# Thread: Simple guide to using a DMM for measurements

1. ## Re: Simple guide to using a DMM for measurements

could you please elaborate about how to estimate run time? say i have a flashlight that is drawing 1 amp but i am using 2x RCR 123 or 1x 18650 how would you guess run time? thanks a bunch!

2. ## Re: Simple guide to using a DMM for measurements

Originally Posted by Zeva
could you please elaborate about how to estimate run time? say i have a flashlight that is drawing 1 amp but i am using 2x RCR 123 or 1x 18650 how would you guess run time? thanks a bunch!
You need to measure the current at the tailcap and it will probably not be the same with 2xRCR123 or 1x18650.

The you divide the mAh with the measured current, but if you have the current in amp you need to convert the mAh to Ah first. Luckily it is very easy, just move the point 3 places, i.e. 2400 mAh is 2.4 Ah and will give an estimated runtime of 2.4/1 -> 2.4h hours.
But that is only a rough estimate, one reason is that the mAh specifications on LiIon batteries are usual very optimistic (I have done some measurements on 18650 batteries). Other problems includes meters that can not measured amps correct, due to pulsing current and that current draw will vary between full and empty batteries.

3. ## Re: Simple guide to using a DMM for measurements

Ohhh isee! one more question..> if my flashlight uses 2x RCR-123 and one RCR-123 is rated at 880 MAH would i double that? or would it be 880 MAH? soo this light is drawing about .6 so i assume thats 880/.6 = 1466.6666? or 2933.333? and i asume thats 1.46 hrs or 2.93 hrs? thanks a bunch !

4. ## Re: Simple guide to using a DMM for measurements

Originally Posted by Zeva
Ohhh isee! one more question..> if my flashlight uses 2x RCR-123 and one RCR-123 is rated at 880 MAH would i double that? or would it be 880 MAH? soo this light is drawing about .6 so i assume thats 880/.6 = 1466.6666? or 2933.333? and i asume thats 1.46 hrs or 2.93 hrs? thanks a bunch !
They will be in series, that gives double the volt and the same mAh, i.e. the calculation would be 0.880/0.6 -> 1.46
But that result would be very optimistic, the 880 mAh is probably more like 600 mAh, i.e. 1 hour would be more realistic.

5. ## Re: Simple guide to using a DMM for measurements

Thanks for the info! my DX DM doesnt seem that bad! :P

6. ## Re: Simple guide to using a DMM for measurements

I have two less expensive older DMM's of different brands which don't have auto-ranging and they both measure approximately the same, I haven't found any difference between the two. I wouldn't mind owning a Fluke, but I haven't found the need for a more expensive meter.

7. ## Re: Simple guide to using a DMM for measurements

circuit & conection resistance will show up under load. is there a problem measuring it (ie wire from +batt to +board) will the flashlight/circuit is running?

8. ## Re: Simple guide to using a DMM for measurements

Originally Posted by kosPap

circuit & conection resistance will show up under load. is there a problem measuring it (ie wire from +batt to +board) will the flashlight/circuit is running?
Do you want to measure resistance in wires and switches?
This is a very low resistance and requires a special technique to measure and is not covered in this guide. If there is a need for it I can add it to the guide.

9. ## Re: Simple guide to using a DMM for measurements

Also never use the resistance setting of a multimeter on a live circuit with current flowing through it.

10. ## Re: Simple guide to using a DMM for measurements

Originally Posted by Mr Happy
Also never use the resistance setting of a multimeter on a live circuit with current flowing through it.
That was exactly what I thought it would be and made me ask...

Originally Posted by HKJ
Do you want to measure resistance in wires and switches?
This is a very low resistance and requires a special technique to measure and is not covered in this guide. If there is a need for it I can add it to the guide.
let me guess ...parallel metering with a know value resistor?

11. ## Re: Simple guide to using a DMM for measurements

Originally Posted by kosPap
let me guess ...parallel metering with a know value resistor?
No, use a known current, measure voltage drop. This way you can measure 0.01 ohm with less than 10% tolerance (in many cases a lot better than that).

