multimeter reading be so far off??
Which terminals of the mm were the leads plugged into?

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Old 12-24-2012, 06:57 AM   #3
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Basic Multi-meter Questions


hey, the voltage spec on the transformer maybe stated under load. most voltages from wallwarts are high with no load on them when measuring.

make sure you have ac and dc straight when measuring. most meters set on dc or ac by the operator (you) and applied to the other source (meter set to dc and trying to input ac will read 0) the meter is saying when set on dc NO DC. that does not mean there is no ac. 0 dc means no dc, not 0 voltage as the circuit may be ac. and visa versa.

always think when using a meter . keep in mind to measure voltage the meter probes are touched to the circuit. to measure current the meter has to be inserted into the circuit. (unless a clamp on). you have to physically break a circuit to measure current

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Old 12-24-2012, 07:16 AM   #4
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Quote:
Originally Posted by Mathlete View Post
Testing out my new Craftsman 82141.

I decided to test voltage of an AC wall power adapter (for my dustbuster)
The little plug block says "Output 15V DC 200mA"

I set my Vdc to 20 and got a reading of 1.
I set my Vdc to 200 and got a reading of 24.2

If this AC adapter is supposed to be giving out 15V, how can the multimeter reading be so far off??
When there is no load on the adapter, you are measuring at the end of the plug that goes into the device (the Dustbuster) and is now otherwise loose, the voltage you measure may be somewhat higher than the stated output. Under normal operation of the device the voltage will be roughly as stated.

Your meter apparently uses "1" to stand for "error" such as voltage outside the range you selected.

(AC adapters for digital devices are probably more accurate in terms of output voltage particularly when the wall wart supplies the +5, -5, +12, etc. DC voltages common to computer circuits.)
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Last edited by AllanJ; 12-24-2012 at 07:18 AM.
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Old 12-24-2012, 07:18 AM   #5
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I agree, that's most likely no-load voltage and it's a cheap unregulated circuit. I'll check a couple here in the office and see how they react.
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Old 12-24-2012, 07:21 AM   #6
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[Board generated a double post for me)
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Last edited by AllanJ; 12-24-2012 at 06:21 PM.
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Old 12-24-2012, 07:30 AM   #7
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I checked 3 adapters here, 1 of which is for an expensive thermal imaging camera. 2 of them are regulated so they were right on rated output voltage while the 3rd was an obviously cheaper unregulated one. It is rated 12vdc 1.0amp but was reading a hair over 17vdc.
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Old 12-24-2012, 09:53 AM   #8
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Quote:
Originally Posted by brric View Post
Which terminals of the mm were the leads plugged into?
My probes are plugged as follows:
Black = COM (middle)
Red = V.Ohm.mA (right)



Quote:
Originally Posted by bernie963 View Post
make sure you have ac and dc straight when measuring.
Wait, this may be it.
I am testing a house current, so it should be AC.
I had it set on VDC-200 on the dial (upper left)
This is what gave me the 24V reading.

So, I need "VAC" on the upper right, yes?
However, when I put it to VAC-200, I get 0.00 (After briefly spiking to 8v for a second)
Does this make sense?
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Old 12-24-2012, 10:09 AM   #9
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Be sure to select AC or DC as needed for volts measurements. Using the wrong selection gives unpredictable results. Wall wart power supplies come in both AC output and DC output versions.

The 200 ma labeling means that the probes in the center and right terminals can be used for in-line amperes (current) measurements up to one fifth of an ampere (200 ma).

The left terminal is only used for in line amperes measurements up to 10 amps and you need to keep the test brief and wait 15 minutes before doing another test.
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Old 12-24-2012, 10:31 AM   #10
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Your 20V range is for voltages up to 20V....if you go over that, it will display a "1" indicating over range.

You should always start on the high range and work down.
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Old 12-24-2012, 10:34 AM   #11
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Quote:
Originally Posted by Mathlete View Post
My probes are plugged as follows:
Black = COM (middle)
Red = V.Ohm.mA (right)

Wait, this may be it.
I am testing a house current, so it should be AC.
I had it set on VDC-200 on the dial (upper left)
This is what gave me the 24V reading.

So, I need "VAC" on the upper right, yes?
However, when I put it to VAC-200, I get 0.00 (After briefly spiking to 8v for a second)
Does this make sense?
Your testing the output voltage of the device ... your output is DC not AC. You need the meter set to DC volts in the correct range. If the meter is auto ranging it will do this for you. Yours is not.
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Old 12-24-2012, 10:36 AM   #12
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That's right, it said DC on the wall wart itself.
So, it gave 24V b/c cheap wall wars are unregulated?
I tried another one labelled "Output: 14V AC"
I got about 15V, so I guess it's working.
Thanks for the help.

Last edited by Mathlete; 12-24-2012 at 10:41 AM.
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Old 12-24-2012, 10:49 AM   #13
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Get some household general purpose batteries and test them ...ie.. AAA, AA etc... put the black probe on the - and the red probe on the + if the are fully charged you should read the stated voltage. Since polarity is relevant if you switch the leads you will get a negative sign in front of the voltage reading.
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Old 12-24-2012, 11:07 AM   #14
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Yea, I did that, and I also tried beep continuity on a fuse.
Resistance should be close to zero if there is continuity.
If there is a break, Resistance should be infinity, which shows up as 1 on the MM.

Looks like I can test voltage and continuity now. Cool.
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Old 12-24-2012, 12:43 PM   #15
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mathletes,

make sure you read and understand my first post this am. setting the meter to the wrong scale ac or dc can give a very false reading. set on dc and trying to measure household current which is 120 vac can read as 0 dc. (which it is) but not 0 it is 120 vac. thinking its 0 and really 120 cAN LEAD TO DEADLY RESULTS.

think always, double check your meter settings and understand them before touching any wires, we want you alive, not dead.

bernie

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