Power curves

How do I use a Multimeter?
I just found one in my garage.

tomcat said:

im making a maglite one what resistance for 2xAA to run at like 230mw (no lm317 of course) im so crap at eletronic maths.... never my strong point. my multimeter says 750mw no load on the pair of AAs..

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How can a multimeter say 750mW? And no load at that?

You didn't go and measure the current from the two batteries, did you? If you did, you shorted the batteries, as if you would put a piece of wire between the + and -.. That's not very good for the batteries. Especially if you do it for a while, because they violently discharge, only limited by their own internal resistance.

You can only measure voltage on the batteries that way (by touching the + and - with the multimeter). You can only measure current by putting the multimeter in series, between one pole of the battery and the load. Otherwise, the current you get is the short circuit current - the maximum current your batteries can put out.

And a pair of AAs should be well capable of putting out a few Ampers in a short circuit, so i really don't understand the reading you got..


You should really build or buy a driver circuit, and power it from a pair of rechargable 3.6V Li-Pos.. That would not only be safer, but the power would remain the same for the entire life of the batteries, from 8.4V when they're both full, down to 6V when they're both empty. You can get such Li-Pos in AA or even AAA size from DealExtreme..

If you're not carefull, you're just gonna kill your LD.

Dude_With_Lasers said:

How do I use a Multimeter?
I just found one in my garage.

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You grab both of those ends of the cord in your fist, and fling it around like a pair of nunchucks..


Or, if you want to measure the voltage for some strange reason, you put it in the voltage range (select the next higher voltage, than the one you're measuring) and simply put the probes across the + and - pole..

Don't do the same in current measuring mode! You'll short the battery.. You can only measure current by putting the multimeter in between one of the poles and the load (in series!!!). The multimeter acts as if it was just a piece of wire, connecting the battery to the load.


A more elegant solution is, to solder a 1% - 1 Ohm resistor in series with the load, and just measure the voltage drop across it, in the 2000mV range. The voltage drop across a 1 Ohm resistor in mV is the same as the current in mA flowing through it. It's better and safer, because you don't have to disconnect the load from the circuit to put the multimeter in between...


But i think for questions like these, a new thread should be made, asking "How to prevent killing my battery?" or "What is a multimeter and how to prevent killing my battery with it?".. This one was supposed to be for the LD power curves..

Igor, did you even notice how old these posts are?

chido said:

Igor, did you even notice how old these posts are?

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Nope...


I was looking for the power curves for an open can, and someone pointed me in this direction, and when i got to the end of the thread, i thought the posts were new..


Sorry..


My multitasking abilities are not what they used to be anymore.. I'm obviously getting older.. Confusing threads and all.