What's the right way to provide both 5V and 3.3V from external power?

Other Hardware General Electronics
1

When moving my circuits from a prototype on breadboard with Arduino, to either stripboard or custom PCB (with just the components needed - i.e. no Arduino board), I normally only need 5V and as I have a box full of old wall-warts that supply approx 7V, I use one of those with a 7805 (or similar LDO), which works fine.

Now I'm starting to play with nRF24l01+, which requires a 3.3V supply. So, which is the best way to get power to this device?

  1. Voltage divider from my existing regulated 5V supply
  2. Use a 3.3V LDO regulator from the incoming 7V wall-wart
  3. Use a 3.3V LDO regulator from the regulated 5V supply
  4. Something else?

I can see that options 1,2 and 3 would all work, and I'm assuming that option 3 would be more efficient than 2 in terms of heat loss, but is there something I'm missing (e.g. any noise issues/advantages by running the output of one regulator into the input of another regulator).

Any advice welcome :slight_smile:

2
  1. Voltage divider from my existing regulated 5V supply

No this would not work. The regulation is very poor when driving anything but the simplest circuits, you should avoid this.

  1. Use a 3.3V LDO regulator from the incoming 7V wall-wart

This increases the power that has to be burned off by the 3V3 regulator where as:-

  1. Use a 3.3V LDO regulator from the regulated 5V supply

Shifts some of the power that needs to be burned off from the last solution over to the 5V regulator. Depending on circumstances this can be a good or a bad thing. However not withstanding this, this is what I do most of the time.

3

Grumpy_Mike:
However not withstanding this, this is what I do most of the time.

Then I'll probably do the same :smiley:

Thanks for the comprehensive answer - I didn't appreciate that a voltage divider wouldn't be suitable in this situation and that's almost certainly something that would have tripped me up in the future!