how to measure current ot the led

[aljsk8]

how do i measure current going to an led

i have a multimeter and i know where to set the dial etc and its digital so i know how to read it

all i need to know is do i just turn the light on and put the probes on the positive and negitive while its running?

will this damage the led?

is this the correct way?

id like to try this in the next 1/2hr if possable

thanks

 

[greenLED]

You gotta place the DMM leads in series (as if it were part of the wire connecting the LED to the rest of the circuit) with one of the LED's contacts.

|-----O DMM O------LED------------|
|_____________Batt______________|

 

[aljsk8]

dmm?

so i take it in normal flashlight terms this would require de-soldering one led leg/ tab/whatever and putting the probes between the leg and the solder bit on the driver?

i was hopeing i could just put it across in paralell while its all still in one peice - i take it this wont work?

thanks

 

[Doug S]

how do i measure current going to an led

i have a multimeter and i know where to set the dial etc and its digital so i know how to read it

all i need to know is do i just turn the light on and put the probes on the positive and negitive while its running?

will this damage the led?

is this the correct way?

id like to try this in the next 1/2hr if possable

thanks

Your possible risk is not to the LED. Some, but not all, electronic driver circuits for LEDs can be damaged if operated without a load. As Greenled as said, the meter needs to go in series with the LED, if you even monentarily operate the circuit without the load, example, you meter probes slip off of where you have them connected, your circuit may be toast.

 

[aljsk8]

so if the led is attached to the driver curcuit and is switched on - is it not under load

this is what i had in mind

ignore the fullstops - they were to get the picture to work

--------
/........./
/........./
/ dmm. /
/........ /-----------(+)---------\
/........./ ................................o (led switched on)
/........./------------(-)--------/
--------

so if the led is on and you hold the probes on the leds is it not under load?

Alex

 

[greenLED]

dmm?

DMM- fancy-shmancy for multimeter

...this would require de-soldering one led leg/ tab/whatever and putting the probes between the leg and the solder bit on the driver?

Exactly

i was hopeing i could just put it across in paralell while its all still in one peice - i take it this wont work?

It'll give you "a" reading, but not "the" reading you're after.

so if the led is attached to the driver curcuit and is switched on - is it not under load

Nope. "Under load" means the light is switched on. Some circuits burn up if you apply power and the LED is connected.

 

[MrAl]

Hi there,

The best way to measure current is indirectly. That is, connect a 0.1 ohm resistor
in series with the LED, then turn the unit on, then measure voltage across
the resistor and use ohms law to calculate the current. The 0.1 ohm resistor
should be SOLDERED in place so that it can not ever come loose, because if it
does, as mentioned above the driver circuit may burn out in less than a half of
a second.
One of the problems with using the meter without a resistor is that if you set
the meter on the wrong current range or a lead comes loose the circuit burns
out and you cant use the flashlight until it's repaired.

The formula to use with a 0.1 ohm resistor is this:
I=V/0.1 or
I=V*10

so if you measure 0.035 volts across the resistor then that means there is 350ma
flowing through the LED.

Play it safe and use a resistor, and solder it in place before turning on the light.

 

[Illum]

 

[MrAl]

Hi there,

I would be careful about that last circuit...some drivers can not take a short circuit
on the output.

 

[jrmcferren]

In the last circuit, disconnecting the LED is an unnecessary step, electricity will follow the path of least resistance (in the case of a meter shorting across an led) which is the meter. The AMPs will read higher for a resistor based supply, don't know the results for an electronic supply.

 

[abvidledUK]

Not quite to the LED, unless it's direct drive...

Just replace the tail cap (switched or twisty) with the leads from the multimeter, in DC High Current (10A) mode, to get a direct measurement of the current taken from the battery, through the LED and any electronics.

ie, tail cap off, leads one on neg of battery, one lead to end of torch casing thread.

 

[TJZ]

MrAl, greenLED, Doug S, jrmcferren, abvidledUK,

I have a question if anyone of you knows the answer.
Sounds like you know a lot on this subject.

When you take a current reading in series how does having
the wrong current range on the meter burn out the circuit?
Is there voltage induced from the meter going to the circuit also
when measuring current in series and if you have the wrong current range it
is even higher and it can blow out the circuit?

And if you have the proper current range on the meter, does this mean
no voltage is induced from the meter going to the circuit?

Thanks,

Tom

 

[abvidledUK]

Usually the lower current ranges on the multimeters have some sort of low current fuse, to protect the fine wires in the meter from burning out. Fuse rated at less current flow than the fine wires.

High current range usually measures voltage across built in resistor, converting to amps.

