Powering of PICAXE from 12V with 4V7 zener?

[lbenson]

I found a circa 2004 design for RV LED lighting using a PIC16F627P here: http://members.shaw.ca/swstuff/einstein.html

The PICAXE-18 used a PIC16F627A (I'm not sure of the difference between the "P" and the "A"). The circuit showed VDD off of the nominal 12V supply with a 680 ohm resistor and a 4.7 zener diode, thus:

I know that newer LEDs would benefit from an updated driver circuit, but my question is--would this dirt cheap powering arrangement be appropriate for the picaxe (assuming, as per recent threads, that no ADC is desired), e.g., for a PICAXE-08M?

I don't know anything about RV power supplies (other that, in North America they tend to be 12 volts)--but perhaps this kind of lighting would be subject to the same kind of sparklies which we are warned about with respect to other in-vehicle circuits.

 

[womai]

In principle that sort of power supply will work, but it's quite wasteful - it drains the batter no matter how much electrical current the Picaxe actually uses:

I_supply = (12V - 4.7V) / 680 Ohms ~ 11 mA

Contrast that with a linear regulator that uses only microamps or less when you don't draw any current from it.

In addition, that supply will only be stable up to 11mA Picaxe supply current (basically the 11mA is distributed between Picaxe and Zener diode, with the Zener taking all that the Picaxe does not use).

You'd also make sure that the resistor that can take the power dissipation:

P = U x I = (12V - 4.7V)^2 / 680 Ohms ~ 80mW, not a big deal.

A 7805 regulator can be had for much less than a dollar, so why scrimp there?

Wolfgang

 

[Andrew Cowan]

For a low current, low accuracy, low efficiency power supply, it will be fine.

A

 

[Dippy]

Totally agree.

Many quirky things like this should only be considered for 'special' applications.
(e.g. not enough pocket-money for a regulator).
It's the usual design used by schoolboys after they have discovered Zeners.

I'd like to see that combo supply the power to a PICAXE if a few of the 'output' pins were providing significant number of mA !

As said above, it is wasteful and should be the last method considered for portable battery powered devices.

 

[fernando_g]

Talk about a "retro" regulator
The zener-resitor combination was commonly used back when monolithic ICs were relatively expensive, in the late '70s.

Nowadays, the cost difference may be a few pennies, if any.

-A monolithic regulator has the following advantages:
-Improved line regulation, and significantly better load regulation.
-Higher ripple rejection.
-Overcurrent and overtemperature protected.
-And if you employ a regulator in a T092 or even better, a SOT23 package, the board area difference is negligble.
-Higher efficiency

 

[lbenson]

Ok, thanks--"retro" it is, then. I just hadn't seen it before.