Driver Current not what I expect.

[iNcHeS]

Hey guys,

Out of pure interest I decided to hook up some drivers I had today to a multi-meter to determine if the current supplied to the LED was 'as advertised on the tin' so to speak.

I tested a 4.5A 12x7135 driver first and to my surprise on high the DMM was only reading approx. 2.56A. I then took another modified 11x7135 that supposedly provides approx. 3.9A and only ended up with approx. 2.04A.

I tried the test on the both drivers measuring the current between the driver and the LED and the battery -ve and the driver (tail cap).

I'm using a DX Ultrafire 18650 battery (yellow with red and white stripes) which could be the first problem. Any other suggestions?

Cheers.

*EDIT: I did the same test with a light I had that runs a 1.4A 7135 based driver and was getting 1.38A on high. So either the battery can't handle the higher current, or those high current drivers are rubbish.

Please help!

 

[HKJ]

Both battery, DMM and probes can be a problem. Usual a DMM has between 0.2 and 0.6 volt in burden voltage and some probes are not suitable for current measurements (I have measured 0.1 ohm for one probe on a set).

 

[iNcHeS]

I just narrowed it down to the battery I was using, found a few other batteries and the currents look more promising!

Thanks for your help!

 

[uk_caver]

Bearing in mind what HKJ has said regarding meter/probe resistance, with a 7135-based circuit, assuming the chips are all genuine and all connected*, a meter could be usefully used as a voltmeter to measure the voltage drop across the 7135s, from ground to LED-.

If that voltage is meaningfully above the 7135 drop-out voltage of ~0.12V, that would suggest that the 7135s should be giving full output (and that the power source is up to the job).

For borderline cases, whether low power source voltage or high LED Vf (or high resistance connections) it could be that using the meter in the circuit as an ammeter would show low current, but taking it out of the circuit and measuring the voltage across the 7135s would suggest that with the ammeter not in the way, things were working OK.

Measuring the voltage drop across the 7135s can also give some idea of the 'headroom' available, and how far a particular cell might be able to be discharged before the setup couldn't maintain full output.

[edit] (*and all powered full-on. If the 7135s are not permanently 'on' the voltage (or average voltage) across them can be high without much current flowing - in a typical variable-power setup where the 7135s were being driven with a PWM signal by a microcontroller, the lower the power level, the higher the average voltage between ground and LED - would be.)

 

[iNcHeS]

Ah that makes perfect sense, thanks so my uk caver!