Please help me with my LM317-T LED driver. Answered
A while ago I bought a 10W 12V LED on line to see if I could build a small projector. Then somewhere, I hear you need to build a driver for it, otherwise you WILL blow the thing. So I start googling and tinkering and find a few basic formulas, P=IV, ==> P/V=I ==> 10w/12V=0.833A
So I need to get a current of 833mA at 12v to run this light, next step.
According to most sites, the cheapest way of doing this is with a simple LM317-T Regulator.
The ADJ voltage on this is 1.25v, so, V=IR ==> V/I=R ==> 1.25v/0.833A= 1.5 Ohm.
So I pick up a few regulators and a fist full of resistors at my local electronics shop, dust off an old soldering iron and thow it all together.
Then I plugged it on to a 12v 1.0A Power Adapter. I'd heard the 317 had a Vdrop of about 3 volts, so I took some readings to make sure everything was ok. Got 620mA between the Power/LED, the LED/LM317, and LM317 to the Power, all is good.
Voltage was 12.13v across the Power, 9.20v across the LED, and 2.91 across the LM317v (0.95 over the resistor). All the numbers looked fine.
Here's where it got tricky. Knowing the LED was rated for 12v, and the drop over the LM317 was ~3v, I dug up a 15.2v 1.2A Power adapter, and plugged that in instead.
When I took the readings however, my current was up to 810mA, but the voltage was still down at 9.56v. The Power adapter was at 15.20v, and the 317 was dropping 5.67v.
This is the bit I don't understand.
How can I get my volts back?
I know the LM317-T regulates its current via the resistor, but I'd assumed the voltage was dependent on whatever you plugged in, minus a constant 3v drawn by the regulator. This assumption is obviously wrong.
Is there a way I can get the voltage back?