Passive voltage drop from 6V DC to 5V DC for 3W circuit

arduinodclow-powervoltage-regulatorzener

I plan to put an Arduino onboard of my hub-generator powered bicycle circuitry.

Currently, I have a circuit consisting of:

  • 6V/3W AC hub generator (variable frequency and power);
  • Rectifier bridge;
  • 6v/5W Zener regulator;
  • 6800 uF electrolytic capacitor;

With this circuit, I can safely drive anything needing 6V up to the 3W limit.

Now for the Arduino, I'd want to provide 5V at Vin, and for that I would like something like this:

  • link Vin to GND with a 5V Zener (and possibly a generous capacitor);
  • insert some voltage drop between 6V output and Vin;
  • drive external loads with transistors (BC 548 have served me well for up to 100mA per transistor);
  • (of course) ground everything together;

But I am not sure if voltage-dropping is a good practice, or if I would find a suitable diode with the desired drop.

If everything I proposed is b*shit, I would like some guidance, preferably involving only simple components. Absolute efficiency is not a must, and I can leave with some lost current through the regulators (zeners for example).

Update: I plan to add a battery to this circuit, but it should be able to run only from generator by design, anyway.

More update: I have read something about "Low Drop Out" regulators (LDO). Can I build one myself? Is there any off-the-shelf, widely available component for that? Can I use some of these together with the Zener and the capacitor, or it wouldn't make sense then?

Yet another update: a coworker suggested the TPS-76650 (Texas). Are there "non-proprietary" components with similar characteristics?

Best Answer

For any application, this very basic (and not much reliable) "dc-dc step down", could be done using a resistor between your 6V output and Vin. Considering:

Vout = 6V
Pout = 3W
Iout = Pout/Vout = 0.5A

for a total voltage drop of Vr = 1V, the resistor value should be:

R <= Vr/Iout = 2 Ohm

Higher value of R won't let you use the total 3W of your power source. If value of R is too low, your circuit will have high Ibias when in standby mode (arduino not connected). Consider the case where R-->0, and see that your 5V zenner will drain more current, and also make your 6V power source more unstable.

At the same time, you should take into consideration the resistor's dissipation power:

Pr = ((Vr)^2)/R = ((1V)^2)/(2ohm) =~ 500mW

where Vr = Vout-Vin.

Finally, calculate the zener's dissipation power.

edit: for a 500mW resistor, it would be safer to use a 1W resistor. This way it won't overheat, reducing it's variation and hence the output stability.

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