How can I convert 4.5 volts to 3.2 volts using a voltage divider?

Question

I'm powering a laser with these specifications:

650nm laser diode, 3.2 VDC

Current consumption is 40mw at 3VDC.

I've got a 4.5 volt wall wart that I was going to power it with. A friend of mine said to create a voltage divider with two resistors. I use this site:

http://ourworld.compuserve.com/homepages/Bill_Bowden/r2.htm

putting in 4.5 v as my in voltage and 3.2 as my out voltage and 0.25 as my amps but I get really odd values for the resistors (5.2 and 12.8). That doesn't seem right and I don't see resistors for sale out there with those values. What values of resistors should I use?

Thanks

Answer 1

You do not need 4.5 volts DC to light up the 3.2 volt LED. Just get two pieces 1.5 volts battery,join them to obtain 3 volts (it has 3.2 volt when battery is new). Connect it directly  to the 3 volt LED. It shall light up.

If you insist to use a 4.5 v power source, you need a resistor in series with the LED. The 1.5 volt power shall be wasted on the resistor to produce useless heat.  The resistor is 100 ohms at 0.5 watt is good enough to do the job. Cost you 10 cents.

Answer 2

pick the closest value, probably 5.6 and 12. they need to be 1 watt resistors.

but those values won't work very well. first of all they will draw about 260ma out of the battery.

Better is to use a few rectifier diodes in series with the output. 2 diodes will reduce the 4.5 volts to about 3.1 to 3.3 volts, probably good enough for the laser. And they won't draw any additional current.

Or you could use a LM123 regulator. Download the data sheet, it has schematics on it.

edit: Lee is correct, I didn't look deep enough into your question. If it's just a semiconductor diode, like an LED is, you can't drive it directly with a voltage. You need a voltage higher than 3.2 by a volt or two and a series resistor.

Measure the output of your supply, some of the cheap ones are very poorly regulated. Check that it is DC and not AC.

40mw and 3 volts is 13 ma. you want to drop 1.5 volts at 13ma, which is 100 ohms.

.