Welcome to Laser Pointer Forums - discuss green laser pointers, blue laser pointers, and all types of lasers

Buy Site Supporter Role (remove some ads) | LPF Donations

Links below open in new window

FrozenGate by Avery

LED driver questions

Zeebit

Zeebit

0
Joined
Aug 27, 2012
Messages
1,110
Points
0
Hey guys,

I've done some reading but I still have some things to clear up.

I will be making an LM317 based unbranded 3W White LED driver.

Considering the dropout voltage and the LED's forward voltage, how much input voltage is needed by the driver?

Can I use a red LD test load to test the driver for the white led?
 





The LM317 has a dropout voltage of around 1.5V. A white LED will usually be around 3Vf, meaning you'd need a minimum input to the driver of 4.5V.

No need to use a test load on linear drivers, simply short it across your DMM on the 10A setting and you'll get the current reading.
 
Actually, a constant current LM317 drops about 3V, and in the worst case senario, a white LED will drop 4V, so a minimum of 7V is needed.
 
Last edited:
I will be using a wallwart to power the circuit. Do I need to use a voltage regulator or will the circuit be enough to load down the supply to the correct voltage?
 
It depends totally on the wallwart. If it is DC, then it should be fine without any other circuitry.
 
Yeah its DC. I guess >700mA should load it down enough.

Thanks Fire and Things!
 
Has anyone used the $1 thermal adhesive found on eBay? Is it just thermal compound or does it harden?

I know that the Arctic Alumina would be the best choice but if the cheapo works then why not.

images
 
Adhesive would imply that it will harden, but ask the seller. They often don't know what they are selling on ebay.
 
Last edited:
It goes by many names thermal paste, compound, plaster, glue, adhesive. I know that the seller will most likely not know the real deal.

I guess I'll give it a shot. Its only a dollar anyway.

Btw, do I need to use a star shaped pad (or whatever its called) before mounting the LED the heatsink?
 
No. But if the LED gets electrical contact with it, then you need to watch out for any other electrical contact that may be touching it.
 
I'd be careful using a wallwart directly, I've seen a few even 10V overspec! Would be a good idea to measure them first to make sure. I still wouldn't recommend it though.
 
The wallwart I'm using is putting out 20V when set to 12V if there is no load. I've hooked up the driver without the LED and the input voltage dropped to 12V. I'm using 2 LEDs in series.
 
No. Things said test loads are not needed with linear drivers. I simply hooked it up to the wallwart and shorted the output to the amps terminal on my dmm.
 
You must log in or register to reply here.


Back
Top