Is this a sensible and cheap way to get 3.3v for the N64?

[bacteria]

Saw an article, had a play.

Took a 7805, connected a battery pack (about 8v at it happens) to the input pin (pin 1); to pin 2 (ground) connected a 461 ohm resistor; to the other end of the resistor added a 208 ohm resistor (well, actually a 150 ohm and 82 ohm ones) to pin 3 (7805 output pin). If you connect ground (pin 2) to your multimeter and the other (positive) multimeter connector between the 461 ohm resistor and the 208 ohm ones (ie between the "3" and "5" on my breadboard pic below), you get 3.38v. The 7805 outputs I believe at 1.5 amps, so would be enough to power an N64.

By adjusting the resistors you can get higher voltage; according to the article, you can get 12v out of a 7805 this way too.




Questions -

1) is it reliable and constant (under load from an N64, as the N64 with expansion pack needs about 1.3 amps as I recall).
2) will the 7805 overheat; if so, a small heatsink ok?
3) is it efficient or inefficient?
4) is it a viable alternative to the TI card?

[codeman]

So are you seeing the 3.3v directly at the output (pin3) of the regulator? Or is the 3.3v in the middle of the resistor chain you have set up?

From what I understood it seems like you're taking the voltage output from the middle of the resistors, which wouldn't work as a power supply. Once a load is connected to that point in the circuit there will be a large voltage drop across the resistors. It wouldn't be a constant 3.3v like you need.

It's true that you can get a range of voltages out of the 7805 if you set up the feedback properly, but the output current limit of the 7805 is only 1A, maybe 1.5A. The 7805 is a linear voltage regulator, which is much less efficient than the TI switching regulator. You'll only get about 60% efficiency at absolute maximum for a linear regulator...and it's usually closer to the 40% range. If you use a linear regulator you'll simply be wasting power within the regulator itself and your battery life would be reduced.

Does that answer at least some of your questions?

[marshallh]

Your output voltage is probably going to fall on its face once you connect a load to it, resistors are current based not voltage based. You can properly set up a 7805 to output any voltage above 5 volts, but not under.

The popular LM317 "adjustable" voltage regulator is just a 1.3v fixed voltage regulator with an adjustable gain. You can use this exact same setup for the 7805.

The best solution of course is to use a switching regulator, it's why the TI ISRs are so popular.

[bacteria]

Something told me the idea sucked, thought I would ask anyway! Might try it at some point on an N64 board and see what the voltage drops to, just out of interest. Won't harm the N64 after all, just won't light up.

I bought some LM317T variable voltage regulators, got three in the post today - don't seem to get any output voltage at all from them. Would the one I test be faulty or do the LM317T's work differently to the standard "in", "negative", "out" like a 7805?

I have a few TI cards, fortunately; I was just trying to come up with an alternative!

codeman - the voltage is from the middle of the resistor chain; pin 3 on the 7805 still registers at 5.02v. 1.5 amps is all that would be needed; but if only 60% efficiency, not much use.

[codeman]

The 317 works a little differently from the 7805. The 317 has an input, a feedback and an output pin. To get the output regulated to the voltage you want, you need to have a feedback connection from the output. If you go to google and type LM317 you'll get a lot of sites to look at that will explain it better than I can!

The voltage that it will drop to is pretty easy to calculate. If you're getting 5v on the output then I'm assuming it's still hooked up to be a fixed 5v regulator. To find the (approximate) voltage you would get at your measurement point it would be: 5v - (N64 current)*(resistance value). Based on what you said your circuit was, the resistance value would be 208 ohms.

[XCVG]

First off, as said before, this won't work. Secondly, even if it did the heat output alone would be reason enough to abandon the idea.

I bought some LM317T variable voltage regulators, got three in the post today - don't seem to get any output voltage at all from them. Would the one I test be faulty or do the LM317T's work differently to the standard "in", "negative", "out" like a 7805?

That won't work, there's some weird adjustment system thing. Here's a nice diagram (me not like capacitor arrangement).


(from here)

A 220 (R1) and a 370 (R2) ohm resistor will get you 3.35V. Changing R2 to 360 ohms will give you exactly 3.3V. I actually built a circuit but never got to use it (didn't even get to test it). I have no idea where it is now, probably with all the other YAP64 parts. The LM317 is a linear device, which means heat (you need a heatsink, mount it to the main sinks maybe) and handles about 1.5A. There is a version that comes in a TO3 package (metal can) and handle 5 amps.

[bacteria]

I take it though that this will still be very inefficient (40% as mentioned by codeman before), or is it ok?

It is more efficient to use a LM317 setup as per the pic in previous post or 7805, or are they both very inefficient and get hot under load? Sorry, after background info, and what scenarios it is useful and beneficial to use them please.

