Regulator and resistor design makes 5v instead of 3.3v?

sbf · Post by **sbf** » Sun Nov 27, 2005 12:28 pm

Hi, I'm just in the final stages of my portable n64 project, and I have just made a regulator circuit, and found it doesn't reduce the voltage as expected.

I have connected a 2 X 8505 regulators each with a 3 ohm resistor on the output legs (as per Triton's drawing), and the voltage drops to about 4.9v or so, any idea on how to get it down to 3.3V ,
(the regs are in parallel.)

Any ideas on this?

Higher value resistor? has anyone suceeded with a regulator?

Cheers!

daguuy · Post by **daguuy** » Sun Nov 27, 2005 1:21 pm

i believe the voltage drops according to the load. if there's no N64 attached, it'll be around 5v. if it's drawing a certain amount of current, the voltage will drop. i'd suggest a more efficient way to drop the voltage though

nos_slived · Post by **nos_slived** » Sun Nov 27, 2005 1:29 pm

That is not only extremely unefficient, but also unsafe. The load on the 3.3V line jumps so much that you're likely to fry it.

sbf · Post by **sbf** » Sun Nov 27, 2005 2:41 pm

So it looks like a dc dc voltage convertor is the way ahead then?

It's going to run a modded psone screen and N64.

Can anyone suggest a good configuration?

Post by **marshallh** » Sun Nov 27, 2005 5:33 pm

You might check out the Free DC-DC converter thread. You cna get a sample from TI that will work for the 3.3v line.

daguuy · Post by **daguuy** » Sun Nov 27, 2005 5:39 pm

i got some of those samples and they worked great at first. then after playing zelda for a while it got all staticy and bad so i turned it off. then i played pod racing and it worked. then i put in an expansion pack and everything was all staticy. if it's a malfunction of that specific DC-DC convertor or N64, it's not a problem because i have extras but havn't tested them yet.

nos_slived · Post by **nos_slived** » Sun Nov 27, 2005 11:05 pm

You're using the 2A model, right? Have you heat-sinked them? You might try running 2 in parallel. You have the capacitors really close to the converter?

Post by **Gamelver** » Sun Nov 27, 2005 11:22 pm

if it's not rated for 1.5A or higher, it'll get screwed up occasionally.....check to make sure you have the right regulator, and if it's not 1.5A or higher...try 'em in parallel, like nos said.

Maniac_Man · Post by **Maniac_Man** » Mon Nov 28, 2005 7:00 pm

instead of using a dc-dc converter is it possible to use a resistor?

timmeh87 · Post by **timmeh87** » Mon Nov 28, 2005 8:45 pm

basically, no

Post by **marshallh** » Mon Nov 28, 2005 8:48 pm

no.

1. Resistors don't alter voltage directly, they limit current. By knowing how much current goes in and out, you can calculate what the voltage will be.
BUT the N64 varies widely on what it draws from the 3.3v line. You could be drawing 0.6A while running Super Mario 64, and swap to GoldenEye, and it jumps to at least 1.4A. Even while runinng a game it fluctuates so much the voltage would vary as well. It probably woudln't even start up.

2. Resistors can't handle the 1.5A for the 3.3v line - even if you did get one it would be bigger than the DC-dC converter.

3. They're incredibly inefficient - all excess current is turned into heat! With switching dc-dc converters, the power is kept (more efficient.)

Hope this helps.
Marshall