resistor help

[Dorsai]

Before the thread goes too deep maybe you should just look at some ready made products. Not sure why your going through the aggrevation of building one , they are quite cheap for what you need. Try this..

http://www.newegg.com/app/ViewProduc...999-627&depa=9

[slinks]

yeah i know i could just get a cig power adapter for it. But im trying to avoid yet another one of thoes in my car and just hard wire it from the 12 volt source. Would you seriously want that plus the charger sticking out from your dash?

[Deer Slayer]

You could do it with parts from radio shack, but it's going to take a little space. I would do it with an LM7805 +5v voltage regulator, and a TIP 3055 or 2N3055 transistor. You would take the output side of the regulator and hook it to the base of the transistor, you'll need a heatsink for both the regulator and the transistor. Then ground the center leg of the regulator, and I forget the rest of the wiring. You need a resistor that goes from one leg of the transistor to the output of the regulator, but I forget what it is and which leg of the transistor. The TIP3055 will provide 2 maybe 2.5a and the 2N3055 will do 3+a. I would do the 2N3055 just because it's heavier duty and won't cost that much, $2-3. I'll post the schematic when I run across it. You might want to put a diode in there to prevent reverse current and surges. You could probably get away with just the LM7805 and a big heatsink, but I'd put a fuse on it just in case it shorts. If you don't get what I said, buy one. If you screw this up you'll cook it. :eek:

[slinks]

sweet thanks for the reply. ill look up thoes parts and see what i can figure out. and about the cooking it i baught the super warrenty that covers everything from best buy.

according to radioshack the 7805 can only handle 1A @ 5v i see no listing for the lm7805.

[Deer Slayer]

You can use that to supply it, it's a TO3 case +5v regulator, it does 1.5a with a nice heatsink. Radioshack doesn't have it but it would be a single component solution. I couldn't find an exact schematic but a 20ohm resistor should work between the base and emitter on the 2n3055. The base and emitter are the legs so you just solder a resistor between them. The colector is the case and that's where you supply the +12v. You just attach the output of the regulator to the base directly, and the emitter of the 2n3055 is the output. Here's a link to the datasheet for the 2n3055 which shows the pinout for it http://www.st.com/stonline/books/pdf/docs/4079.pdf . I would put a little resistor on the center leg of the regulator just to keep it from burning up if it shorts internally. Experiment a little with it, I think about a 100 ohm will still be 5v out. If you get the resistance too high the voltage will go up from 5v. The 20 ohm should be a 5w, or it will probably burn out, you can use 5 100 ohm 1w resistors in paralell instead of a 5w 20ohm. I'd also put a diode across the +12v in to the output so it will be a direct short in case of reverse current. If you get it backwards it will put +12v to the output . Then put one across the output to ground so it's a dead short if the voltage is reversed. That will short the output and blow the fuse if it's backwards. That just protects the circuit from spikes and whatnot. Put a 2a fuse on the input and you're set. The output will be +5v at the emitter and the ground just goes straight through. A 16v 1000 mfd cap at both the input and output will help keep the voltage clean. Those are optional along with the diodes. You'll want to put a heatsink on the 2n3055 and the 7805. I've built many little powersupplies like that for different things and they work really well. Good luck, DO NOT HOOK IT UP UNTIL THE OUTPUT IS VERIFIED TO BE 5v. :eek:

[slinks]

Hey thanks alot. ill hit up digikey for the LM7805CK im sure they have it ive got to get a couple of things for a headphone amp anyway.