Taking 5v from a 12v power supply

Hello everyone,

Simply put, I would like to know if it's possible to take 5v from a 12v 30Amp power supply. I ask because it seems rather silly to buy a 5v power supply when I have more than enough power already.

I've done some research on linear regulators, but honestly haven't been able to formulate a plan for this problem.

Linear regulators such as the 7805 have one significant problem; the more voltage you have to lose the hotter they get, especially when you have to take significant current. You end up requiring a huge heatsink to get rid of the surplus energy.

The alternative is to get a switched-mode unit, like those from Pololu. They are reasonably cheap, available in many countries, and run almost cold while supplying a fair amount of power. They are also very, very small units.

... if you'll need less than 1A (e.g. Arduino+display), a 7805 with TO92-housing and standard cooler would be sufficient ...

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Viktor
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I'm using some external stepper drivers in conjunction with a smoothie board. The external drivers take 5v logic level inputs while the smoothie board only sends out 3.3v. I was going to use a 3.3 to 5v logic converter from spark fun [www.sparkfun.com]

But I need to power it with 5v and 3.3v in order to get it to work.

... you can use a 3.3V voltage regulator or a voltage divider after the 5V to support the needed 3.3V circuitry - it only 'adds' the drawn current to the 5V-regulator, so calculate the combined load here ...

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Viktor
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How could a voltage dividea help if I'm trying to step my 3.3v up to 5v? I thought you could only use a voltage divider to reduce

Oh wait, so your saying after I take the 5v from the 12v power source, I can use a voltage divider to peel off 3.3v from the 5v?! That's a great idea, and I'm pretty comfortable with implementing them.

But I guess I have a pretty fundamental question on power supplies. If the only thing I have connected to my power supply is my 5v regulator, does that mean 30 Amps will be sent to it since nothing else is on?

... no - the drawn current follows Ohms law -- only the current 'used' by the 5V-circuit and the following 3.3V circuit will flow.

But it could be, that you have to add a load of some Watts to the 12V-output for stbilizing the PSU, if it's not self-regulating ...

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Viktor
--------
Aufruf zum Projekt "Müll-freie Meere" - [reprap.org] -- Deutsche Facebook-Gruppe - [www.facebook.com]

Call for the project "garbage-free seas" - [reprap.org]

I would try it without any conversion as a 5 volt logic often only needs 2.4 volts to be considered high and the 3.3 volt logic should be higher than that.
If you need logic conversion then draw the 3.3 volts from the smoothie board.
Getting 3.3 volts using a voltage divider is risky as voltage would vary with load.

George