The 15v I mentioned was a theoretical highest battery voltage. I've seen cars and boats get to 15.2v when the alternator is at full output. In any case use the highest voltage you think it would ever be. Calculate the resistor needed to make 5mA flow through the Zener at that "high" voltage. Repeat the calculation using the lowest expected voltage (? 12v ?) and 2mA. Choose a resistor whose value is somewhere between the 2 calculated. That way you'll always have 2+ mA flowing through the zener but never too many mA. The exact value isn't critical.BlindAssassin111:
- Going back to the zener diode, you mention that I would need a 2k ohm resistor to have 10V across it, but the diode is a 5v, and seeing as max voltage is 14.4V, that would mean no break down if that much is lost over the resistor. Wouldn’t using a slightly smaller resistor be better? because if the diode breaks down at 5 then you need less than 2k resistance to get above that…or should I be using a 3v zener? I am thoroughly confused…
So long as the zener has 2 mA flowing through it, it’ll breakover and be at/near it’s rated voltage. The voltage across the resistor is then Vtach - Vzener and the current through the resistor as determined by Ohms law. Remember that these types of current/voltage calculations involving a diode are just a “good enough” approximation to the truth.