I know it's been addressed many times before, but I'm going to ask anyways:
I got a moderndevice.com bare bones board with a 5v regulator on it i want to run in a car. I have some 12V zeners hanging around that I intend to use for transient suppression, but I have another problem.
I would rather not apply the battery's 12V+ to the regulator as I'd need a big heatsink for my application (moderndevice.com 8X8 LED board, 64LEDs), so i was wondering what people had to say about using silicon rectifiers to drop voltage (maybe 4 of them in series) and dissipate some of the power elsewhere.
Probably easier to use a 7809 9vdc or 7808 8vdc regulator to drop the 12 to 9 or 8, and it can be mounted on a heatsink. Even 4 diode would probably only drop around 3vdc and a running car has about 14-15 vdc from the alternator output, so the 7805 would still probably run pretty hot depending on how much total current your using.
But don't use the 12V Zener: it'll be on all the time.
27V is a good value: the 78xx can safely handle that much, and it only clips when there's a real transient.
A good vehicle input circuit is:
Series 1N400x diode, to eliminate negative spikes.
Smallish resistor, to dissipate some of the heat of voltage reduction and limit the amount of current rushing into the Zener during a transient,
Zener, and
A fat filter cap to absorb some of the transients and smooth any ripple.
I'd go with the 7808 for pre-regulation, rather than a 7809: the more heat you dissipate off the board, the better.
Oh, and don't forget that the input voltage will drop to near, or even below, 8V during crank. You should expect your regulator(s) to drop out, and probably reset the Arduino.
Does a 7805 work by burning voltage down to 5V, or does it work by switching the input voltage on and off rapidly in a duty cycle that's regulated with a feedback loop to keep the charge across the output capacitor around 5V? It seems like switching would burn a lot less heat.
Does a 7805 work by burning voltage down to 5V, or does it work by switching the input voltage on and off rapidly in a duty cycle that's regulated with a feedback loop to keep the charge across the output capacitor around 5V? It seems like switching would burn a lot less heat.
A 7805 is a classic linear regulator adjusting the output voltage by changing it's effective resistance to drop the needed voltage across it as heat. A switching regulator works by changing the duty cycle to adjust it's output voltage so is much more efficient way to regulate voltage.
In my opinion, the solution of using 2 regulators in cascade is not useful. At last, the result is the same, one regulator "suffers" 3 volts X 600mah, and the other suffers 4 volts x 600mah, and the result in dissipated power is the same as making a single regulator reduce 7v X 600mah. The heat emitted by both will be the same.
To avoid heat, used a switched regulated supply, as someone pointed, you can use a cigarette USB adapter, and in case you need, you can modify it to supply some more power with few changes, i.e. winding another in-parallel coil in the output toroid.