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Little circuit help with volt reg.

I am using designing a new PCB for a tidy way of connecting all my 5v doo-dads. It is a pretty basic set-up, 5v- 3a low dropout reg, the obvious caps and some nifty screw terminals. This is all pretty straight forward. Now I also need 3v to replace the (2) AA's in walter's mini ampified speakers. Would it be cool to just use a resistor bridge on this same 5v regulator board to get my 3v? There is really no current here, maybe just a few ma. I would think somewhere around a 22k and a 10k should put me at about 3.3 volts or so. Is this going to screw with the other 5v outputs going to everything else?

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How about using 3 power diodes to drop the 5V to 3.2V. Same method OddBot used in this walkthrough to drop from 7.2V to 6V.


or a zener diode....they make those at 3.1v I think....
Really? Dropping volt-by-volt with diodes is a better solution than a resistor bridge? I don't question this, I am just curious why.

Well, a voltage divider will work OK.

However, if you build a voltage divider, you will have current running through it all the time, using power. With your 22k and 10k example, that's:

I = V / R = 5 / 33k = .15 mA.

Granted, that's not much, but it will always be on. Also, the 22k resistor can be a low power resistor, but you'll be drawing all the current for the 3V line through the 10k resistor, so you may need to make that one a higher power rating.

If you drop across the diodes you'll only use current when you draw current on the 3V line.

Thanks, good answer. That is exacly what I needed to know.

Do you think a 3.3v regulator built into the board would do the trick?

It would make Walter a little future proof as well considering so many things like GPS, wireless modules, gyros etc don`t take 5v.

What about a low dropout voltage regulator? LM3940 for example.

That looks like a good choice, especially since Chris' current requirements for the 3.3V line are quite low. It looks like an inexpensive part. Maybe you can even get a sample, Chris.

If you needed more current, the right diodes would be simple and could easily handle the power.