12. ## Re: Simple guide to using a DMM for measurements

right....the skinny imlemation of Ohm law....
thanks

13. ## Re: Simple guide to using a DMM for measurements

Just a question, even if it turns out to be nonsense...

Can I have a DMM with all 3 leads connected while measuring (Current and Voltage alternatively)

My purpose is to monitor the Li-Ion charger with one DMM only. So if I connect the elads correctly will it matter is for example, current is flowing still through the 10A socket while the Dial is turned to voltage metering?

Thanks, kostas

14. ## Re: Simple guide to using a DMM for measurements

Originally Posted by kosPap
Just a question, even if it turns out to be nonsense...

Can I have a DMM with all 3 leads connected while measuring (Current and Voltage alternatively)

My purpose is to monitor the Li-Ion charger with one DMM only. So if I connect the elads correctly will it matter is for example, current is flowing still through the 10A socket while the Dial is turned to voltage metering?

Thanks, kostas
NO, some of the better meters will warn about incorrect probe connection.

With that said, it might work on some meters, but you will have to test it.

15. ## Re: Simple guide to using a DMM for measurements

I think generally it will work (the 10 A socket is just a low resistance shunt internally), but it is risky. If you forget which lead is which and accidentally connect the current probe to the wrong place you will blow the fuse in the meter (if there is one). Also when you turn the dial between voltage and current settings you might pass through other functions like resistance that do not like having a voltage across the probes.

16. ## Re: Simple guide to using a DMM for measurements

Originally Posted by Mr Happy
......If you forget which lead is which and accidentally connect the current probe to the wrong place you will blow the fuse in the meter.
I just thought I'd point out that most relatively inexpensive DMM's anyway, do not incorporate a fuse in the 10A circuit, only the lessor ranges.

Dave

17. ## Re: Simple guide to using a DMM for measurements

Originally Posted by kosPap
My purpose is to monitor the Li-Ion charger with one DMM only. So if I connect the elads correctly will it matter is for example, current is flowing still through the 10A socket while the Dial is turned to voltage metering?
A safe way to do it is to add a 0.1 ohm resistor in series with the battery, the switch the meter between the resistor and the battery.
With some chargers you might even be able to fit the resistor inside the charger, add a switch and two wires out to the meter.

18. ## Re: Simple guide to using a DMM for measurements

hmmm thanks....

19. ## Re: Simple guide to using a DMM for measurements

Just found this guide. Thank you OP. I've not checked everything out but being a visual learner the pictures greatly allowed me to learn faster. I can see some questions I had about how to test a light bulb answered there as with how to put the battery under simulted load via resistor so you know if the battery has enough power to drive the LED.

I like the guide so far even thou I just skimmed it. Dropping a comment of thanks and also so I subscribe to this thread to find it later. Found it off a tagline of some other user here. So far it seems like it was geared with the flashaholic/batteriholic which is what I like with many scenrios of testing... I'll check out more later when I have time. DMM newbie here that only uses it just to test batteries.

20. ## Re: Simple guide to using a DMM for measurements

excellent resource- thank you!

21. ## Re: Simple guide to using a DMM for measurements

An excellent resource for all of us, thank you very much HKJ. One question though. Is there a way to make current draw measurements on a light such as the Malkoff MD2, which has no removable tailcap.

Regards,

Jim - Semper Fi

22. ## Re: Simple guide to using a DMM for measurements

Originally Posted by jmcf1949
An excellent resource for all of us, thank you very much HKJ. One question though. Is there a way to make current draw measurements on a light such as the Malkoff MD2, which has no removable tailcap.
I do not known how the MD2 is build, but I have added two examples on lights without tailcap. Fell free to ask for other examples on measurements, if I have the time and light I will add it.

23. ## Re: Simple guide to using a DMM for measurements

Originally Posted by 45/70
I just thought I'd point out that most relatively inexpensive DMM's anyway, do not incorporate a fuse in the 10A circuit, only the lessor ranges.

Dave
Thanks for the tip, I'll have to check that on mine. I've blown fuses before, but it's been a long time. I think it was in the 400mA range and not the 10A one when I've done it most of the time. If I think it might be over 300mA and don't need a hyper-accurate reading I just use the 10A range, but I thought mine were fused at 10A.

