so many resistors!

daguuy · Post by **daguuy** » Wed Jun 22, 2005 6:47 pm

i looked for a 3 0hm resistor on digikey to add to a 7805 to get 3.3v. i searched for "resistor" and scrolled down to the resistors and there are so many different kinds i don't know what i need

. which kind should i get?

nos_slived · Post by **nos_slived** » Wed Jun 22, 2005 7:06 pm

Digikey has a $25 minimum, so you're better off going to a small electronics store near you.

daguuy · Post by **daguuy** » Wed Jun 22, 2005 7:18 pm

good point. in didn't know about that

rawls · Post by **rawls** » Thu Jun 23, 2005 9:40 am

Digikey doesn't have a $25 minimum. They just add a $5 handling fee to any order under $25. If you are just getting a resistor, though, then just go to Radio Shat.

daguuy · Post by **daguuy** » Thu Jun 23, 2005 9:42 am

i'm getting either 2 1amp 7805s and 2 resistors or 1 1.5amp 7805 and 1 resistor

marksalot06 · Post by **marksalot06** » Thu Jun 23, 2005 9:52 am

what do you need a resistor for?

daguuy · Post by **daguuy** » Thu Jun 23, 2005 9:53 am

getting 3.3v. i saw a diagram that i want to use on triton's site: http://www.gannon.tk/~triton/concepts.htm

Turbo Tax 1.0 · Post by **Turbo Tax 1.0** » Thu Jun 23, 2005 10:07 am

that diagram has the battery going to output i think it should be the other way around

daguuy · Post by **daguuy** » Thu Jun 23, 2005 10:34 am

yeah i'll have to check that out

superdeformed · Post by **superdeformed** » Thu Jun 23, 2005 10:38 am

I don't think using a 3 ohm resistor will work, as the N64 won't be able to pull enough current through it. One way using resistor's and 7805's that I tried was posted by gamelver (I think). It was 2 7805's in parallel (to handle the current) and one, 1 ohm 10 watt resistor in line to the 3.3v line. A 2 ohm resistor might also work but remember that everything is going to get extremely hot, so you will likely need a heatsink on it. If you have the money and patience, a linear regulator will be much more efficient.

rawls · Post by **rawls** » Thu Jun 23, 2005 10:50 am

Yeah, just adding a resistor to the output expects a certain current through the resistor to get the required voltage and will be very innefficient. Using a regulator will be a better option, or you could add a 3.3V zener diode to regulate the voltage. The zener will be almost as cheap as the resistor and more accurate and efficient (though not as good as a regulator).

daguuy · Post by **daguuy** » Thu Jun 23, 2005 10:55 am

what kind of regulator are you talking about? if it's an adjustable one, how would i adjust it to get the right voltage?

rawls · Post by **rawls** » Thu Jun 23, 2005 11:11 am

A very efficient regulator is the LM651 (my post on this page talks about it). Linear 3.3V regulators exist but I just checked on Digikey and they're all out (at least of those which can handle >1A and don't have to be bought in bulk).

A zener diode is just a diode that you put in the opposite direction of current and it lets roughly the voltage it is rated for through. They are a very simple way to regulate voltage.

nos_slived · Post by **nos_slived** » Thu Jun 23, 2005 12:02 pm

You must use different zeners than me, because all through electronics, all the zeners we used would let all the voltage through, as long as there was enough to open it.

rawls · Post by **rawls** » Thu Jun 23, 2005 1:00 pm

You have to set the circuit up correctly (the way I worded my last post was misleading). The zener maintains a (relatively) constant voltage across it, so you have to set it up to ground to maintain your current. The circuit looks like this (using a 5V regulated supply and assuming maybe 1A load):

If you use a regulated load, this will be a decent regulator. If you use an unregulated one, then the voltage will shift around depending on the input voltage. Also, this depends on a constant current; as the current shifts the Vout will change as well. I don't know much of what the 3.3V supply for the N64 is used for, but if the current changes a bunch, you should either use the zener with transistor emitter follower, or just a voltage regulator (the latter being preferable).