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

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

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

Picture of 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

Picture of 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

Picture of 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

Picture of 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

Picture of 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.


ThanayutS (author)2017-04-16

ic lm7805 มันร้อน

bouchah (author)2015-11-06


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?

MakinThings (author)bouchah2015-11-06

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.

bouchah (author)MakinThings2015-11-06

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.

DaveD105 (author)bouchah2016-09-04

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.

MakinThings (author)bouchah2015-11-06

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.

bouchah (author)MakinThings2015-11-06

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.

RaffyTheResearchMan17 (author)2016-04-18

How can i put switch in the line thru the USB port?

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?

yes exactly :D

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( 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 )

Mark Boulton (author)2016-08-07

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?

MakinThings (author)Mark Boulton2016-08-08

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.

Mark Boulton (author)MakinThings2016-08-08

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

MakinThings (author)Mark Boulton2016-08-08

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....

JohnM612 (author)2016-03-11

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 ?

SANGRAM KISHOREH (author)2016-02-08

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..

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.

As you are using 0.5 amp fuse according to the tutorial, will it be able to sustain the 12 v 2 amp as my input?

12v@2 amps won't go through the circuit if you are pulling less than that from the 5v end. You can safely draw up to 1A at the 5v end. If you need to draw more power, then you use the same circuit but with a larger fuse and possibly higher rating LM transistor. I do give a few examples in the materials steps of subsitutions that can be done. To be clear though, what will you be plugging in to the 5v? If it does not draw (pull, require) higher amperage, then higher amperage will NOT go through the fuse and circuit. Also, if you draw 1A from the 5v, only .5A will be at the 12v end, etc.

C.B.MADHANK (author)2016-01-04

Hi Sir

I have to use my FM in my van,which is capable 5V power input,

Please help me how to install it on my dashboard

Carlos-sp (author)2015-11-20

i need help!!

how to convert 12v car power to 3v with high amper?

+100 amps

please help me

MakinThings (author)Carlos-sp2015-11-20

Is this for welding? 3V at 100+ amps will require massive cables (like 10+ cm across) unless it is for very very short duty cycle (like a few ms)

crowe13 (author)2015-05-20

Very nice and detailed! thank you for explaining the circuit out in detail. sadly a lot of instructables are very linear and don't explain anything, I feel educated :) and now have a usb in my car ?

MakinThings (author)crowe132015-10-22

thanks! :)

TheAudioBoy (author)2015-01-05

Hi, is there any way to change the output voltage to 3.3v ?

MakinThings (author)TheAudioBoy2015-01-05

you would need a transistor that outputs 3v (or a variable one with the correct resistor) and to change the resistors to the LED accordingly.

MakinThings (author)MakinThings2015-01-05

many of these could do the trick

sportative (author)2014-06-18

Hi, I wonder if someone can help.

I made this circuit and it is running a bluetooth receiver in my car perfectly. However, I have split the output to two USB cables; one for the bluetooth receiver and one for a phone charger. Whenever I plug my phone in to charge, the bluetooth device stops.
I think the circuit doesn't output the correct amps to run both. Any ideas of how to increase the output beyond 2Amps?

Many thanks!

MakinThings (author)sportative2014-06-18

It may be worth just making a second circuit; the components cost pennies, and I know that the 12v can take it for sure.

On a side note, are the two USB in parallel or in series? It would be important that they not be in series and could definately cause problems.

coventrian (author)2014-04-17

I've made it using this voltage regulator

I used this one as I want something that is high output, however it's only giving me 0.44 amps. Any idea why i'm not getting a higher output? What factors would affect the output current apart from the voltage regulator it's self?

MakinThings (author)coventrian2014-04-17

How are you measuring this? Is there a load on the circuit? Are you measuring at the input (12v) or output (5v)?

coventrian (author)MakinThings2014-04-30

I was measuring with an ammeter in series between the voltage regulator and my phone. I also have an app which I can use to compare different chargers, although this reads lower because the phone is consuming some of the power.

I have also found the solution. I noticed that in the phones status it shows as USB charging (same if it's plugged in to a PC), where as I want AC charging. The phone thinks it is connected to and AC power source if the data pins are connected / shorted (they are the two in the middle). This is known as the USB "dumb specification". I now have full speed charging :) . If you decide to do this, be sure that the voltage regulator can supply MORE than the max charge rate of your device, and that you adjust the fuses accordingly. My phone is android, I seem to remember that iPhones will not charge at full speed unless there is a chip at the other end, therefore this may not work.

Thanks julienrl for a great post and quick responses.

MakinThings (author)coventrian2014-04-30

nice find! Thanks for sharing!

MakinThings (author)coventrian2014-04-17

The link you sent me gives me an error. Can you give me the product numer?


coventrian (author)MakinThings2014-04-17
Jedrokivich (author)2013-10-25

This is great, thanks :) Used it to add bluetooth to a speaker system I'm building

MakinThings (author)Jedrokivich2013-10-25

cool, I guess you used a bluetooth device that listens and outputs to a standard audio output that needed 5v?

mole6999 (author)2011-03-10

Do you know of any resettable fuses or a regular fuse I can use that can be put on a breadboard. Been loooking on digikey and there are so many options I am lost.

