Current draw discrepancies

Billson · Jun 28, 2004

I've read a lot of posts describing current draw of lights in BST. I'm wondering why some leds being powered at 700ma are drawing more than an amp from the battery. What causes this?

BatteryCharger · Jun 28, 2004

Most likely you're reading about LEDs that are RATED at 700ma, being overdriven at 1 amp.

hacky · Jun 28, 2004

Or maybe they use a step-up converter, which boosts the voltage from - for example - 3V to 3.7V, and because of this, it requires a higher current to drive the LED at 700mA. So the LED is driven at 700mA, but the converter draws (for example) 1000mA from the batt. Correct me if I'm wrong.

Billson · Jun 28, 2004

This thread is what promted my question. It could be a typo but I recall having read threads like this before.

hacky · Jun 28, 2004

That one is using a step-up converter, because a singe CR123A cell will provide only 3V, and the LuxeonIII LED requires more (around 3.5V).

So there is a converter circuit, which boosts the voltage from 3V to 3.5V, and provides 670mA (constant) current to the LED. But the converter has to draw more current in order to achieve this voltage boost, that's why it's drawing 1.4A from the batt.

pbarrette · Jun 28, 2004

Hi Billson,

And if that still doesn't make sense, consider this:

Voltage step-up circuits basically suck extra current from the source (battery) and convert that current into extra output voltage.

So if you have an LED that needs 3.5v and you want to run it at 700ma, but your battery only supplies 3v, then you can "steal" extra current from the battery and convert it into extra output voltage to hit your 3.5v goal.

So, in the thread you mention, that extra (1.4A - 670ma) = 730ma of current coming out of the battery is being turned into extra voltage to power the Luxeon.

pb