By Mike Sokol
Welcome to my J.A.M. (Just Ask Mike) Session, a weekly column where I answer your basic electrical questions. If you’re a newbie who’s never plugged in a shore power cord (or ask – what’s a shore power cord?), or wonder why your daughter’s hair dryer keeps tripping the circuit breaker, this column is for you. Send your questions to Mike Sokol at mike (at) noshockzone.org with the subject line – JAM.
Dear Readers,
Since I’ve been promoting the use of digital meters over the last few months, now I’m getting questions on how to use them. Well, I’m here to help.
First of all, never poke the meter probes into an outlet before you’ve set the meter to the appropriate scale.
In most cases the 600-volt AC scale is the best one to use since you could be measuring anything between 0 and 240+ volts. And as I’ve noted many times in my advanced classes, it really doesn’t matter if the black and red probes are reversed when measuring AC voltage. That’s because in the U.S. the voltage polarity reverses itself 120 times every second. The AC scale on the meter accounts for this rapid polarity swapping, so you just need to plug the meter probes into the appropriate slots on the outlets to get an accurate reading.
Here’s a basic tutorial video on how to use a $20 Southwire multimeter to measure AC voltage. Watch the short video HERE or click on the picture.
And if you learn better from reading about a topic rather than watching a video, then stay tuned next week for a basic written tutorial on meter usage.
OK, everyone. Remember that electricity is a useful and powerful force, so we all need to pay attention to safety precautions while using it.
Let’s play safe out there….
Mike Sokol is an electrical and professional sound expert with 50+ years in the industry. His excellent book RV Electrical Safety is available at Amazon.com. For more info on Mike’s qualifications as an electrical expert, click here.
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It takes two changes in direction in order to make a complete cycle. This means that it takes 120 changes in current direction to produce 60 cycles
Dennis, you’re 100% correct. Each full cycle is comprised of 1/2 positive cycle and 1/2 negative cycle. The the current crossed zero 120 times per second, which makes 60 complete cycles per second (Hertz or Hz) in the USA. In Europe it’s only 50 cycles per second. For you musicians out there in the US it’s right around B flat on a bass guitar.
Mike: I want to add a small chest freezer in a Grand Design 3740BH. I have removed the couch that came with the RV and I am under weight even if the freezer is full. What I would like to do is tap off of a 12V light and install an inverter so while we are traveling doing some boondocking the freezer can still operate. The light that I want to get power from has a white and a black wire, is white +and black and do you think this will work?
Here’s the problem with that plan. Whatever amount of current your chest freezer needs at 120-volts AC will be multiplied by a factor of 10x on the 12-volt DC side. So if your freezer draws 4 amps of current at 120 volts, it will draw 40 amps of current on 12 volts from the battery side. I’m sure that your 12 volt DC light circuit will not be able to supply 40 amps of current.
Mike, Isn’t the 120V correct for “shore power” or your RV generator, but not always for all inverters? While “pure sine wave” inverters may have a 120v output and a digital meter may read 120v. Don’t lower cost inverters (not “pure sine wave” give a lower reading with a standard VOM? My old inverter reads 87v AC with my Harbor Freight digital VOM yet 110 with an old analog vom I have not calibrated.
A standard digital voltmeter without True-RMS measurement will indeed read lower than the equivalent voltage of a stepped-sine inverter, up to 40% low if the inverter is making square waves. But that would be a very old inverter (think 1960’s or so). I would guess that a modern modified-sine inverter might read 10% low using a non-RMS meter, but I’ll have to confirm that. Interesting question.
Mike, thanks for recognizing that a lot of us would rather read than watch a video.
A picture may be worth a thousand words but a thousand words is often worth more than an hour of video.
I’m working on the text version for next week’s JAM session. So stay tuned!!!
“That’s because in the U.S. the voltage polarity reverses itself 120 times every second.”
How does the 60Hz get doubled to 120?
I bet that was a “typo” where Mike was thinking about voltage and writing about cycles.
No, I’m correct in saying that 60 Hz power reverses its polarity 120 times a second. That’s because each complete cycle first swings positive, then swings negative. Since there are two of these voltage changes per cycle and 60 cycles per second for 60 Hz (Hertz) AC power, there will be 120 polarity reversals per second. Mind blowing, isn’t it?