[Alie]

The lm7805 isn't the best way to do the work, you can use voltage even over 9 volts (but a least 8-9 because of the dropout of the regulator, so you can't use it with a battery 7.4v for example).
It's only a regulator that is usually used after a diode bridge and a capacitor in order to stabilyze the ripple, but it can't ever be better than a switching regulator (if you have an oscilloscope you can see that by yourself).
If I remember well the model packaging that you are using can support up to 1,5A under load, but it will be like an oven at that point so you have to put on an heatsink (that stoles space in your portable).


This is a typical application, the diode bridge reverses the negative half wave so you have this:


Then the capacitor keeps up the voltage when it's tryng to go go down:


Do you see the VR? The one between the cyan lines? That is the oscillation the regulator tooks off.

After all i don't think it's convenient to use that, it's much better to use a switching
(excuse me for my bad english, i'm italian)

PS: Gamelver sold me a psone screen about ten days ago, i've tried to ask him if he has shipped after i paid but he doesn't reply. What can i do?

[codeman]

The LM317 will still have bad efficiency as well. All linear regulators work by basically using a pass-transistor as an adjustable "resistor". So it senses the output and adjusts the resistance necessary to keep the output at the voltage you want. So it's basically just burning power to keep a constant output voltage.

So as was said, for any high current load you want to use a switching regulator because of its high efficiency. Linear regulators are great when you have a low current load and thus the power loss will be less. It still won't be very efficient, but in lower current cases it might not be worth the extra circuit complexity and expense to put in a switching regulator, so you just live with the linear one.

Linear regulators also create a much cleaner output voltage. So if you're trying to power components that are sensitive to power supply voltage fluctuations then you'll want to use a linear regulator. It's actually pretty common to use a switching regulator to do a large step down in voltage to take advantage of its efficiency and then use a linear regulator to "smooth it out" if you have sensitive components. Sorry for the rambling...

[bacteria]

Just had a thought, if the regulators are inefficient as they convert surplus voltage into heat, and the milliamps you use one for the hotter it gets, then logically, if you have a few of these circuits connected together, say 3, then if the draw is 1.2 amps, each 7805 would only have to work at 400mA at maximum, barely producing any heat, if any; and be more efficient (as heat not being produced). Don't know if this logic is logical, thought I would ask.

I'm just trying to find an alternative to using the TI cards, that hopefully isn't too inefficient and an easy circuit to make; as these TI cards are not easy to get - I can't get them in the UK, had to get a fellow member to send me some a year back; so I have some for my own use but that is it. If that means your battery life is 10% or 20% lower as a result, then hey, better than no solution at all! If you can't get a TI card and want to make an N64 portable, at least it means you still can then, even if it means you get 3 hours playtime instead of 4 for example - that's my interest in this - easy viable alternatives.

Comments??

[XCVG]

An LM317 will work, but as stated before, HEAT. Also ignore what that italian guy said, that only applies if you connect to a transformer.

I suppose you could wire a few LM317s together BUT they won't have exactly the same voltage and may backdrive. The problem can be mitigated with a few diodes and caps. I think.

[evilteddy]

Bacteria, you could try looking through the other voltage regulators in the LM range, specifically the buck switching regulators. From a bit of searching a while back I found the LM2679 which takes input from 8-40 volts and there is a 3.3V IC so the circuit is left fairly simple. I realise that the input voltage is probably too low for many of your projects but it was good for my 11.1V batteries. Most importantly it is a switching regulator and comes in a sane package that we can solder to (TO 220), while many packages are SMD components with pitches of 30 mil.

Having a quick look at the website a LM2596-3.3 in a TO 220 package is probably what is best suited to you. Here's the link.
It's fairly similar to the other regulator in that it is cheap to make the surrounding circuitry. If you can't find any around you though just look through the national semiconductor website until you find a suitable IC that is available at Mouser UK or digikey UK if there is such a thing.

EDIT: Just checked and you can get the adjustable version from Mouser UK but you'll probably find better options that me in my 5 minutes of searching.

[XCVG]

I feel really awesome

That's the LM-whatever he was talking about. The specs look good, I wanna try one.

EDIT: Oh wait, wrong package. What about this one?

EDIT 2: The manufacturer's datasheet is down, but I found one.

EDIT 3: Digi-Key UK has PTH08080 regulators.

[Alie]

Also ignore what that italian guy said, that only applies if you connect to a transformer.

Correct, that was only the standard application for the lm7805.

[XCVG]

Also ignore what that italian guy said, that only applies if you connect to a transformer.

Correct, that was only the standard application for the lm7805.

Ummm yeah, nothing we do is "standard application" at all.