This will show you how to make a 12v to USB (5v) adapter. The most obvious use of this is for 12v car adapters, but anywhere you have 12v you can use it! If you need 5v for anything other than USB, simply skip the steps about adding the USB ports ;)
Step 1: Material
You will need
- an inline fuse holder
- a 0.5 amp fast blowing fuse
- 2 different colors of wire (so that you don't get confused later on)
- a L7805CV voltage regulator for lower amps. For higher amps (many devices at once), use a TO220 (and a bigger fuse)
- a 220uf 16v Capacitor.
- Female USB ports. Header boardsare a great and easy way to do this.
- if you choose to add an LED, you need an LED and an appropriate resistor for 5V. This depends on your LED's rating. Any LED will do, here are the values for the most common LEDs : 1.2v = 220ohm, 1.6v = 180ohm, 2v = 180 ohm, 2.2v = 150ohm. If you have a strange LED or want to make it brighter, please use this resistor calculator.
More information on the fuse: you can use a higher rated one if you use a different transistor that
can take higher amperage. Because we put the fuse on the 12v side, (which can vary from 11.5-12.5 volts, we have to use a value 2.5x smaller than what we want on our USB side. So, if you want 1.5 amps for your USB ports, then you select a 0.6amp fuse, if you want 2.5 amps at 5v, you select a 1 amp fuse, if you want 3.75 amps, you select a 1.5 amp fuse, etc.). Also, if you really want to protect your circuit badly, just put one on both sides.
Of course, you can also use an existing 12v-5v converter in your car, be it light duty, medium, or heavy duty; or use these nice power adaptor that are meant to be integrated into other projects or this beefier waterproof, super high power alternative with a heatsink. They are nice because of their screw terminals.
Step 2: Prep the Transistor
The Transitor has 3 pins, we will call them pins 1 2 and 3 (when you are looking at the transistor and it's heat sink / metal plate is facing AWAY from you). Pin 2 is the GROUND (-).
- Pin 1 will be connected to the power supply, passing by the fuse. there are different size of fuse holders, the size does not really matter as long as it has the same ratings. There might be a 1$ difference or something.
- Pin 2 will be connected to the ground (-) so we will just add a wire
Step 3: Add the Capacitor
The capacitor will be connected to pins 2 and 3 (the shorter leg goes to the ground \ pin 2)
This capacitor's role is to mitigate startup power-spikes.
Step 4: Wiring the Ports
The USB will be connected to pins 2 (- ground) and 3 (5v +). You can use this diagram; use the picture called "receptacle". I have used salvaged USB ports, if you order them, they will probably be a bit easier to solder. The advantage to this is to have a solidly joined pair as I have.
If you have more than one port, connect all the pins 1 to pins 1 and pins 4 to pins 4
*more detailed note on why the ports are wired the way they are, skip if you don't care* In order to keep the voltage steady at 5v, your ports should be in parallel rather than in series in order to keep the voltage constant. What does this mean? quite simply you make sure that the red wire goes to every positive port you have (do NOT go "wire to +" and then "from minus go to next +"). Does each red wire have to leave from the same place? no, the importance is just that they all touch each other.
Step 5: LED Indicator
If you are adding the LED, place it the same way as the capacitor, but put the appropriate resistor in series with it (this calculator will tell you how, use 5v as the voltage in the 1 LED calculator) (aka, make it an extension of either one of the legs). You might want to put this on wires so you can move the LED to a better position later on.