MakinThings (author)mole69992013-05-03

I am not sure if resettable fuses come in that size, but if they are fast blowing, then why not! I added a list of fuse values in my instructables that can go with different transistors.

thirdGEARchirp (author)2012-02-01

I built this 12v to 5v transformer. I bought all of the parts you specified. I built it exactly as your instructions implied. After I finished it was working great. I put it on a shelf in my locker. I went back about a week later. For some reason the 12v is going in, but nothing is coming out. I decided to build another one. the EXACT same thing happened. would there be a certain reason why after not using the 12v to 5v transformer for a week it would short out?


use a lower amperage fuse. I revised my instructables. With my old fuse values, it was possible to fry the transistor (the 5v device was safe given the max output of the transitor).

strange, I have never heard of this. Could it be that it got a static shock? Static electricity will RUIN electronics (I fried my 50 000v capable multimeter with a static shock once).

If it happens again, I would consider returning the transistors you bought.

somabva (author)2010-06-11

Cool project. Just a couple of questions julienrl: Under no load, my circuit works great. Once I add a typical load to the circuit, it gets very, very hot. Is this to be expected? The spec sheet seems to allow for operating temps up to 150 deg C, but I had planned to enclose this circuit in my car's dash and am now hesitant. I don't want to light my dashboard on fire. The chip was definitely hot enough to melt your hot glue casing, so I'm assuming I did something wrong. The only thing I could think of as I read the spec sheet was that perhaps I had put the capacitor in the wrong place. If I followed your directions correctly, the capacitor went over the ground and output pins. But the spec sheet seems to indicate that the capacitor should be placed over the input and ground pins to insure stable operation under load. Or am I misunderstanding the purpose of the capacitor in your circuit? I don't do this sort of thing very much...

MakinThings (author)somabva2010-06-23

putting a capacitor simply helps to prevent peaks. Circuits that require more precision usually just put 2 capacitors instead of one (one that goes input-ground and one that goes output-ground); in this circuit, putting the capacitor on the output is what was intended. As for heat, these transistors are cheap and abundant, however, if you can get your hand on a slightly better quality one (we are still only talking about 5-10 bucks max), you can get much more efficient ones. If I remember correctly, the transistor used is only efficient at 40 or 60% or something (meaning the rest is converted to heat). You can get solid state transistors which have much less tolerance for error but that have 80+% efficiency ratings (bringing the temp down). Operating temperature means that this chip can function in such temperatures (so if you have a lot of them in a small enclosed area for example, they would still function), however, in this use, it should not reach those temperatures. You can however expect the hot glue that is directly on the transistor to melt away if you used low-heat glue like I did; this is okay since the glue prevents proper heat dissipation and is only really wanted on the contacts to prevent shorts. You will notice that in my picture I had no glue on the actual transistor (well a bit of over spill, but that is not really an issue). Although the transistor can get hot enough to hurt you if you touch it, it should not damage things around it; that being said, don't go sticking it in a bundle of wires! When you think about it, they don't get hot enough to de-solder themselves. It should be easy enough to put it in a space where it has 1-2cm of breathing room around it, and if you want extra precautions, you can use a standard project box. These boxes can absolutely withstand the heat this transistor produces; you can also use a heat sink (with or without the box). These heat sinks are readily available and made just for this kind of use; they screw on the hole on the metal plate (that's what that hole in the built-in heat sink is for!) If you need more clarification, feel free to ask! :)

jgosselin (author)MakinThings2011-11-02

I am currently building a dual port setup for installation into the dash of my pickup. that said i have had an issue finding the capacitor you list locally at all. that said you say just about any capacitor will work. If i am going to sub-statue a cap what value should I try to carry over to the new cap. Also i am interested in lower heat as this is going in the dash. I've attached an aluminium heat-sink to the transistor with some thermal paste. dropped the heat to a level where i can touch the transistor and or heat-sink without getting burnt.

Also you say that for more precision you'd use 2 capacitors one on the input to ground and one on the output to ground.. would you recommend this for an automotive in-dash install?? I charge some very expensive objects off the 5v USB ports i just want to be sure its clean power and not likely to damage my devices.

Thanks in advance

MakinThings (author)jgosselin2011-11-02

having the two capacitors will never hurt. The risk of a dangerous spike is near nothing once you have your first capacitor, the second one would be important for precision circuits such as measurement instruments. Then again you lose nothing at doing it!

If you want less heat, there are also more efficient transistors I have talked about before. They are a few dollars more, but will drop the heat down significantly, but a heatsink would still be useful.

As for the caps, just make sure that they are higher than your car battery can spike to, this is normally 13-14 volts when fully charged, that is way a 16+v cap is recommended. you can use one as big as you want, jsut make sure you don't end up with something that arcs in your car! I would say don't go over 60v-ish to be safe (that is about what telephone wiring has) since you will probably have exposed wires in close proximity.

jgosselin (author)MakinThings2011-11-03

i went and scoured radio shack and the only 220uf caps they had where 35v will those work ok?

MakinThings (author)jgosselin2011-11-03

they will work perfectly

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