I am building a digital voltage gauge for my motorcycle for fun. I am buying a nice little display which has common ground, so that I can provide 5 and 12 volts from the same source. I was hoping to use a "7805" regulator to control the 5v, but do not want to provide the varying 12+- volts directly to the 7805. I would like to drop about 7 volts before I apply the power to the 7805. I looked at Using diodes to limit current to LEDs which seems to indicate that using diodes forward voltage drop is not a good idea for leds, and maybe using a zener diode is not either. If I just use a series resistor, I would have to guestimate and experiment to get in the ballpark (as I don't know the current draw), and, as I understand it, the voltage dropped over the resistor would vary with the applied voltage. Here are my questions:
- Is using a diode(s) to drop the voltage independent of current a bad idea here? (Is it always a bad idea?)
- Do I still get he same IIR loss for the voltage dropped whether it is a resistor or diode?
- Is there a conventional "best practices" approach I should be using?
- Lets go out into left field for a minute: other than heat-sinking the 7805, can I pot the whole mess with RTV silicon to vibration and moisture resist?
Always nice to know if the alternator is working OK. Not an electronics guy, just a tinkerer.
Thanks!
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Thank you Nick and Thomas. I agree that a motorcycle is about the harshest environment one can choose-greater vibration, direct exposure to rainwater, probably greater variations in regulated voltage due to greater variations in engine (and so alternator) speeds. I had hoped to fuse and put the dropping resistor at the battery connection to provide the greatest protection for anticipated faults in the wiring run and/or the digital readout itself. Based on your comments, I will fuse at the battery connection, run fine (#28?) wire to a MIC2945 and let it do all the voltage dropping. I anticipate a mounting plate fastened directly to the 2945 heat sink, with the entire remainder of the assembly "glued" to the mounting plate with RTV silicon. Waterproofing is a concern, for which I will rely on the RTV silicon.
Best Answer
First, why would you do this? The 7805 will be perfectly fine with 12V. If you're passing a lot of current, use a heatsink. The diodes will only move power loss from the regulator to the diodes themselves.
In this case it should be fine, but diodes do not drop a fixed voltage; the voltage drop varies depending on current in a non-linear fashion.
So for example getting 5V from a 12V source using 10 diodes in series would be bad, and the voltage could range from as much as 7V to as little as 2V, depending on the current draw. The 7805 has a minimum voltage of 7V to output 5V at 1 amp, so as long as you can guarantee this 7V you should be okay.
Note that the output of the 7805 will vary depending on input voltage by a few millivolts - it will be slightly higher at 12V than at 7V.
The same loss happens no matter if you use resistors or diodes. The energy must go somewhere and it is converted into heat by either resistors or diodes.
The best practice here is just to feed 12V into the 7805. Introducing more components into the circuit will make for more chance of failure.
Why is a heatsink not practical? Here, I'm unsure if potting would work. Best to ask someone else.
You should be cautious when using the 7805 in an automotive circuit. The battery line is a harsh place. When you turn the headlamp(s) on your car/bike off, for example, the excess energy in the alternator has to go somewhere and it does - it appears as up to a 60V spike on the 12V line. And in some rare circumstances you can get a negative voltage on the battery line. Both of these situations will destroy a 7805, and the output could short, connecting the battery line to the 5V output, giving your load 12V instead of 5V.