Using your Multimeter – Part 1
Up until now we have been talking about a lot of theory and learning vocabulary for electrical systems. Today we are taking the first step in learning how to make use of our new found knowledge to make electrical measurements. There are two important things to know:
First, always keep safety utmost when you are working on electrical systems. Second, and maybe too obvious, you will need a meter of some type, which is suitable for the measurement you are making.
There are several common measurements you may want to make while using your RV. These include testing fuses, checking available campground power to verify it is correct before plugging in your shore power cord, checking the condition of your battery to determine the charge level. Obviously there are a lot of other things you can do, but let’s talk about these and see if we can find a meter that is suitable for the above checks.
We’ll get into the selection process in more depth later, but for now, and in order to check the condition of fuses, you will need a meter that can measure resistance. To check the campground power post you will need a meter which can measure AC volts up to 240 volts AC. To check the condition of your battery, you will need a meter that can measure at least 12 volts DC.
A final condition I would impose on such a meter would that it be inexpensive, and reasonably rugged as well as simple to use. Such a meter can be found at Home Depot, www.homedepot.com for $19.99.
This meter comes complete with color coded test leads and several “RANGES”. The various ranges on the meter enable you to make different measurements such as described above.
If you don’t have a suitable multimeter I suggest you consider getting one to carry with you in your RV. Get a meter and study the various parts of the meter. Next week we will begin discussing the “how to” part of owning such a meter. BTW, for what it’s worth, I have this exact meter in my motorhome and find it to be more than adequate for all the checks I need to make. If you have any questions please let me know.
Larry
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First of all, I would like to thank you Larry for this wealth of information. I have a very limited understanding of electrical knowledge, but I seem to know a lot more than most people I know. My strong point is logic, I can usually study something long enough to figure out how it is supposed to work, and once I figure that out I can then figure out logically, why it is not working.
This information helps me add to the logic of how and now why, things work the way they do. I always took things apart when I was younger, to see what made it work. This information just adds the knowledge I need to become more adept at working on electrical circuits etc…
Thanks for sharing your knowledge, and keep it coming.
Steve
Steve, thanks much for those kind words. One of the reasons I love working with electrical systems is that electricity is very logical and always follows the rules. Ohm’s law is deceptively simple on the surface but is so useful in understanding how electricity works. Hopefully I will be able to continue to provide useful information here.
Larry
Hi Larry…I recently purchased a multimeter to use as needed. When we arrived at a campsite I went to test the voltage with my meter but I found the probs where not long enough to reach into the outlet to get a reading. I attached the clips to see if I could reach further but they were too big to fit. Have you any suggestions on how to extend the probs to test the 240, 50 amp outlet? Thanks very much…I am ejoying the sharing of your knowledge. tom
Tom, typically the probes are about 5/8″ to 3/4″ long which should be sufficient to reach into the plug and make contact. However, it requires a bit of “fishing around” to touch the internal copper contacts. It’s a technique that requires some practice. Try working with a wall outlet at home first until you get the “feel” for it, then move up to checking the outlet at the campground. It takes a little patience but is worth the effort.
Larry
[...] RV.Net Blog Admin I hope everyone has been following Larry Cadman’s great series on electrical theory and multimeters , there is a lot of great information in them- good, solid theory, backed up with practical [...]
I have some Christmas light’s pluged in out side the fifth wheel and it quit working along with three out let on the same side of trailer . The breaker’s are all good where would I check to fix this problem? I sure do thank you out there for helping me. Have a great New Year
Make sure that circuit doesn’t include a GFI outlet that has tripped which will shut down the enitre circuit.
Darby, Darrel is probably correct. Often RV manufacturers connect outside outlets to the GFI outlet in the bathroom. Check to make sure that outlet, or any other outlets that are GFI type, are not tripped. I usually try tripping the GFI outlet to make sure it trips, then reset it. There are two buttons on the GFI outlet marked “test” and “trip”. Good luck and happy new year.
Larry
I, too, have had difficulties on occasion over the years with VOM probes being too short to reach the conductor that I am probing for. (this seems to be more prevalent with the really inexpensive meters. ones in the $7 to $10 range.) My solution is to get a couple of un-insulated alligator clips and very carefully crimp the end that is designed to have the wire crimped into it down to whatever size snugly fits the too-short probe. (I usually do this resizing with an ordinary pair of pliers) You then slide the alligator clips on the probes and clip a straightened out paper clip, a 3D finish nail or similar item in the jaws of the alligator clip and you have a probe long enough to reach whatever you are probing for. These removable alligator clips make many of your measuring jobs much easier, too.
A “disclaimer” is in order here. One must be very careful when using an adapter such as I have described here because much more conductive material is exposed making it very easy to either short the two conductors together and make a “big flash and burn” or give ones self a dangerous shock. Also, one must still only handle the probing device by the insulated handles that are part of the original probe.