How do you check mA going to a LED?

[Lunal_Tic]

I thought I got it right. I changed the + lead to the "A" port on my meter and used the DCA range at 2 but when I put it across the Lux with power going to it just turned off the light and blanked the meter screen.

The magic smoke is still OK since the light still works. I tried to get a voltage reading too but with the + lead in the V/Ohm port, and the voltage going to the LED was 3.22V.

Does any of this make sense? How do I get the mA of converter under load?

Thanks,
-LT

 

[greenLED]

For measuring current going to the LED, you need to place the DMM leads in parallel with the LED. That is, break the circuit, attach one DMM lead to the (+) wire, the other to the LED, and power the light. I think... /ubbthreads/images/graemlins/smile.gif I usually have to fiddle a bit with my DMM and a couple of crocodile clips to get this to work.

Here's a thread I bookmarked a while ago. I seemed to have deleted the one where I got what I wrote in my first paragraph.

 

[jtr1962]

The problem with using a multimeter to measure current is that it introduces a proportionately large resistance on the order of a few tenths of an ohm or better. The best way to measure current of something like a Luxeon being driven by a converter is to insert an 0.1 or 0.01 ohm, 1% resistor in series with the Luxeon, measure the voltage drop across the resistor using the millivolt scale of your multimeter, and then use Ohm's law to find the current (I = V/R). For a current of 350 mA the voltage drop across an 0.1 ohm resistor will be 0.035V, or 35 mV. For currents larger than about 1 amp it's probably a good idea to use an 0.01 ohm resistor instead to minimize the voltage drop.

 

[Lunal_Tic]

Thanks for the info. I think I'll try the "simple" /ubbthreads/images/graemlins/grin.gif way first since I'm not a EE nor do I have a test bench (yet) with a bunch of resistors laying around.

It used to be I just wanted to know if the light was bright and lasted a good while. Now it seems I'm slowly sliding into the dark abyss of testing . . . .Aaaaaarrrrrgggggghhhhhh.

-LT

 

[balazer]

For measuring current going to the LED, you need to place the DMM leads in parallel with the LED. That is, break the circuit, attach one DMM lead to the (+) wire, the other to the LED, and power the light.

You mean series, not parallel.

 

[rantanplan]

You should use the "20A" port on your DMM, because this circuits uses a much lower internal resistance. That means the influence of the DMM on the measured values is still existant, but less ... this port has no fuse, but under normal circumstances it´s difficult to get more than 20A in a "flashlight-related" environment .

 

[Lunal_Tic]

You should use the "20A" port on your DMM, because this circuits uses a much lower internal resistance. That means the influence of the DMM on the measured values is still existant, but less ... this port has no fuse, but under normal circumstances it´s difficult to get more than 20A in a "flashlight-related" environment .

Thanks for that info. I would have never guessed that was the case. I just thought one had a fuse the other didn't

-LT

 

[eebowler]

You mean series, not parallel.

lol balazer, you caught it too

 

[andrewwynn]

If you have a Jbin emitter you can find the current using a formula: http://voltage2current.rouse.com is the CPF thread where i describe it. It's pretty darn accurate and of course once you have voltage and current you can calculate power since it's simply the product of the two.

Depending on what circut you have.. most have a sense resistor between the LED- and the BAT-... so you can temporarily attach a fine wire or if big enough even just use a meter to measure the mV drop and multiply by 10 you'll have the mA (presuming that the Rsense is 0.1ohm.. most are).

-awr

 

[rantanplan]

Thanks for that info. I would have never guessed that was the case. I just thought one had a fuse the other didn't

-LT

I checked the resistance values of my second, older DMM (2A fused, 20A unfused). The 2A-port showed 0.2 Ohm internal resistance, the 20A-port 0.02 Ohm ...

Current measurement in a DMM works exaktly like jtr1962 has described. The DMM measures the dropped voltage over a internal resistance (from (20)A-Port to Com-port) and convert it to a current value for display. The 20A-Port uses a large shunt resistance, that can handle 20A (for a short time) and has therefore a low resistance. The A-Port uses a larger resistance, because it´s made for measuring much smaller currents. Remember, you measure actually the voltage drop and a small current over a small resistance result in a very small voltage drop, that would a require a accurate, but expensive meter ...