24. ## Re: Simple guide to using a DMM for measurements

Originally Posted by ampdude
Thanks for the tip, I'll have to check that on mine. I've blown fuses before, but it's been a long time. I think it was in the 400mA range and not the 10A one when I've done it most of the time. If I think it might be over 300mA and don't need a hyper-accurate reading I just use the 10A range, but I thought mine were fused at 10A.
There is one problem with a 10A range without fuse: if (by accident) the meter with probes in the 10A range is connected directly to main voltage, the meter might explode.

Using the low ampere range can give some measurements errors when working with low voltage (i.e. flashlights), because of voltage drop in the meter.

25. ## Re: Simple guide to using a DMM for measurements

Hello, and thank you for this valuble information!! I have one question however.. on the section where you explian the "power Draw" it states: "To calculate the power draw, the current must be multiplied with the battery voltage, as can be seen below it is about 1.2 volt under load and the light uses two batteries, the power is then (voltage*current): 1.2*2*1.6 -> 3.8 watt. Most of this power goes into the led where about 20% is converted to light, all the remaining power is converted to heat."
but the photo below is of the tailcap measurements. I didnt get the reference to the "1.2v under load"... Are you saying to measure the battery " while in use" ?? and where did you get the "1.6" in the problem? was it, one battery measured 1.2v and the other measured 1.6v?
Thank you!!

26. ## Re: Simple guide to using a DMM for measurements

Originally Posted by aingel2k1
I have one question however.. on the section where you explian the "power Draw" it states: "To calculate the power draw, the current must be multiplied with the battery voltage, as can be seen below it is about 1.2 volt under load and the light uses two batteries, the power is then (voltage*current): 1.2*2*1.6 -> 3.8 watt. Most of this power goes into the led where about 20% is converted to light, all the remaining power is converted to heat."
but the photo below is of the tailcap measurements. I didnt get the reference to the "1.2v under load"... Are you saying to measure the battery " while in use" ?? and where did you get the "1.6" in the problem? was it, one battery measured 1.2v and the other measured 1.6v?
Thank you!!
The 1.6 was the current draw, the battery is down to 1.2 volt when loaded.
Both alkaline and NiMH drops to around 1.2 volt while having a high load (i.e. 1+ amp) (This number is not an exact value, just a rule of thumb).

Tailcap measurements are (at best) a rough guide, but as long as you understand that, they are useful as a guideline to the current draw.

27. ## Re: Simple guide to using a DMM for measurements

Can you point a good sub \$100 multimeter? I want the fluke buy it is too expensive

I have 2 cheap \$30 multimeter and both get different readings. The differences are huge, ranges 0.07v and 0.4A.

28. ## Re: Simple guide to using a DMM for measurements

Originally Posted by richardcpf
Can you point a good sub \$100 multimeter? I want the fluke buy it is too expensive

I have 2 cheap \$30 multimeter and both get different readings. The differences are huge, ranges 0.07v and 0.4A.
I can not point you to a specific meter, but there exist hundred of different meters in that price range, many (if not all) of them good for hobby use.

Some thing I would look for:

• CAT III/CAT IV rating, this means the meter is safety rated for professional use.
• Accuracy specification are more than just x%+y digits, there must also be specified some frequencies (for AC ranges) and some temperature limits. I.e.: "50 V range: 0.3%+25 at 45 to 65 Hz within 18 to 28 degrees" is a much more reliable specification than "AC: 0.3%+25"
• The meter warns when the battery is low.
• The 10A/20A range has a fuse.
• The meter has a max reading of at least 3999, not only 1999. For measuring LiIon batteries an even higher value is better.
• I also like that the meter has "True RMS", but it is not required for hobby meter.

I do not know if you can get all of the above below \$100.

29. ## Re: Simple guide to using a DMM for measurements

I have added a chapter on low ohmic measurement.

30. ## Re: Simple guide to using a DMM for measurements

Interesting the differences in mOhm measurements of tailcap using the DMM only, and the DMM, resistor, and power supply. Fluke alone read 20mOhm resistance, and DMM, resistor, and power supply setup ended up showing 34mOhm resistance.

Bill

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