Proto Board input voltage

Circuitsoft

I'm hoping to put the Propeller ProtoBoard into my car as an accessory. In my application, the prop needs little current (only two cogs running, 80MHz, probably significantly downclockable), and the only other power is for a 10K pullup and 10K load on an I/O pin.

So, I figure ~45 mA. The LM1086-5 should handle at least 25 volts input. At 25v in, power dissipation in my application would be 900milliWatts, which I think would be just on the edge of what the protoboard can dissipate. 15v (very high for running car, more like 14.2) would have 450mW dissipation in the regulator.

What I'm more concerned about is C3, which the schematic only lists as 10uF, and has no voltage rating I can find. Can I provide 15v in to the protoboard without popping a cap?

cavelamb

The only correct answer is, "There is one way to find out for sure"...
Is there some reason not to put a simple 3 terminal regulator in series
and drop town to 6 to 9cDC?

Circuitsoft

Only that I don't have one readily available and the rest of the device is done. I did find a 5.1v 500mw zener that I put in series with it.

RDL2004

For the last several decades and until recently, most AC to DC adapters (wall warts) have been un-regulated. With low or no loading their output voltage can easily be 20-30% higher than the rated output (or more). Since there is a very good chance that this is how a typical end user will power their proto-board, the sensible rating for that particular capacitor would be 35-50 volts (2x maximum expected voltage plus some padding). I would consider 25 volts to be a bare minimum.

kwinn

Common electrolytic capacitor voltage ratings are 6.3V, 10V, 12V, and 16V. The one on my Proroboard is 10V so you might want to get the input voltage below that. Your zener will dissipate 0.255W at 50mA so it should be ok, and it will drop the 14.2V from the car to 9.1V which should be safe enough.

Car electical systems are noisy so you should add some filtering to the circuit. An inductor and capacitor are a good idea.

Circuitsoft

The electrolytic can is only on the servo power which I don't have connected at all. The cap I'm worried about is the ceramic chip over by the 5v reg.

kwinn

Parallax documentation says 6 - 9 VDC as the power requirement so with the zener in the circuit you should be ok. I still recommend adding a filter circuit using an inductor or choke and capacitor to reduce electrical noise. The noise spikes can be very unhealthy for passives and semiconductors.