Simple guide to using a DMM for measurements

[HKJ]

I did things just like on your picture. I'm bit confused, if i'm doing thing just like your pic, I'm I measuring volt battery or current draw? :thinking:

That way you measure the current draw only, but not at a specific voltage. The voltage you measure before or after, on the battery, is without load and will be considerable higher than when you measured the current draw (especially when we are talking about the Preon ReVo at high).

When doing this kind of current measurement it is best to use a fresh (or freshly charged) battery and then only specify battery type, not voltage.

 

[njet212]

Thanks for the answer HKJ, appreciate that!

 

[fisk-king]

Great work & thread:thumbsup:

 

[njet212]

HKJ, last time was asking about how do you measure standby current on a flashlight and you did answer it already. But today i tried to look at it again but i realized your post was missing.

Would you mind to explain it again ? the flashlight i'm going to measure is New Jetbeam RRT-0 Infinite Variable Brightness version. I was wondering how much is the standby current because it's suck all the juice on my AW RCR123 just only in a week !!


Thanks

 

[HKJ]

HKJ, last time was asking about how do you measure standby current on a flashlight and you did answer it already. But today i tried to look at it again but i realized your post was missing.

Would you mind to explain it again ? the flashlight i'm going to measure is New Jetbeam RRT-0 Infinite Variable Brightness version. I was wondering how much is the standby current because it's suck all the juice on my AW RCR123 just only in a week !!

That light is supposed to have a real on/off switch, i.e. no current draw when switched off on the tail switch. If you want to measure the current draw with the ring in off position, you can just use the "Current consumption" chapter, but you will need to use the "mA" socket, not the "A" socket.

 

[njet212]

Thanks for your answer HKJ :thumbsup:

 

[brightnorm]

HKJ,

Many thanks for posting this. It is extremely helpul and beautifully illustrated and explained.

Brightnorm

 

[HKJ]

Added a "High current measurement" showing how to use a clamp meter.

 

[Walterk]

Very nice ! And clearly illustrated, thx for this thread.

 

[Viking]

I have a cheap Velleman DVM92 multimeter.


Specs are:


3 ½ digit ( maximum display 1999 )
Accuracy DC voltage +/- 0,5% of rgd +/- 1 digit
Accuracy DC current 20mA +/- 0,8% of rgd +/- 1 digit
Accuracy DC current 200mA +/- 1,2% of rgd +/- 1 digit
Accuracy DC current 20A +/- 2% of rgd +/- 5 digits


Is the DMM accurate enough for measuring flashlight batteries , or will you recomend me buying a better one.

 

[HKJ]

3 ½ digit ( maximum display 1999 )
Accuracy DC voltage +/- 0,5% of rgd +/- 1 digit

Is the DMM accurate enough for measuring flashlight batteries , or will you recomend me buying a better one.

For LiIon batteries the charge voltage is usual 4.2 volt +/- 0.05 volt, as long as you batteries does not read above 4.2 volt on you DMM they are below 4.25 volt (According to the DMM specification below 4.23 volt).
Most chargers will be more than good enough and you do not need to check the batteries each time you take them off the charger.
Where it is important to check batteries is when you run them down, many LiIon batteries can get dangerous if you discharge them to low, to check this does not require much precision from the meter. If the battery measures below 2.5 to 3 volt (After they have rested some time), there is a risk they may explode when charged. This is a cumulative effect, each time they get discharged to much the risk for explosion during any subsequent charge will increase slightly.

Batteries that show 0 volt on the meter might not be discharged to much, it is probably the protection that is tripped. If you charger can reset a protection (See my charger reviews), give the cell a second in the charger, then take it out and check the voltage again. If the charger cannot reset protection you need to wire it parallel with another LiIon cell for a second.

And if you DMM's reading suddenly are a bit beside the expected values, you probably need to replace the battery in the meter.

 

[Viking]

Thanks for the answer

 

[Tohuwabohu]

The tailcap is removed from the flashlight and the meter is substituted for that connection, i.e. one probe pin is placed on the battery and the other probe pin is placed on the battery tube, where there is some bare metal. Placing the probe pin on anodized metal will not work.
As can be seen, the two meters do not agree on the value. That is because the current draw is not really DC, but drawn in very fast pulses, the meters does not sum this the same way. This is only a problem on some flashlights.

