DCmA readings of Multimeter



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equeen Regular
Joined: 04 Feb 2011 Posts: 384
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Posted: Sun Nov 11, 2012 4:51 pm Post subject: DCmA readings of Multimeter 


One more lesson for today, please.
I've never used the ACmA or DCmA feature of my Micronta Digital Multimeter. The manual was printed in Korea in 1982 with skimpy details especially for a nonelectrical person like me. Soooooooo.
If I tried to read amperage with it, I understand that my leads have to be in the circuit of whatever I'm testing. Okay. But....and here's the question(s):
1. The maximum capacity of the unit is 200 DCmA.
Whatsat?
2. I suppose that the maximum amperage of my tractor battery is about 650 CCA. Will my meter measure amperage of my lights or any other part of the electrical circuit on my tractor?
Hush, Chevy1908 or whatever.
Thanks everyone else. 

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Royse Tractor Guru
Joined: 17 Jan 2011 Posts: 10445
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Posted: Sun Nov 11, 2012 4:57 pm Post subject: Re: DCmA readings of Multimeter 


200ma should work out to be .2 amps if my math is working.
The only way to really use them for useful readings on your tractor is by selecting
the proper size shunt to use in parallel with the meter.
Generally way more hassle than its worth in my opinion.
I just use an old dash gauge to get a rough measurement of headlight current etc. 

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GeoTH,In Tractor Guru
Joined: 05 Aug 2009 Posts: 8502 Location: terre haute
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Posted: Sun Nov 11, 2012 5:32 pm Post subject: Re: DCmA readings of Multimeter 


#1. Not saying that one doesn't exist, but I've never seen a mulimeter measure AC ma.
#2. The only reason I would measure DC ma is if I were looking for a leaky load that will slowly drain a battery.
#3. My cheap HF electronic multimeter can measure 10 a DC. Polarity is not critical when you are using an electronic DC ammeter. What you need to remember is electrons out the negative of a battery and returns to the positive post of the battery. So when using any meter, if you connect the negative of the meter on the negative post of the battery, the meter will have the correct polarity. This is also true with measuring DC V.
#4. All EVOM meters are basically the same, so looking for a manual on how to use your old meter isn't important. If you know how to use one EVOM, you should know how to use all them.
5. I have an old analog DC ammeter out of a car that I use to measure larger DC amps, 50 amps.
Still wouldn't try to measure starter amps with it.
6. I use an ammprobe to measure AC amps with. It can't measure AC ma.
7. I recently bought a meter that can measure 15 amp AC, frequency, and power factor. Wasn't impressed with the accuracy of ammeter. However, I was more concerned with measuring frequency of my generator and power factor of a well pump.
8. I wouldn't use an EVOM to measure measure starter amps with. Starter amps will toast an EVOM. Think of it this way. Look at the size of the wires going to the starter. Look at the wires on your EVOM. No way 100's of amps will flow through a small meter.
9. The only way I'm aware of to measure large DC amps is to install a shunt resistor in series with the starter and measure the voltage across it.
10. One tail light will draw 25 amps. One head light may draw 48 amps, depending on the watts of the bulbs. Sounds like you may need a larger meter. I'm surprised your meter can't measure 10 amps DC. Take the watts of the bulb and divide it by 12v to get your amps.
Hope this helps.
George 

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equeen Regular
Joined: 04 Feb 2011 Posts: 384
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Posted: Sun Nov 11, 2012 5:51 pm Post subject: Re: DCmA readings of Multimeter 


I'm surprised your meter can't measure 10 amps DC.
......
Scale for DCmA is 2, 20, 200 and some lesser (I suppose) number. Scale for ACmA is same except not the lesser number.
Scale for ACV is 2, 20, 200, 2000. Scale for DCV is same, plus the lesser number mentioned above. 