 

[aljsk8]

abvidleduk - im interested in the tailcap off method - this is a lux iii so i dont want to start dismantling it but taking the cap off sounds good

1 question do i turn it on when i do all this? (im guessing yes)

will it give the same results as cutting the led and doing the in series thing?

thanks

 

[aljsk8]

thanks for all the help every one - its very usefull especially the pics (illum) but i think its confused me more as to what i need!

i think i need to clarify why im doing this as im not sure if it is the LED current i want

i thought the driver current and the led current would be the same - but mabee not?

anyway heres what i want to do...

i have a stainless steel civictor (lux iii) i want to put the ssc p4 led in it

so i want to know what current the driver is at the different modes (med, low, high) so i can guestimate the lumens (if its 350ma on high then i can guess ill be getting around 90 - 115 lumens on this mode) and i also wat to know how hard the ssc will be being pushed - for the issue of heat - as stainless is a poor conductor.

so will this tailcap method work?

also what is dc high 10a mode? my multi meter just has numbers around it like 2, 20, 200 etc but not high i dont think
ill post a photo of what ive got later so people can help me

i didnt know this was so complex! (well its probably not - im just stupid!)

Alex

 

[abvidledUK]

abvidleduk - im interested in the tailcap off method - this is a lux iii so i dont want to start dismantling it but taking the cap off sounds good

1 question do i turn it on when i do all this? (im guessing yes)

will it give the same results as cutting the led and doing the in series thing?

thanks

If the switch, or twisty operation is in the tailcap, it will automatically complete the circuit when you put the 2 multimeter leads across the neg battery and the end threads you have now exposed.

If side or front switch or twisty, yes, turn it on !!

It will only show total current taken by torch, so current to led's may be less depending on electronics, if any.

 

[abvidledUK]

.

 

[aljsk8]

heres the problem -

i want to learn to mod! - i wont learn anything buying another torch!?

also - my ss civictor is what i want - no1 has a stainless steel 1aa torch with with a cree or ssc in it (apart from the 2 ever made) - so i cant buy one - and if i could im guessing it would cost a bomb!

although my electrical knowledge from a "theory" point of view is crap
"practically" im not to bad - im very good at handywork - so the "taking out the led, putting in the new one" wont be a problem

to be honest i dont need these numbers i can just buy the thing stick it in and it will work

but id just like a rough idea of "how" it will work before i try it

sorry if this post sounds ranty - your all right im just a bit dumb with all this amps, volts stuff! lol

 

[MrAl]

-------------------------------------------------------------------------

STARTQUOTE=TJZ
MrAl, greenLED, Doug S, jrmcferren, abvidledUK,

I have a question if anyone of you knows the answer.
Sounds like you know a lot on this subject.

When you take a current reading in series how does having
the wrong current range on the meter burn out the circuit?
Is there voltage induced from the meter going to the circuit also
when measuring current in series and if you have the wrong current range it
is even higher and it can blow out the circuit?

And if you have the proper current range on the meter, does this mean
no voltage is induced from the meter going to the circuit?

Thanks,

Tom ENDQUOTE

-------------------------------------------------------------------------

Hi there Tom,

The problem with using a meter alone to measure current is that
on some of the ranges it might switch in a rather high value resistor.
This extra resistance causes the driver to put out a very high voltage
which burns the output transistor out.

The safest way is to use a 0.1 ohm resistor and measure the voltage
across it with a digital meter set on low volts dc.

BTW, some driver circuits can put out over 100v with no LED connected,
for a very short time after which the output transistor blows out due to
overvoltage between collector emitter and/or collector base.

 

[Doug S]

TJZ Asks:
MrAl, greenLED, Doug S, jrmcferren, abvidledUK,

I have a question if anyone of you knows the answer.
Sounds like you know a lot on this subject.

When you take a current reading in series how does having
the wrong current range on the meter burn out the circuit?
Is there voltage induced from the meter going to the circuit also
when measuring current in series and if you have the wrong current range it
is even higher and it can blow out the circuit?

And if you have the proper current range on the meter, does this mean
no voltage is induced from the meter going to the circuit?

Thanks,

Tom

Tom, expanding a bit on MrAl's excellent and entirely correct answer.
You ask specifically how meter range can be an issue. If you change range in the course of your measurement, be aware that some meters have ''break before make'' contacts which means that the driver will be momentarily unloaded during the range change. In some cases this is all it takes to fry it.
Regarding your question about induced voltage, in the strictest engineering sense, if measuring pure DC, No. There will be a voltage *drop* through the meter since it inserts a small amount of resistance. The higher the ammeter range selected the lower this resistance. To minimize the perturbation caused by insertion of the meter into the circuit, use the highest range possible consistant with the resolution you need. If you check the specifications section of your meter manual it should give the ''voltage burden'' of the meter at various ammeter ranges. Many meters have a burden of 0.25V at full scale, e.g., if measuring 100mA on the 200mA scale the drop through the meter will be 0.125V. The resistance of the meter leads can add additional drop. Be aware that the DC ammeter function does best measuring DC. If the DC has a high ripple component or is high frequency pulsing DC, the measurements can be erroneous. Under these conditions voltage *is* induced in the meter's internal inductance. It can cause the erroneous readings but will not be a cause of harm to the circuit.