Using a regulator to control DC voltage

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When designing a new circuit, we have to make sure we provide the right voltage level to all the components being used.  Some components may require a specific voltage like 1.8V, 3.3V or maybe 5V.  If you do not respect that constraint, your components won’t survive for a long time!

I find using voltage regulators is the simplest and least error prone solution to that problem.

regulator

Most of the time, I will use a member of the LM78XX regulators family .  You should find a regulator suitable for the voltage you are looking for:

  • LM7833: 3.3V
  • LM7805: 5V
  • LM7806: 6V
  • LM7807: 7V
  • LM7808: 8V
  • LM7809: 9V
  • LM7812: 12V

As you can see, the XX in the LM78XX part number stands for the regulator output voltage.

If you need a negative voltage regulator, simply look at the LM79XX family of regulators.  The last 2 digits, in the part number, also indicate the output voltage.

One thing to be aware about using a LM78XX in your design is that it requires to be supplied with 1.6V to 2V over its output voltage.  So, a LM7805 regulator will have to be supplied with (at least, more is better) 7V to work properly.  This will influence the number or type of batteries you will want to use to power your circuit.

The other thing to take into consideration is the fact that a LM78XX requires capacitors to be placed both on its input and output lines.

Capture d’écran 2015-11-01 à 10.03.30

Typically, you will want to use a 330nF (same as 0.33uF) on the regulator input line and a 100nF (same as 0.1uF) capacitor on the output line.  This will help the regulator to stabilize the voltage and will reduce the noise the regulator generates on the line.  I read that you may need additional decade capacitors beside the output line capacitor when working with high frequency circuits to prevent all the PCB traces to radiate at that frequency.  But I haven’t done a project with such constraints up until now.

There are alternatives to the 78XX linear voltage regulators family including switching regulators.  From what I have seen, it seems that switching regulators are much more expensive but also more power efficient.  So, if your project battery life is more critical than its cost, you may want to explore that path.  I know there’s also the Zener shunt regulators option.  I will look into those two alternatives more in details some day!

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