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Indiana Ken Long Time User
Joined: 10 May 2011 Posts: 855 Location: Northern Indiana
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Posted: Sun Nov 11, 2012 6:17 pm Post subject: Re: DCmA readings of Multimeter 


You cannot do much with 200 ma around a tractor. Many VOMs have a 10 amp scale where you put the red wire in a socket labeled 10 Amp. If your meter does not have that you can use what is called a shunt. The shunt is placed in the circuit you wish to measure the current draw (amps) and the meter is used to read the voltage drop across the shunt in millivolts. The current is calculated using Ohms law.
It is not that hard: To make a shunt take a length of 14 Ga Romex (house wiring) cable and remove one of the wires. You will need a length of 47.5" for the shunt and add two inches at each end to allow for connecting into the various circuits you want to measure. Strip the insulation for 2.5" at each end and mark two places exactly 47.5" apart on the bare wire. The marked positions are where the meter leads are placed.
The resistance of 47.5" of 14 Ga wire is 10 milliohms, this is your shunt. Ohms law states: I = V / R, where I is the current in amps, R is the resistance in milliohms and V is the voltage in millivolts measured across the shunt. For example: Lets assume you measure a voltage of 10 millivolts across the shunt. From ohms law, I = 10 millivolts / 10 milliohms which equals a current of 1 amp for the circuit being measured. In other words each 10 millivolts equals 1 amp, therefore 200 millivolts would indicate a current of 20 amps, for example.
To use the shunt simply wire it in the head light circuit, for example, and turn on the lights. Set the meter on the millivolt scale, place the meter leads at the 47.5" marks and read the millivolts. Calculate the amps. using ohms law.
One nice thing about the shunt is if you place it in a dead short or a very high ampere circuit by mistake the shunt may be toast but your meter is not harmed. If the shunt becomes hot, you have too high an ampere circuit for the shunt.
Good Luck. 

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GeoTH,In Tractor Guru
Joined: 05 Aug 2009 Posts: 8502 Location: terre haute
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Posted: Sun Nov 11, 2012 7:25 pm Post subject: Re: DCmA readings of Multimeter 


equeen,
Think you are a little confused, 10 amps is equal to 10,000 ma.
Your 200 ma is only .2 amps.
I would think your meter can measure more than 200 ma.
Again, I've never seen a multimeter measure ac amps. Not saying yours don't.
Could you post a pic of your meter?
George 

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equeen Regular
Joined: 04 Feb 2011 Posts: 384
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Posted: Sun Nov 11, 2012 7:49 pm Post subject: Re: DCmA readings of Multimeter 


George, hope the photo comes thru.
Please straighten me out.


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JMOR Tractor Guru
Joined: 13 Aug 2007 Posts: 17433
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Posted: Sun Nov 11, 2012 8:00 pm Post subject: Re: DCmA readings of Multimeter 


George Marsh wrote:  (quoted from post at 23:25:35 11/11/12) equeen,
Think you are a little confused, 10 amps is equal to 10,000 ma.
Your 200 ma is only .2 amps.
I would think your meter can measure more than 200 ma.
Again, I've never seen a multimeter measure ac amps. Not saying yours don't.
Could you post a pic of your meter?
George  Well now you see a 1988 Micronta that measures AC milliamps & amps up to 10.


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GeoTH,In Tractor Guru
Joined: 05 Aug 2009 Posts: 8502 Location: terre haute
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Posted: Mon Nov 12, 2012 4:55 am Post subject: Re: DCmA readings of Multimeter 


Thank you for the pic. This is definately a first for me, a Radio Shack meter that measures AC milla amps, DC milla amps and no 10 amp range. All I would do with this meter is measure resistance, ohms, AC and DC volts. The amps scale, for the average person, is worthless, sorry to say.
HF has a similar meter for little to nothing, yes nothing, they sometimes give them away sometimes. I get them and give to guys at Christmas as stocking stuffers. HF meters are accurate enough for 99% of the guys that work on tractors, trucks and cars. It can measure 10 amps DC. HF also sells an ammprobe for around $20 if you need to measure large AC amps. It too can measure volts and ohms. I keep one in my truck along with HF volt meter.
I also have an old analog radio shack meter in truck, just in case it's very cold. Can't trust a digital meter when left in the cold and you bring it inside to measure voltage. The condensation screws up the meter readings.
George 

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