REALLY Simple Guide To Figuring Out Voltage In Maglite

• OK....let's use a practical example. I want to use 2 of AW's C Li-Ion cells in a 2C size STOCK Maglite to light up a WA 1111 6V bulb, using this 1D size light I bought a while ago from FiveMega. I put the bipin bulb into a gold bi-pin bulb holder I also got from FiveMega, and put that into the stock Maglite bulb holder, and tighten down the retaining ring. Next, I look at my bulb testing thread here, and click on the 1111 bulb test data here.

• I know that each of the AW C Li-Ions when charged will have about 4.15V, and two of them in series gives 8.3V. When I look down the left "Applied Volts" column, I notice that when 8.3V is applied, the bulb flashes (dies). I am now worried that my brilliant idea of using 2 x Li-Ions at 8.3V (fully charged) will kill my bulb, and I wonder if I should use a different setup. :thinking:

• However, what needs to be taken into account is the amount of resistance (to the flow of electrical current) that is related to the various parts of the flashlight, and which effectively decreases the voltage actually seen at the bulb. There is a basic law you may have heard of called Ohms Law which we will use to understand this. Ohm's Law says:

• Now to see the effect of the flashlight parts having resistance and lowering the amount of voltage delivered to the bulb, I need to get some data in 3 measurements.

• 1) With my DMM (digital multi-meter), I measure the voltage of my two C Li-Ions outside of the light, which I know are not fully charged. Touching each together, like they would be inside the light. I get a reading of 7.94 Volts. This is known as "Vbat" (battery voltage)

• 2) With light switch off, I put the Li-Ions in the light. I then put the tailcap in position, with spring touching back of Li-Ion, but do not push down and screw it on. Now I turn on my DMM with it set to measure Amps. I turn on the switch for the light while it is resting on a brick...but with the tailcap off, there is not a complete (closed) circuit, so light stays off.

Now I take my black (Neg) DMM probe and touch it to tailcap edge where there is bare aluminum just above threads. I simultaneously touch my red (Pos) DMM probe to the back edge of flashlight tube, and my light comes on because I have now closed the circuit. I read the current as 3.52 Amps. When I take away my DMM probes, the circuit is now "open" and the light turns off.
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3) I now turn off the light switch, screw on the tailcap, and screw off the head so I can see the bulb in the bipin holder. I use the DMM's Pincher leads, and clip them onto each leg of the bipin inserted into the gold bipin holder.
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Now I turn on the light, and get a DMM reading of 6.65 Volts. This lower Voltage reading is known as Vbulb.
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• So now we have readings of

• The difference between Vbat & Vbulb is 1.29V (7.94 - 6.65), which is the voltage drop because of resistance in the light (including bulb). Now we use Ohms Law to see how much resistance is in my setup, since we know the current, and how much the voltage dropped.

• There are 1,000 milli-Ohms (mOhms) in 1 Ohm, we could also say that I have 366 mOhms of resistance in my setup. In effect, this resistance is both "protecting" my bulb by lowering the voltage delivered, but is also decreasing my lumen output.

• Now I know that when using fully charged Li-Ions at 8.3V, I would only see about 7V at the bulb, so it is not going to flash. Good News! But now if I want to have around 7.5V delivered to the bulb for better lumen output, then I need to do some things to reduce the resistance down to about 220 mOhms in my light setup.

cat said:

Thanks, LuxLuthor.

I noticed that the unfused positive jack in my DMM says 10A. What happens if it gets more than 10A ? The DMM becomes the fuse?
I was wondering whether that meant I couldn't use it to measure vbulb on a 646** setup.

I thought it might mean that I need a Fluke multimeter. :( A few months ago, I thought M6's were way too expensive. Now I've spent more on MAG stuff, and I think I need a bench power supply. :( And it seems like I need a high-end RC charger to charge torch batteries! :(

That reminds me, I was thinking how I was going to buy Fluke test leads that that cost 3 times more than my DMM. :duh2:

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LuxLuthor said:

rizky_p - You just line up batteries and measure voltage as shown in pix...red (+) probe goes to (+) end of battery. These are two Li-Ion Pila brand batteries.

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rizky_p said:

Hi Lux, thanks for helping a newbie like me..

One question, how do you measure the VBatt? is it like measuring forward votage on LED?

thanks

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LuxLuthor said:

Cat - Like a fuse in your car or any other product, when you pump too much voltage and/or current through it, the fuse will "blow" and either break the circuit, or end up not being able to test the function (like current in this case).

Fuses are designed to protect your DMM.
You don't have to use a Fluke or Fluke probes, that's just what I had on hand to demonstrate this topic. You can attach wires to the bipin legs and measure voltage of wires (just don't use too thin of wires, and don't let them touch each other when wrapped around bipin legs).

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js said:

ALWAYS ENSURE THAT THE DMM LEADS ARE IN THE CORRECT HOLES TO MEASURE THE PARAMETER YOUR DIAL IS SET TO MEASURE.

If you switch from voltage to current measuring, you must move the red lead.

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cat said:

This DMM (I think it's quite typical of a cheap DMM) has 3 jacks one above the other on the righthand side: the black COM one, the "normal" red positive one, and then the red one marked 10A UNFUSED.

The background to my question:
A while back (and it seems like a long time ago because of the steep learning curve) I read something about measuring vbulb by poking the DMM probes into the socket and having to have thin enough probes. Then, recently, I saw Petrev's photo showing it being done with some well-shielded crocodile clips clipped to wires running connected to an unmounted Kiu socket.

When I noticed the other day that my DMM said 10A unfused, I wondered if it was capable of what I'll want to do when all my parts arrive: measure the vbatt with bulbs that will be drawing a bit more than 10A.

So,...DMM test lead clips on wires from the bi-pin bulb, DMM set to measure voltage, red test lead in the normal and the bulb is a 64458 that might be drawing more than 10A... will that not be too much for the DMM to handle?

My local electronics component shop has only got test leads with those tiny crocodile clips with...unsuitable plastic sheaths. So, look at online catalogs...and the most suitable seem to be the hook probes (similar to your pincher probes.)

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