 

[wquiles]

rantanplan,

Since most good cables for meters that I have seen and measured are about 0.2ohms or so, how did you measured 0.02 ohms across your internal meter? What meter/equipment did you used?

Will

 

[greenLED]

lol balazer, you caught it too

brljsdhl... yup, series, not parallel!

 

[rantanplan]

oh, well ... I used a adjustable bench supply set to 1A, connected it directly (with two 1m long cables, 4mm connector/jack on each side) to the ports of the DMM ... short circuit

Then I connected a second DMM (set for voltage measuring) to the jacks of the cable (near the DMM of course) and did some difficult calculations with the voltage drop.

The test subject is a cheap DMM, 10 years ago its price was around 50 DM (25$) ... not, what I would call "high-tech"

I wanted to show up the difference, especially the magnitude ... the exact values may be different, of course.

 

[evan9162]

I'll vouch for rantanplan's method - I've done the same myself to characterize the total resistance of my DMM. I used several test currents to get an average, but the idea is the same. My DMM has 0.06 ohms, lead-to-lead on the 20A scale. It's a good thing to know, especially when measuring currents that could be affected by the prescence of the meter.

 

[Lunal_Tic]

OK I finally made a rig where I can get to all the leads. I connected the meter - to the + on the battery. Then the meter + to the + on the LED. I used the meter + plugged into the 20A port and the meter - into the COM port.

On the DCA section of the meter, seen above, I set to "20µ 20" and got a 0.25-0.27 reading. None of the other settings would register anything. Any ideas what this means?

TIA,
-LT

 

[Lunal_Tic]

Drive by

-LT

 

[rantanplan]

Other dial settings than "20µ 20" show zero current flow while the leads are connected to "com" and "20A" ???

That´s pretty normal.

The current flow over the 20A-port is measured only when the "20µ 20" setting is chosen. Other range settings (2A,200ma,...) only affect the (fused) A-port. That´s a disadvantage of the 20A-port because you only have a single (coarse) resolution. Not suitable for really small currents. "20µ 20" means actually "up to 20A on 20A-port or up to 20µA on the A-port" ...

 

[Lunal_Tic]

So what does the 0.25-0.27 reading mean? What does it tell me about the converter? IIRC this converter is supposed to be about 700mA but I'm getting odd results.

In a light that I built with a SYOH and this type converter I could only get about 1:45 runtime on 4x 2500mAH set up to run parallel (think that's right, only getting ~2.5V) but this particular converter is giving me ~5 hours with a Q3J LED and the same battery setup. I would have thought that the SYOH would have run longer since it's got a lower vF.

Ideas?
-LT

 

[rdshores]

The .25-.27 reading is the current in Amps or 250-270ma. If you have the meter hooked up correctly in series between the converter and the led, that is the current flowing through the led. What converter are you using? Check the current that is going into the converter the same way, with the meter on the 20A scale and the leads hooked up in series between the battery and the converter. Then switch the meter leads to the COM and V jacks and take voltage readings across the led and also the battery. Put the meter on the DCV 20 position.

 

[rantanplan]

0.25-0.27A ... 250-270mA. A bit less than the advertised 700mA . Should be noticeable that it´s to dimm.

First, what type of converter (specs?) is it? I assume it´s a boost regulator, that pushes a lower Vin to the higher Vf of the LED? Then I don´t understand what you´ve said about a parallel setup of 4x 2500mAh rechargeables and ~2.5V?

Common boost converters use a ~2-3V input to fire up a single Luxeon, that means NiMH rechargeables parallel connected (=1.0-1.3V) don´t have enough voltage for the circuit. The 2.5V you´ve mentioned, is this the voltage at the Luxeon? Or have you tried using only two rechargeables connected in series? (that should give you 2.4V for the convertor and it should work properly) :thinking:

... Moulder, Scully ???