Simple guide to using a DMM for measurements

[hazna]

I just got my cheap DMM from DX. Having a few troubles in in measuring the voltage now. When I first got it, I was able to measure the voltage on lithium-ion cells. However after running some tests with on a charger (to measure the current from it), I can't seem to measure voltage anymore. I can still measure current though. Did I fry something when measuring the current on charger? I actually left the DMM attached to the charger, while it was charging.

 

[VidPro]

However after running some tests with on a charger (to measure the current from it), I can't seem to measure voltage anymore. I can still measure current though. .

the small side of most DMMs is usually fused, with a easily replaceable fuse. the High Amps side is often not fused, because it would add in to much resistance.
the small side is for doing voltage, and lower amps and all, and will pop the fast acting fuse anytime you jam to much power into it to protect the small side from total aniliation.
so did you check the fuse?

 

[hazna]

Well I opened up the DMM to look for the fuse, after jiggling a few things around (namely the bit that connects to the battery), seems to be working again!

 

[Billy Berue]

Thanks for this, very very helpful. A couple quick questions.

1. Can you safely measure voltage of Li-Ion cells while they are charging? If so, is it as simple as touching the probes to each terminal while it's in the charging bay? Or is this a recipe for some sort of disaster? Would doing so provide you with the voltage of the cell, or would it simply be measuring the voltage of the charging current?

2. Is it possible to measure the voltage of a Li-Ion cell while in actual use in the flashlight? I have successfully used the tailcap method to measure current while in use, but just curious if there was a way to measure the actual voltage. I am aware that you can use a resistor to simulate the load of the specific flashlight, but I am curious if the voltage can be measured more directly.

3. How do you measure Vf and If in a regulated light?


Thanks again, and sorry for all the questions. I usually like to ask questions before playing with electricity.

 

[HKJ]

Thanks for this, very very helpful. A couple quick questions.

1. Can you safely measure voltage of Li-Ion cells while they are charging? If so, is it as simple as touching the probes to each terminal while it's in the charging bay? Or is this a recipe for some sort of disaster? Would doing so provide you with the voltage of the cell, or would it simply be measuring the voltage of the charging current?

While charging the voltage on the LiIon is higher than the usual no load voltage and how much higher depends on the charge current. I.e. you can not use it to see when it is completely charged.

2. Is it possible to measure the voltage of a Li-Ion cell while in actual use in the flashlight? I have successfully used the tailcap method to measure current while in use, but just curious if there was a way to measure the actual voltage. I am aware that you can use a resistor to simulate the load of the specific flashlight, but I am curious if the voltage can be measured more directly.

To measure the voltage you need a probe on each end of the battery, the easy way to do that would be to take the battery out of the flashlight and then run wires into the light from the battery.

3. How do you measure Vf and If in a regulated light?

Both requires access to the led.

Vf is easy to measure, you put the meter on volt and connect it to the same place as the wires from the driver is connected to the led. BUT shorting the probes to the flashlight tube or to the some metal part in the light might blow the driver in the light.

If requires to unsolder one of the wires to the led and connect it to the A terminal on the meter, the COM terminal must be connected to the place where the wire was soldered in. Again the risk of damaging the driver is high.

Also note that opening the light probably will reduce the cooling of the led, i.e. be careful not to cook the led.

 

[Billy Berue]

While charging the voltage on the LiIon is higher than the usual no load voltage and how much higher depends on the charge current. I.e. you can not use it to see when it is completely charged.

Thanks, HKJ. I have measured the charging current of my WF-139 using the aluminum foil method in your guide. Is the voltage difference between a Li-Ion cell while charging versus resting typically constant (given a constant charging current)? I suppose I could determine this for myself with some experimentation, but I thought I'd ask anyway. If the difference is constant at a given charging current, then I should be able to monitor the voltage while charging to determine when to pull the cells off the charger. Is there a formula or something that can be used to mathematically determine what that difference should be?

If none of that is possible or practical, then I suppose I could use some trial and error to determine the voltage while charging at the point that the cells reach 4.2v (as measured once taken off the charger) to determine when they should be pulled from the charger.

Looks like my question is drifting a bit from the main topic, so I'll try to refrain from any more follow-ups in this thread, but will start a new thread if needed.

