Multi-Meter Readings and Use

Multi-Meter Readings and Use

OK, I am just a hobby guy and am working my way around the proper use of a multi-meter.

I have a new All-Sun EM830 meter.

Question 1: I take a 40 watt (listed on packaging) light bulb intended for 110 use. I set my meter to 200 ohms and measure the resistance. I get a reading of 31.8. I assume this is 31.8 ohms.

Realizing that getting the decimal correct is important and I want to double check the logic of my reading. So I plug the values of 110 volt and 31.8 ohms into an online calculator. The answer is 380.5 watts. Since the label states 40 watts, I seem to be off by a factor of 10 on my ohm reading. The reading should be 318 ohms

I am confused as to how to read the meter to get 318 ohms instead of 31.8

Question 2: I want to measure the amps being used by a 12 volt DC heating pad powered by a 12 volt DC deep cycle battery.
The pad has a controller in the circuit so I can't directly measure resistance and calculate amps. So I figured I would put the meter inline with the pad running and measure amps directly.

Regardless of how I set up the meter range, I do not get any measurement and the insertion of the meter does not complete the circuit so that the pad operates. I tried just simply creating a circuit with a 1.5 volt battery and a 1.5 volt bulb (a simple flashlight) I can use a piece of bell wire to jump from the battery (-) to the bulb (-) to complete the circuit and the bulb lights. If I remove the jumper and insert my meter set to read amps, I get no reading as the circuit is not completed (the bulb does not light) regardless of what amp setting the meter is on.

I can't decide of the meter is bad or if I am using it incorrectly.

dan

 

First off you need to use 120 volts for the voltage of the general receptacles, not 110. Standard home voltage has been 120 volts for quite a few decades.

Question 1:
It sounds to me like you are using the meter correctly to measure resistance. However, it will depend on the type of bulb you are trying to measure. You can really only measure resistance on an incandescent bulb. Is this an incandescent, CFL, or LED?

Question 2:
To measure current draw on a DC circuit you need to put the meter in series with the load. You need to put the probes on the top and bottom holes and set the dial to the 10A setting.

 

Thank you for answering so quickly!
Question 1:
I understand about 110 and 120. If I run the calculation for a 40 watt INCANDESCENT bulb the calculated resistance is 360 ohms. I am still having that decimal problem I described in original post. ?

Question 2:
I had probes in the first and second holes for the first measurements. I opened the case and found the 250ma fuse blown. That would explain all of my observations. I guess I blew that fuse on first measurement, I will replace fuse and also try the flashlight experiment with the 10 amp hole to gain experience before trig the heating pad again.

 

I agree, You should be reading about 360 ohms on a 40 watt incandescent lamp. You might want to check/replace the battery on the meter. That can give you some odd readings.

That is the bad thing about those meters. If you have the dial on the wrong setting you can burn things up.

 

Negative. The Resistance of Tungsten is highly temperature dependant. Expect about an order of magnitude change from cold to white hot.

 

Interesting. And Thank You!
So I am measuring the filament cold and therefore should expect to see a much lower resistance than when the filament is hot with current flowing. ?

The meter may indeed be reading correctly. Of interest to me is that the calculated value is almost exactly 10X of the cold reading. Is that the order of magnitude you reference.

In this case, then I could never estimate the current through a heating pad by measuring the cold resistance and calculating amps with a known voltage. Best to just measure the current dirtily and be done with the error factors. ?

In order to check the accuracy of the meter, can I use known value resistors? Do they not rise in temp like a filament and therefore the reading would be true?

 

Good point Telecom Guy. But, how many people have tungsten lamps in their home?

 

I've got some 0.1% temperature stable resistors around if you are interested.

 

Yeah, really. I even put LED back up lights in my 2002 cars!

 

OK, on to Question 3 as part of this thread. =)

I went thru all the gymnastics in order to measure current draw through the outdoor pet heating pad powered by the battery, I wanted the draw to make an estimate of when I need to trot down to the barn and bring the battery back for charging.

Once meter was functional again, I measured the charged battery at 12.75 volts and the current to the pad in fully on-demand mode was 1.75 amps. If the current is constant, I assume the usage will be 1.75 amps per hour. (so thats what an Ah means.) I am using a marine deep cycle battery -AutoCraft Marine 27DC-2.

I get a different answer every time I ask, (How many Amp Hours does this battery have and What is the effective number at 20 degrees F I should use for the Amp Hours in this battery =Y ) I want to divide my 1.75 measured current into Y to get an idea of how long I should wait before going to the barn and recharge the battery.

So, you two guys seem to know your stuff so I am interested in what you have to say.

 

I would guess that battery is around 80-90 amp hours. But since it's a hybrid rather than a true deep cycle, how deeply do you really want to discharge it? Let's say you're willing to go down to 50%. (How quickly do you want to wreck your battery?) That would mean you'd need to recharge it every 22 hours, approximately.

This is ignoring a _lot_ of stuff which will change the results slightly. But does it really matter if it's 20 hours or 23 hours? Seems about the same amount of work to me. I'd put a voltmeter on the battery while it's in use and monitor it for a more precise state of charge to know exactly how things are holding up.