# REALLY Simple Guide To Figuring Out Voltage In Maglite

#### LuxLuthor

##### Flashaholic
I get a number of questions in PM's from people trying to figure out how to use and select the proper bulb and battery combination voltage for their particular Maglite setup. I remember not too long ago how confusing all of this was, so let me do my best to give a few SIMPLE pointers. PLEASE, NO EXPERT, DETAILED INPUT OR COMMENTARY from all of those electrical engineers who sleep on superconductors!
• 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:
• V = I x R or Voltage (Volts) = Current (Amps) x Resistance (Ohms)
• 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.

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.

Now I turn on the light, and get a DMM reading of 6.65 Volts. This lower Voltage reading is known as Vbulb.

• So now we have readings of
• Vbat = 7.94V
• Amps = 3.52A
• Vbulb = 6.65V
• 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.
1.29V = 3.52A x ??? Ohms so then solving for Ohms:

1.29V / 3.52A = 0.366 Ohms
• 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.
8.3 Vbat - 7.5 Vbulb = 0.8 V

This smaller drop in voltage would both protect my bulb, and give me better lumen output...so I want to try and reduce my resistance to: 0.8V / 3.6A = 222 mOhms
How to reduce your resistance is another topic.

#### sed6

##### Enlightened
:thumbsup: Another great post Lux! So much of the info you put out is truely useful for us newish flashaholics. I have more of your posts bookmarked than anyone else I can think of. Thanks Lux!

#### littlejohnle

##### Newly Enlightened
Wow, great write up. Lots of very useful info.
So thats how you measure the current!!

Another thread that may have to be stickyed.

#### LuxLuthor

##### Flashaholic
Thanks guys. Be careful if you are measuring current in a higher level (like 9-12Amps), because with the initial spike of voltage/current when first turning on your light you can blow the 10A fuse in the DMM. Been there, done that. Measuring higher current is done with a "shunt" like you see in pix of my destructive bulb testing setup, but is a more complex scenario.

#### cernobila

##### Flashlight Enthusiast
Lux,

Now I know what is going on.......after all this time on here I finally made a significant step forward in my knowledge in this area.

now, how can I tweak my light to give me the ideal resistance.......I am ready for the next step......YEAH.

#### cat

##### Enlightened
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:

#### rizky_p

##### Flashlight Enthusiast
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

#### js

##### Flashlight Enthusiast
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:

DMM's 10 Amp current measuring lead position IS fused, and will blow if you put significantly more than 10 Amps through it. I blew the 10A fuse on my Fluke early in my hotwire career by moving the lead for measuring current, changing the dial to measure current, THEN I decided I wanted to know the starting, unloaded, hot voltage of the pack, so I dialed up DC Volts and put the leads across the battery contacts. POOF! Instant blown fuse--because while I did change the DIAL from DC current to DC volts, I did not change the LEAD POSITION!

So, be careful to ensure that you have the right lead position for the right dial position and right current range. If you change the dial to measure current you must switch the leads. If you change it again, you must switch back.

Lux,

Thanks for taking the time and effort (and pictures) to make this thread!

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#### mdocod

##### Flashaholic
hello js,
There are actually quite a few truly unfused multi-meters out there that will turn their internals into a big gooey fuse if you aren't careful. And there are quite a few who's low current measurement setting is the one that is fused while the high current side is unfused. (go figure)....

My old Mastech is a perfect example of this, the current measuring "plug" is rated "20A Unfused" on the front, and in fact, I have measured as high as 32 amps with it.. (probably not good for it)... I have searched it's internals through and through for a large 20A+ fuse for that channel and it does not exist, there is a small fuse on the "200mA fused" plug. I even had friend who has a similar multi-meter make a "mistake" (plugged into current metering tried to check for line voltage, POOF) with his recently, and ended up with a destroyed multi-meter. Luckily they are only about \$10-20... In some cases, it's easier to just find a replacement cheap multi-meter than it is to find a fuse that will fit some of them! LOL...

#### LuxLuthor

##### Flashaholic
Guys, with respect please keep this topic on a simple level. Thanks

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).

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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#### sb_pete

##### Newly Enlightened
Thanks Lux.
I gotta get me some o' them pincher leads, any suggestions?

Thanks again
-Pete

#### rizky_p

##### Flashlight Enthusiast

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.

isn't that supposed to be the VBat? not the Vbulb?

thanks

#### LuxLuthor

##### Flashaholic
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

That is Vbat which was what you asked. I showed measuring Vbulb in first post with the pincher leads.

#### js

##### Flashlight Enthusiast
Lux,

Sorry. You're right. I'll edit my post down to the simple core of it, which was simply this:

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. If you switch back, you must move the red lead yet again.

This is simple. This is important. This is why I posted.

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#### cat

##### Enlightened
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).

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.)

#### cat

##### Enlightened
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.

...From the "normal" /default jack to the one that says 10A UNFUSED?

And (see my previous post), if I am measuring voltage (or trying to) and the vbatt is 21.something V and the bulb is a 64458, will the current draw of slightly more than 10A be a problem?

#### cat

##### Enlightened
Lux, are we limited to Voltage here? (I have a newbass question about measuring resistance.) :tinfoil:

#### js

##### Flashlight Enthusiast
Another simple but VERY important warning:

HALOGEN LAMPS CAN EXPLODE! Wear safety glasses if you are working with bare, powered up lamps. DO NOT TOUCH THE GLASS WITH YOUR FINGERS, either, as the oil will cause an explosive failure when the glass heats up. If you do handle a lamp, clean it with isopropyl alcohol and let dry before powering it up.

#### LuxLuthor

##### Flashaholic
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.)

If you are using a DMM that does not have a fuse when measuring up to 10Amps (i.e. is "unfused"), then yes, subjecting it to higher than 10A current will damage your DMM. Don't do that 64458 current measurement. This brand has unfused 10A like you describe. Note paragraph titled Current Measurement - 10A DC under "Measurement For Managers."

Fire away on resistance measurement question.