 

[HKJ]

Thanks, HKJ. I have measured the charging current of my WF-139 using the aluminum foil method in your guide. Is the voltage difference between a Li-Ion cell while charging versus resting typically constant (given a constant charging current)? I suppose I could determine this for myself with some experimentation, but I thought I'd ask anyway. If the difference is constant at a given charging current, then I should be able to monitor the voltage while charging to determine when to pull the cells off the charger. Is there a formula or something that can be used to mathematically determine what that difference should be?

If none of that is possible or practical, then I suppose I could use some trial and error to determine the voltage while charging at the point that the cells reach 4.2v (as measured once taken off the charger) to determine when they should be pulled from the charger.

I have a few comments to this: LiIon are not finished with charging when they reach 4.2 volt, they need to stay some time at the voltage before they are full.
But the WF-139 does not use the standard charge algorithm, I believe that it keeps the charge current up, until the batteries reaches 4.2 volt without current (It does turn charge current off at regular intervals to measure the voltage). This way to charge is faster than standard, but might be a bit harder on the batteries.

And as long as AW recommends the WF139 charge I have no problems with using it for my LiIon batteries.

You could probably make a table with the voltage measured during charge and the charge %, but that table would only be valid for one type and manufacturer of batteries.

 

[Billy Berue]

HKJ, I have another question, this time regarding the measurement of current draw. I understand the fairly straightforward tailcap method you described above:

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.

But I was reading another of your posts in wattnot's review of the Fenix TK40 over on the Flashlight Reviews forum, where you described what appears to be another method of measuring current draw:

...I open the light between the head and the tube. This makes it easy to make the connection for current measurement: One alligator clip wire between tube and head, one alligator clip wire between outer rings and the meter between the center positions.

I can't seem to quite visualize what you are saying here. If not too much trouble, I'd love to see a photo of the setup for that technique.

In fact, in a subsequent post further down in the TK40 review thread, you actually show a photo of what seems to be a similar technique that you were using to measure the alleged "parasitic drain" of the TK40. But in that setup, the head appears to be connected directly to the battery, and the body tube and tailcap appear to be uninvolved in the circuit. I assume that what is being shown in that photo is not the same technique that I quoted above for measuring current draw of the light while it is actually running.

Hope that makes sense. And thanks in advance for your help.

 

[HKJ]

I can't seem to quite visualize what you are saying here. If not too much trouble, I'd love to see a photo of the setup for that technique.

In fact, in a subsequent post further down in the TK40 review thread, you actually show a photo of what seems to be a similar technique that you were using to measure the alleged "parasitic drain" of the TK40. But in that setup, the head appears to be connected directly to the battery, and the body tube and tailcap appear to be uninvolved in the circuit. I assume that what is being shown in that photo is not the same technique that I quoted above for measuring current draw of the light while it is actually running.

In the photo where I measure the standby drain, I do not need to switch the light on, that is the reason that I can do without the body tube and switch.
To enable use of the switch, I must put the battery container into the body tube and also make a connection between the threads on the body tube and the treads on the head.
Or I could make a momentary short between the outer ring in the head and the threads in the head, that would simulate a press on the switch (A flat screwdriver is very useful for that).

Also remember to use the 10A/20A terminal on the meter to measure current drain with the light on!

Here is the photo with the standby current measurement, just add a flat screwdriver between outer ring and threads, when it is touching both the ring and the threads, it simulates pressed switch:

 

[Billy Berue]

Or I could make a momentary short between the outer ring in the head and the threads in the head, that would simulate a press on the switch (A flat screwdriver is very useful for that)....

Here is the photo with the standby current measurement, just add a flat screwdriver between outer ring and threads, when it is touching both the ring and the threads, it simulates pressed switch:

Excellent...thanks, HKJ. I will try this when I get home tonight from work. Just to make sure I understand, when you say "momentary short," you simply mean to use the flat screwdriver to touch the inner threads of the head to the outer ring in the head and then to release the screwdriver after a quick moment (i.e., just enough for the circuit to register the simulated tailcap click), right? In other words, I'm not supposed to leave the screwdriver in contact with the ring and threads for the duration of the measurement, correct? Sorry to ask questions about what must be so obvious to you. But I really don't want to fry my new TK40. :duh2:

 

[Justin Case]

The difference appears to be due to the fact that the TK40's multimode electronics are all in the head. So that's all you need to power up and probe for parasitic resistance. The battery tube and tailcap are just glorified wires and unnecessary to include in the circuit. A few other systems have the electronics in the tailcap (e.g., the OpticsHQ multifunction tailcap). In that case, you need to insert your DMM in series with the tailcap to measure parasitic current.