Hello HKJ,
have you ever tested the resistance of the probe leads of the cheap DMM?
I got a Best DT9205A today to compare it too a good multimeter. The DT9205A is not very accurate but usable but the probe leads are crap, their resistance is far too high. I measured 0.46 ohms - ten times the value of some probe leads from another meter and more than 50 times the value of a pair of short leads with banana plugs on both ends.
I don't think that fast pulses cause the wrong reading but the voltage drop across the probe leads makes the regulated light draw a higher current.
Of course the effect of the high probe lead resistance gets worse with pulsed currents.

 

[HKJ]

Hello HKJ,
have you ever tested the resistance of the probe leads of the cheap DMM?
I got a Best DT9205A today to compare it too a good multimeter. The DT9205A is not very accurate but usable but the probe leads are crap, their resistance is far too high. I measured 0.46 ohms - ten times the value of some probe leads from another meter and more than 50 times the value of a pair of short leads with banana plugs on both ends.
I don't think that fast pulses cause the wrong reading but the voltage drop across the probe leads makes the regulated light draw a higher current.
Of course the effect of the high probe lead resistance gets worse with pulsed currents.

The test leads is probable part of the reason.

I have been testing some leads, switches and dmm's:

JetBeam M2S Tailcap	70 mohm
ThruNite TN11 Tailcap	85 mohm
4Sevens Quark X AA2 Tailcap	82 mohm
Fluke 289 10A range	33 mohm
Fluke 189 10A range	55 mohm
Fluke 179 10A range	35 mohm
Vichy VC99 20A range	48 mohm
Best DT9205A 20A range	18 mohm
Fluke test lead (x1)	38 mohm
Fluke test lead (x1)	51 mohm
No name test lead (x1)	257 mohm
No name 10A test lead (x1)	22 mohm

Doing a test with 10A current and measuring the voltage from test probe tip to test probe tip:

	10A	Tot. Res
Fluke 289 + Fluke test lead	1.02 volt	102 mohm
Fluke 189 + Fluke test lead	1.16 volt	116 mohm
Fluke 179 + Fluke test lead	0.97 volt	97 mohm
Best DT9205A + No name 10A test lead	0.59 volt	59 mohm

The connection resistance from test probe tip to current source is not included.

 

[tobrien]

so the ZTS battery tester (the one that supports 3.7V li-ions) can do protected cells fine?

 

[HKJ]

so the ZTS battery tester (the one that supports 3.7V li-ions) can do protected cells fine?

Yes, there is no difference between a protected and unprotected cell in this regards.

 

[tobrien]

Yes, there is no difference between a protected and unprotected cell in this regards.

oh okay, thank you! I had been misinformed in the past that the protection circuits kept those cells from being accurately tested by the pulse load.

 

[dragosios]

I see that many of you ask why some multimeters do not display zero.
For TrueRMS meters this is normal. They will not display zero, it's about how the processor makes the measurement.
On Flukes seen here: 87, 179, 189, etc, calibration manual tells you which value is the calibration check on each scale. Eg: a Fluke 189 that has 5.0000 on it's input and reads between 4.9997 and 5.0023 then it's in spec. Next value is ten times the first one, in this case 50V and 500V DC.
Good probes are below 0.1ohms each. Cheap chinese one are higher than that.
Measurement of very low resistance can be done conveniently with an AC ohmmeter, or ESR meter.

 

[loquutis79]

I want to check the open voltage of my battery charger, and I think this is what you call it when its plugged in and no battery installed. [Sorry, not much electrical knowledge]
My charger is labled at 4.2V output. Do I simply plug it in and attach each DMM probe [auto range set to AC V?] to the pos. and neg. sides of one channel?
When I did the above I get a reading of .027V?? on the auto range setting.

Does this make sence?

 

[flashlight chronic]

My DMM set on DC reads 4.87v off my WF-139 charger.