12v to USB Adapter \ 12v to 5v Transformer (great for Cars)




Introduction: 12v to USB Adapter \ 12v to 5v Transformer (great for Cars)

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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

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.

Step 6: Encase

I like to encase circuits in hot glue, because I find hot glue is easy to apply and easy to remove, but will not be removed by accident.



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    67 Discussions

    The amazon link for the listed capacitor is out of stock. Does anyone have a link for another one I could use?

    ic lm7805 มันร้อน


    I built this circuit for my truck, it only has 1, 12 volt outlet. What I made has 2 12 volt sockets and a 4 slot USB outlet bank off of a CPU board. Each 12 volt outlet is fused and the USB outlets are fused as 2 pair, so I have 4 fuses total. Each individual circuit has an LED after the fuse so I know if the fuse blows. I bought the components that I did not have on hand from Digi-Key, (2) LT1084CT-5#PBF-ND transistors, (2) P5139-ND capacitors, and (4) C4SMF-BJS-CROU452-ND LED's.

    I had the correct resistors on hand for the LED's, for the 12 volt outlet LED's 470 Ohms, for the 5 volt LED's 82 Ohms, for each LED. Have the capacitor going from the 0 volt to the +5 volt coming out of the Transistor.

    That's the circuit, here is the problem,

    When I plug in my Samsung Note 3 phone to charge using any of the USB ports it charges fine. If I try to charge and Iphone it will not charge, it does not recognize it is connected to a power source. The output power on the USB ports is 4.9 volts. If I plug a premade 12 volt to USB into one of my new 12 volt outlets the Iphone will charge, the output voltage on that USB outlet is 5.1 volts. I am happy that my phone charges from the USB ports but sort of bummed that the Iphone will not. Is the 4.9 volts too low for apple products?

    Are the Transistors supposed to put out and actual 5 volts or is 4.9 acceptable?

    5 replies

    In some cases, such as with samsung tablets, and the iphones, there is a verification process before charging is allowed. In the case of the samsung tablets, the data pin needs to communicated an O.K., for the iphone, I am not sure what the requirements are, but I do know that it does check the power source.

    Was this with the alternator running (car on)?

    What is the voltage of your 12v input? The output should really be a fixed output, regardless of input voltage, but that is within a certain tolerence. As imput voltage chagnes, current draw changes, and temperature changes, the output can normally change by .2v. 4.9 is quite a bit more, but looking at the health of your battery is worthwhile. If yoru battery is outputting 11-ish volts, it is likely on its way out.

    I am not sure what the output of my truck outlet is I did not check it at the outlet. The voltage of 4.9 volts was taken both with the truck running and not running, same voltage both ways.

    I do not have the data pins connected to anything only the + & - DC voltage.

    use 2 resistors at 100 Ohm and connect one D- and one on D+ data pins on the usb connector to the positive wire. this should give your iphone the OK for charging. it also helps to charge android phones faster cause the also check for an OK signal.

    which is good, they use different approaches and you could fry devices if it is not built properly. You can buy adaptors for 1-2$ that make that bridge for the samsung tablets so any port can be used to charge.

    I doubt it is the voltage, it's just that it is the first thing to check.

    I also powered the unit up with a variable DC power supply on the bench to test before installing in the truck. I started it off at 12 volts, had 4.9 volts output. Bumped it up to 12.4 volts, still 4.9 volts output.

    What do you mean by as witch in the line through the USB port? A switch to stop the 12v from going to the conversion circuit?

    Sure, just put a switch in one of the wires of your 12v line that feeds your circuit. I think conventional placement would be on the negative (black wire) leg.

    Wrong totally a bad idea if you are wanting to put a switch to go across the 12v rails to stop the 12v current running into the 12-5v converter then you will need to solder the wires to positive( Ie...one wire from the switch to the positive terminal of the battery or Psu and the other wire to the positive input of the 12v to 5v converter. putting a switch across a negative wire will never work switchs are designed to work on positive current only (worked this out by many years of tial and error work with circuits )

    Great Instructable.

    I have a small MP3 player I hook onto my car radio and would like to make this for my car and avoid the huge cig lighter adaptor. I get a lot of noise from the cig lighter supply, would this be better or do I need to add any type filter please?

    3 replies

    Noise? If you mean power spikes and dips, then yes, the capacitor should handle those, but if your mp3 somehow has poorly isolated power that feeds back through the audio jack, I don't know what to say... Can you describe the issue? Also, modern cig lighter adaptors are tiny these days, it's why I recently added to link to one of those cheap flush mounting ones.

    The noise is only when I use the power supply. If I run it on the MP3's internal battery the noise is gone.

    It's hard to diagnose like this, but is the noise different/change when the engine gets revved? If so, the capacitor should do the job, you could even put a bigger one if necessary. If not, it's because the power of not isolated, no clue what to do then....

    Would anyone be able to help me, I'm going to make this little project but need it to work with Apple devices. What will I need to make this happen or is there another project like this that will work with Apple products ?

    sir, my input is from an adapter that gives 12 v 2 amp.. i want to get output as that of the usb i.e., 5 v 100-150 miliamp for running the portable speakers.. plz provide some suggestions or circuit diagram to get it done..

    1 reply

    That is what this is. Your 12v 2A has a capacity of 2A, it will only output what is required. If it is indeed a fixed 2A output (uncommon and very specific type of device), you would need to be able to dissipate the heat, so I would just add a bigger heat sink. Lastly, again, the ma of the 5v will change based on what your device pulls. If you actually need fixed current and fixed voltage, that is, once again, a whole different issue.