 

[HKJ]

Excellent...thanks, HKJ. I will try this when I get home tonight from work. Just to make sure I understand, when you say "momentary short," you simply mean to use the flat screwdriver to touch the inner threads of the head to the outer ring in the head and then to release the screwdriver after a quick moment (i.e., just enough for the circuit to register the simulated tailcap click), right?

That is correct for turning the light on/off, to change brightness you need to short a bit longer (Exactly like you need to hold the switch down longer to change brightness).

 

[Billy Berue]

Again, thanks. Your posts are extremely educational.

Sorry, but one more question. And I think you addressed this somewhere else, but now I can't find it. Anyway, I suppose the fairly direct connection of the battery to the head imparts very little resistance to the circuit, depending on the quality/gauge of the connecting wires. I believe my DMM applies some internal resistance when measuring current draw, which I guess in theory would introduce a touch of voltage sag. Since the TK40 has constant output, that small voltage sag would cause it to draw a small amount of additional current. I'm guessing this would be small enough so as to have an insignificant effect on the measurements produced by this technique, particularly if you were measuring current draw while the light was powered on. I suppose it might be more significant to the measurement if you were trying to accurately measure "parasitic drain" while the light was powered off, since the current being drawn in that scenario is so low to begin with.

My question is: if you followed the alternative method of measuring current draw (i.e., the method that enables the switch, quoted below from your previous post), would the additional resistance imposed by the tailcap and body tube yield a different measurement of current draw?

To enable use of the switch, I must put the battery container into the body tube and also make a connection between the threads on the body tube and the treads on the head.

I'm guessing that, in theory, it would yield a slightly higher current for a constant power system like the TK40. But, I'm also guessing that the difference in current draw would not be significant unless you were trying to measure parasitic drain.

I'll try to resist from asking any additional questions so as to not wear too thin on your patience.

 

[HKJ]

My question is: if you followed the alternative method of measuring current draw (i.e., the method that enables the switch, quoted below from your previous post), would the additional resistance imposed by the tailcap and body tube yield a different measurement of current draw?

In this light the switch does not carry any significant current, i.e. you result will be exactly the same, independent on how you connect the switch.

The wires, connection and meter will add a resistance, that translates into a voltage drop and the light will compensate by increasing the current. But the charge state of the batteries will have a much larger effect on the voltage and current drain.

These "tailcap" measurements can never be anything but a rough guide to the current consumption in the different modes, a much better way to evaluate the current consumption is making a graph like this:

This graph is not for TK40.

But again there are some factors that will spoil precision, both drivers with a slightly unstable current (this is very common) and current changing with the temperature of the led and drive (The temperature depends on how long time the light is on and at what power level).

 

[Billy Berue]

HKJ, all I can say is... :bow:

Really, really appreciate your patience and thorough explanation.

 

[Billy Berue]

In [the TK40] the switch does not carry any significant current, i.e. you result will be exactly the same, independent on how you connect the switch.

I'll actually try it both ways for myself later tonight, just as an educational exercise/experiment. The CPF geek in me is really coming out right now.

 

[ECKO32]

After picking up some tips i decided to play with my cheapo dx meter
and do some cceq tests,Thanks so much for the informative guide and the pics really helped take some of the guess work out:twothumbs:twothumbs:twothumbs

 

[Vikas Sontakke]

What is the typical resolution of DMM when measuring resistance?

- Vikas

 

[Vikas Sontakke]

Wow, what a great job HKJ! I don't know how I had missed this.
Fabulous work and thanks for putting such effort in to it.

- Vikas

 

[HKJ]

What is the typical resolution of DMM when measuring resistance?

- Vikas

Most meters are 3½ digit, i.e. a 1999 readout, but you have better meters with 3999, 4999 or 5999 readout. They use all the digits to show a ohm value, but the precision is not that good, more like 1% of actual reading.
Meters with two wires can usual not measure with better resolution than 0.1 ohm for low ohmic values and even that is doubtful, as you can see on my pictures.
If you really want to measure low ohm values you need a 4 terminal meter or do you own 4 terminal measurements. It is easy enough to do 4 terminal measurements, you just need a stabilized power supply and a resistor, then you can get 0.001 ohm resolution without much trouble.