Help and insight on voltage regulation

I know resistors limit current flow, but they also step down voltage, right?

I was wondering, how do you know what resistor you need if the current varies or doesnt matter how much the device receives? (based on V=IxR) Or am I supposed to use a different device to step down the voltage?

In this case, im going to buy a digital DC voltmeter for my car. It requires 5V/9VDC power supply. So im wondering what resistor I need to step the voltage down from 12-14V, down to that figure.
I also have a blinking circuit controlled LED light that works off of 2 AA batteries that i'd like to run off of 12-14V.

I appreciate anyones help

I would suggest a automotive step down for those. They take 12 to 14v in and then you set a voltage out, 3, 4.5, 6, 7.5, 9v out. You could use a resistor, but getting the right value would be hard. You would have to know the amperage draw on both items and then figure a value out. The problem is a car battery will run somewhere between 12.6v and 14.4v so the value of the resistor would ahve to be set for the highest voltage and then might be to high if the car wasn't running.

Are they pretty cheap? If they're too expensive, I wont bother. Also do you know any good places to get them from (online?)

Thanks

There are probably cheaper places, but I used to get them at Radio Shack, they have about 10 differnt ones in a bunch of power ranges.

Ok, sounds good....and what exactly are they called so i can look them up in the catalog?

RS calls them DC power adapters. They are in the part # 273-1801 range.

Brock,
Are you referring to those battery eliminators that plug into a cig lighter? Like for running a CD walkman or something?

If so, they're available at most Dept. stores too. Like Walmart, Kmart, Target, etc...

Should be easy to take it apart and wire it in permanently as opposed to plugging in in.

Yes, they are basically those battery eliminators. I didn't think of WalMart, but I bet a lot of places like that carry them now since they are used for portable electronics, most commonly protable CD players. Some have the cig plug then a box then the output wires, those would be easier to hardwire in to a car.

Good call Gadget.

Hmm, the thing is though....in order to run two devices at two different voltages, i'll need two units. This could get kinda pricey. Thought there may have been little diode sized components to step down the voltage.
Would somethin like this work?

Don't think I'd take that approach unless you've got lots of time and some $$ to play with. The chip *might* work, but my reading of the product information leaves me doubtful. But then, my SMT PCB skills are pretty weak to begin with.

It looks like you'd need to build a circuit for each Vout you need. Read about what happens if you have long input or output leads. The chip doesn't have much lee-way on voltage input, which means it'd be easy to fry the chip ...

I'd go with Brock's suggestion. I think you'd find the actual cost (in time and money) to build the circuits you need will far outstrip the cost of jury rigging manufactured units in the long run.

Maybe someone with more experience here will chime in and correct me if my assessment is wrong ...

Good luck, regardless and let us know how it turns out!

Mark

For dirt simple regulation requiring very little soldering or circuit knowledge, you can't beat the LM317 regulators.
http://www.national.com/ds/LM/LM117.pdf

Way down near the end of the datasheet are some example applications. A simple voltage regulator uses one 317 and two resistors. Hard to beat that.

$2 at Radio Shack, around 60 cents or so on eBay. With shipping charges, you are probably best off going to Radio Shack. They will also have the resistors you need to make it work.

That LM317 is an elegant solution to this application. Good call!

Mark

These might be similar to what you're talking about. Takes ~~>this<~~ for example. This one is "adjustable"...but how do you adjust the output? If you do with resistors, then why not just forget the regulator and use resistors? Also, how would u hook one of these up with the 3 prongs.

Sorry for all the questions...just trying to understand

Thanks Mark.
I don't know why it took me so long to think of it.

Nightshift, if you have any questions about using a LM317, let us know. They're very easy to work with, even if you have no prior experience. Do you solder?

Yep, that's the one.



This pic is set up for variable voltage.

I will have to do a little more searching to tell you what values of resistors you need for 5 and 9 volts fixed.

BTW, Radio Shack also has FIXED voltage regulators for 5 and 9 v, but I don't know if they require any extra components. (276-1770 and RSU10911048).

Yeah boy, i solder
Just bought a new iron not too long ago...
I just need to know how to work the components and find out which one i need (by the number) and im good to go. Thanks for your help.

OK, for a test setup I grabbed a 317, my box of resistors, meter, breadboard, and 12v wall wart.

Vout=1.25*(1+R2/R1)+Iadjust*R2
Using the formula from the PDF, I used a 12K for R2, and a 4.7K for R1.

Problem is, I don't know what value to use for Iadjust, so I just got close, then tried different resistors to get the correct output.

With a 20mA load, the V out is 5.04 volts.

A 2.7K and 15K provide 9.04 volts.

Your results may vary...
Which is why I highly recommend getting the resistor assortment pack (271-312) if you go to Radio Shack.
It's $8, but you should have nearly all the resistors you'll ever need, unless you build more than 5 identical circuits.

Otherwise, you have to get the 4 values I've stated, and they are not all available at local RS.
AND, all LM317's and resistor vary, so you'll likely get different values than I did. It's mostly trial and error to get the right "pair" of resistors unless you use a variable pot for one of them. Problem there is pots can change value over time or the contact surface gets dirty and stops working.

cool, where can i get the LM317? To order the digital meter im wanting...i have to order that from the radioshack website, so anything im gonna need i'll probably put it with my order

276-1778 is the RS part #.

Will you be getting the resistor assortment or going for individual values?

Ugh, it seems the total price of the digital meter and the resistors alone equal the price of a digital LED voltmeter guage made for cars from jcwhitney.

This hasnt really gone to waste though...I can apply this knowledge to LEDs, the LM317 and stuff since its being mentioned in the LED forum.

Ohh, rather , I can still use this for my LED security light in my car that i run off 2 AA batteries. I'll get the LM317 and resistor assortment to make about a 3.2 volt output.

If its not that much trouble, you can see what resistors it takes for a 3.2 volt output on that LM317...or i can experiment once i get the stuff.

I appreciate all your help!

P.S.-Whats the difference between radioshack # 276-1778 ($1.99) and 900-4516 ($0.80) (both LM317T's)?