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

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?

• Just a comment about this, read the Arduino main page (you can read Arduino uno's web page, and it's the same/similar for all). There it says arduino should be powered with 7V from the DC power jack or Vin pin, or 5V from USB. If you are planning to supply from USB, your 5V should be very stable, as Arduino might assume it is that way. – shortCircuit Apr 1 '15 at 18:12
• Here some high-rep fellow suggests to provide 5V directly via USB with a hackish trick (using a stripped USB cable) that seems interesting, but I wonder if that is different from providing power through 5V board pin electronics.stackexchange.com/a/65582/5814 . – heltonbiker Apr 1 '15 at 18:22
• Given the 40V spike you mentioned then: Rectification small series R to zener to ground plus cap to ground then an LDO from there. The zener and resistors deal with occasional spike so the LDO does not have to.SOEM regulators are made to use in automotive environment but are usually noy LDOs. I think NatSemi LM28xx or LM293x seroies are such. – Russell McMahon Apr 2 '15 at 7:50

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.

• 6V and 3W = 0.5A as you have shown. But if your intent is to place a series resistance to drop 1V from the potential then you must not use 12 Ohms, but 2 Ohms. A 12 Ohm resistor will drop 6V, not 1V, a 0.5A. – sherrellbc Apr 1 '15 at 19:13

Unless the design of bicycle generators has changed, I expect the output voltage to vary with speed. Now, if "6V" is the rms value, rectifying it will get you 7 to 8 volts DC. In the message you linked in your comment, there's a paragraph stating the Arduino has an on board regulator that runs from 7 to 12 volts. I think that's the easiest, safest (and maybe the best) way to power this. Just make sure the DC in doesn't go over 12 volts at high speeds.

• I have once plugged my generator to a friend's oscilloscope, and indeed it went quite above 6V (peaking above 40V for very short fractions of a cycle). One possibility then would be to add up one 3V zener and another capacitor to harvest some voltage to 9V and feed it to the DC in which happens to feed a 7805. Another possibility would be to use an external 7805 and feed the board via USB. I guess it would be better to do the first thing (since I would be using the ready-to-use 7805), what do you think? – heltonbiker Apr 1 '15 at 21:01

For anyone interested, I ended up using an LM2940, which is a Low-Drop-Out (LDO) voltage regulator.

It is cheap, easily available on local stores, and very easy to set-up requiring only two common electrolytic capacitors.

With it I could not only power my Arduino, but also recharge my cell phone, with a quite good performance, while riding my bicycle at normal speeds.