How to safely measure current and voltage...

[GrayFox]

I finally sprang for a DMM. Is there a safe way to measure the operating voltage and amperage of my SF E2/KL1/KL4s and McLuxen?

Is there a simple carrier that can be used with 123's ( say 2 and 3) that can be used as a temporary power source?

Is there anything I nead to be especially careful about.

--Jerry

 

[CM]

Are you trying to measure current from the battery (trivial) or the LED (a bit harder). To measure the LED current you need to insert the ammeter in the LED circuit path which means disassembly of the light. Make sure you don't run the circuit open or you can blow the converter. The exception is the KL1 which has a voltage clamp (zener) to keep the converter from trying to pump current into an open circuit. The KL4 may have a zener also, but don't count on it. Just to be safe, I never run the circuits open.

CM

 

[jayflash]

If you want to measure total drain on the battery the DMM leads must be in series with the load. On some lights you can remove the tailcap and put one lead on the negative battery contact and the other lead to the metal flashlight body - thus completing the circuit.

To test voltage under load you must put the probes in parallel with the batteries - one test probe on the positive end and one on the negative. This may require a battery holder and jumper leads with alligator clips going to the lamp assembly.

Note: if your test leads are reversed, the meter will read a negative value but it won't harm the meter. Just make certain that an LED unit has correct polarity from the battery or damage can occur.

 

[Doug Owen]

[ QUOTE ]
GrayFox said:
Is there a simple carrier that can be used with 123's ( say 2 and 3) that can be used as a temporary power source?


[/ QUOTE ]

If you don't quote me, I'll confess to a paper tube (rolled into a tube), a couple of paper clips, a rubber band and some tape (my personal favorite being masking tape...).

How's that for simple?

To connect to the flashlight when not otherwise convenient to make the positive contact, I've used a piece of stiff solid copper wire (12 or 14 AWG ' house wring') with the end stripped back and folded over. Then you warp more masking tape around it just behind to build up the diameter until it's a fairly snug fit in the tube. Strip and inch from the other end, clip lead onto the end of the tube of the flashlight (for the negative connection) and you're ready to go. The tape bushing keeps the wire centered so it hits the 'positive button' in the end of the battery tube.

Doug Owen

 

[MrAl]

Hello there,

For testing the voltage you simply place the meter leads
across the LED while the light is turned on.
You must, however, make sure the meter is switched to
a voltage setting and not a current setting or you risk
burning out the drive circuit (!).

For testing the current you have to do a little more
work. The safest method so far is to disconnect one
lead of the LED and solder a 0.1 ohm resistor in series
with it. When you turn the light back on, you can then
measure the voltage across the LED and relate that
measurement to current by using ohms law.

Measuring current is obviously harder to do and brings in
a certain risk factor. If it's not done right, something
could be damaged permanently. If you havent done
current measurements like this before it's probably a good
idea to practice on cheap lights first.
Normally if something is left open it doesnt hurt, but
with LED flashlights with built in driver circuits an
open circuit could cause higher then normal output voltages
that permanantly burn up the output transistor.

Take care and good luck,
Al