N64 Portable+3.3V DC-DC converter diagram

sniper_spike · Post by **sniper_spike** » Fri Dec 30, 2005 9:38 pm

Here's the current diagram that's giving me the fluctuations on the final unit:

Mostly stolen from a book.

I'll take some pictures of the actual unit if you need.

sniper_spike · Post by **sniper_spike** » Sat Dec 31, 2005 5:59 pm

Today I've been working on getting this sucker stable. First thing I tried was replicating that resistor (with a long piece of wire) that I crossed out in the schematic. I originally got rid of it because it was supposed to be a .33Ω resistor, but since I had nothing of the sort, I disregarded it.

That didn't work, so on to attemp #2. I thought maybe raising the inductor to 440uH (by combining the two 220uH inductors), would stabalize it. Well it didn't, so then I even tried just one 560uH indcutor I had lying around, that didn't work either, but I left it on anyway.

So then to attemp #3. I thought putting a 1000uF on the unit (because I saw some examples with them) would fix it, and that didn't work either. It ended up making the fluctuations even crazier.

Then it struck me, if rasing the capacitor value un-stabilized the voltage even more, then a lower one would stabilize it! So I desoldered that crap off, and then put on a 220uF capacitor that I had lying around, and it worked! The voltage fluctuations are only from 3.25V to 3.35 now! Perfect (considering I just wanted to get done with the darned thing)!

Figured that, there was a lot of stray capacitance and other interferences from all the leads jumping all over the place on the breadboard. This then causing the values to be incorrect otherwise (like on the final unit which has mostly silver leads from a conductive pen).

I guess the trial and error method really works!

Here's a picture of the final unit.

Zero · Post by **Zero** » Sat Dec 31, 2005 6:24 pm

Congratulations on that, Spike!

sniper_spike · Post by **sniper_spike** » Sat Dec 31, 2005 6:25 pm

Heh, so you finally joined\posted. Yay!
And thanks!

imapaintballa · Post by **imapaintballa** » Sun Jan 01, 2006 1:05 am

the diagram must be stickified! great for people wanting to make dc/dc converters! congrats.

MM007 · Post by **MM007** » Sun Jan 01, 2006 8:28 am

I'll ask the other mini-mod about adding it to the list that's already stickied. Also, what is the number of the chip? Either it isn't here, or I simply missed it.

sniper_spike · Post by **sniper_spike** » Sun Jan 01, 2006 9:28 am

There's a picture of it in it's packaging a couple posts back, but it's a LM78S40 anyway.

imapaintballa wrote:the diagram must be stickified! great for people wanting to make dc/dc converters! congrats.

First the diagram must be fixed to the right values.

Edit:
Here it is.

Note:
If this gives really fluctuating voltages for you, because of whatever conditions you may be in. Then try playing around with the inductor and main capacitor (currently 220uF), but don't go crazy with them, keep them within reason.
Look up a few datasheets for example schematics, to give you some ideas on what to change them to.
http://www.datasheetarchive.com/

MM007 · Post by **MM007** » Sun Jan 01, 2006 11:56 am

Thank you! This is for 9.6V. Will it also work for 7.2V?

Post by **Gamelver** » Sun Jan 01, 2006 2:39 pm

I'm adding this to the N64 info thread

.

sniper_spike · Post by **sniper_spike** » Sun Jan 01, 2006 3:44 pm

MM007 wrote:Thank you! This is for 9.6V. Will it also work for 7.2V?

Yes it should, on the National Semiconductor webpage it lists the minimum input at 2.5V, and the max at 40V.

http://www.national.com/pf/LM/LM78S40.html

timmeh87 · Post by **timmeh87** » Sun Jan 01, 2006 5:15 pm

what made you decide that you could just nix the .33ohm resistor? just the fact that you didnt have one? it is generally not a good idea to just take parts out that were in the official schematic. .33ohms might not sound like it does much, but from what i gather there, it looks like its part of an r/c ossilator or something. no reistor, no oscillator. i could be wrong of course...

but the piece of wire idea is a good one. youd probably only need like 8 inches of 26 guage wire to get .33 ohms.

its good that the circuit works anyways, but for anyone else who is reading this in the future, you should probably have that resistor.

sniper_spike · Post by **sniper_spike** » Mon Jan 02, 2006 6:04 pm

I would also recommend that no one try to use one of those conductive pens, they're absolutely horrible. It would be much better for you to make your own PCB (if you have the tools).

timmeh87 · Post by **timmeh87** » Mon Jan 02, 2006 6:37 pm

i can make my own

sniper_spike · Post by **sniper_spike** » Sat Jan 21, 2006 7:28 am

Sorry for the lack of updates, no real reason, just lazyness. Well the DCDC didn't work out so well when I tried it on the actual N64. It kept heating up like a mad man and dropping to 2.5V. After reading through datasheets again, I noticed this little tidbit "Note A: For Io >_ 200mA use external diode to limit on-chip power dissipation." Well 1500mA is just a little bit more, don't you think? I threw on just a normal diode I found in my stash, you know, just to see if it would work (what could go wrong?).
Hooked the DCDC back up, and started up the N64... I heard a pretty loud *pop* and then smoke coming from the IC, not so good. <- That's what could go wrong!

Now I'm going to do a complete rehall, I'll be using the same chip, just a new diagram.

timmeh87 · Post by **timmeh87** » Sat Jan 21, 2006 11:22 am

id guess you hooked the diode up backwards, or completely wrong... did it tell you where to put it? i never noticed that part of the datasheet before