Tom's Guide | Tom's Hardware | Tom's Games
 |
 |
 |
I'm really new to using a multimeter and was wondering if anyone uses one on a regular basis? If so, I have a question and would appreciate your help.
My multimeter is kinda cheap and can measure amperage up to just 200mA. But say that I wanted to test the voltage of a 12v battery that had a circuit connected to it that was drawing say, 1 amp. I know if you test the amperage of this kind of circuit, it will blow the internal fuse. But what if you're testing voltage? Does this mean that you can test the voltage all the way to 500V as long as your amperage doesn't exceed 200mA? As you can see, I'm pretty confused. I messed my other multimeter up beyond repair, I don't want to do this one the same way. Thanks for your advice.
The impedance of a meter when measuring voltage is very high, hence it draws little current. You can therefore safely check any voltage provided it is within the range of the scale.
When measuring current the situation is quite different. The impedance of the meter is very low and it is inserted "in series" with the supply. You do not normally put the meter across the terminals of the supply when measuring current because of the large current it draws, and in most situations it would be quite dangerous to do so.
some other bloke...
0 
Dereks response is spot on, below are some links I hope will be of help.
Stay away from line / mains voltage for now...http://mechatronics.mech.northweste...
http://www.wikihow.com/Use-a-Multim...
http://www.hobbyprojects.com/using_...Keep the old stuff running
0
Thanks alot!!!! I really, really, really appreciate it!! Now I know more about how to use one without blowing the fuse again.
0
Just in case you come back, I've thought of one more important point I should have made, although not mentioned in your question.
When using the ohms range, whatever you are testing should be disconnected from its power source (even if it is a very low voltage).
Failure to do so could cause damage to the meter and in any event would give a totally meaningless reading.Finally, observe polarity on all d.c. voltage or current measurements - particularly important for analog meters (which still have their uses).
some other bloke...
0
Thanks again! I'll keep that in mind. Oh, one more question. If I'm going to test the continuity of a cable, what do I need to set the multimeter to? I was thinking to the setting that looks like the schematic symbol for a diode, but I'm not sure. Thanks!
0
The diode symbol (wait for it LOL) is for testing diodes.
For testing continuity of anything low resistance (or impedance which is the a.c. term) you set it for the lowest ohms setting. Cable fits this category. In this instance, setting it above the lowest will not produce a very accurate reading.
It won't do any harm if you are on the wrong ohms setting, so in general you chose the one which gives the required number of decimal places (or nearest center scale for analog meters).
You may wonder why I keep mentioning analog meters but they are very useful when examining a varying source, or for a quick indication. Watching digits rolling gives little idea of what is happening. Digital meters are better when an accurate measurement is required. I use both and, perhaps surprisingly, tend to often grab the analog meter as first choice.
some other bloke...
0
Thank you so much. You've helped me alot with your advice. I really appreciate it. Please forgive my ignorance, lol.
0
and finally ....
Best first step is to get to grips with electrics - there are plenty of websites around. Ohms law, and what it means, is a good start. Wattage (ie power/heat dissipation) is worthy of a bit of attention later too. At this stage just think of it as W=Volts x Amps. Its all pretty straight forward until you get involved with "a.c. theory/power factor" and similar such delights.
Be aware of "voltage drop". Mostly the meter impedance on a voltage setting is much higher than the source you are measuring (by intent). Here's a neat example of where things can go wrong:
You measure an ordinary flash lamp battery and get a reading not too much below what it says on the cell (say, 1.5V). BUT if the battery has nearly expired its own internal impedance will have gone high. If you then connect a load, such as a bulb, across the battery this draws way more current than the voltmeter, so it drops most of its remaining voltage across the internal resistance of the battery. Your meter then reads a much lower and meaningful voltage.
From the above, a simple voltage measurement of a battery can mislead you. In fact a bulb will usually give you a much better indication of battery condition. Using the voltmeter with the bulb connected at the same time is best of all. If the voltage is still near 1.5V (in this example) then you have a pretty good battery. If it falls to a low value when the bulb is connected then its a dud.
You may ponder as to why the meter needs to be low resistance when measuring current. If the resistance of the meter (in series with the load) was high it would reduce the current flowing and give an incorrect low reading. Voltage would be dropped across the meter, thereby reducing the voltage left to serve the load - not a good idea.
I could go on but I won't LOL....
CU
some other bloke...
0  |
 Delay in Reply Counter Up...
|
Slow computing.net again
|
This post is quite old and has been locked from receiving new replies. Please create a new posting instead.
| Ads by Google |
