low voltage from battery with LM338

[NeilP]

Can any one help me with resistor values for use with an LM338 to run one of these little 2x AAA powered LED rear lights

http://export.rsdelivers.com/product/texas-instruments/lm338t-nopb/5-amp-adjustable-regulator-lm338t/0460900.aspx

I see various suggested used schematics for use with the reg, but not sure if I really need all the added protection of capacitors and diodes or if I can just do what I need ( 12volt in 3 volt out) with the reg and a pair of resistors.

I already have a 12volt source on the bike , that is switched via the light switch so will use that as the input, just need to regulate it down to 3 volt for the rear LED

 

[John in CR]

I think it's even simpler than that and all you need is the correct resistor to supply the LED(s) properly at the new supply voltage. Measure the voltage drop and current with the AAA's and input the results in a LED calculator. http://www.hebeiltd.com.cn/?p=zz.led.resistor.calculator

 

[NeilP]

Oh yes, i could just do it with a resistive dropper, but wanted to go with the reg in case i wanted to add further in future. without having to bother about changing resistor values etc.

I like the reg idea better than the resistor, ok a little more complex..but 'neater' in my mind

 

[bigmoose]

Neil, just so you have the information.

Put a 120 ohm resistor from the output to the adj terminal.
Note that the internal reference voltage of the LM338 is 1.24 V nominal
Now add a resistor from the adj terminal to ground that equals Vout = 1.24(1+R/120)

So for 3 volts output R = 170 ohms if I did the math right. You can ignore the bias current for the accuracy level you are after.

 

[NeilP]

Thanks, that is the info I needed

Thanks

 

[John in CR]

If you're adding raw LEDs later, won't they need a resistor anyway to avoid over current? Then you'd be looking at 2 resistors...connect in parallel, so no change in the first one, so how 2 resistors and a regulator is neater than just 2 resistors doesn't compute in my simple mind. In my mind you already have a voltage regulator putting out 12V.

 

[NeilP]

No, not LED's an LED lamp

By neater I mean more elegant, less crude than just a voltage dropper resistor, rather than physical anc looks. 'neatness'

 

[bigmoose]

Neil,

Here is a schematic of a pcb that I did for my kids 1997 Honda back in 2008. The LEDs in the wing corroded out, and she liked it; so I made a pcb to replace them. This is a quick and dirty solution. I used the inherent Vbe drop of about 0.6 volts to limit the current in each string to about 60 mA's. Now the Vbe is highly temperature dependent; but it worked for this application.

You can string as many LEDs in series as you have battery voltage to drive. You need to watch the power dissipation in the NPN limiter if you have high battery voltage and very few LED's. I used some Piranaha LEDs that I got cheap from electronic's goldmine here in the USA.

 

[NeilP]

That is certainly worth keeping for future reference, thanks

But in this case, all I am doing is powering a standard unit usually powered by 2AAA batteries. The GF always leaves it on, but the headlamp is from the bike battery, which powers off when she powers off the controller and the headlamp.

 

[John in CR]

No, not LED's an LED lamp

By neater I mean more elegant, less crude than just a voltage dropper resistor, rather than physical anc looks. 'neatness'

Then I take it you'll be removing the resistor that is probably inside the lamp.

 

[NeilP]

Er..no, just taking out the two batteries and wiring the output from the reg straight to the tags where the battery is normally connected,