12v to USB adapter \ 12v to 5v transformer (great for cars)

This will tell 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 ;)

You will need

• a fuse holder
• a 1.5A fast blowing fuse (you can use a higher rated one if you use a different transistor that can take higher amperage)
• 2 different colors of wire (so that you don't get confused later on)
• a L7805CV voltage regulator, part number 497-1443-5-ND from Digikey
• a 220uf 16v Capacitor, part number P5139-ND from Digikey.
• female USB ports times the number of ports you want (2-3 would be normal, you can expect devices to use up to 0.5A each), Digikey number AE9923-ND
• 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.

Step 1

The Transitor has 3 pins, we will call them pins 1 2 and 3. 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

Comments

[thirdGEARchirp]

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?

Thanks

[somabva]

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...

[julienrl]

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]

hello,
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
James

[julienrl]

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]

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

[julienrl]

they will work perfectly

[somabva]

Wow, that's a really detailed reply. Thanks for the insights! I was really more concerned with the voltage drop that the heat spike seemed to produce, but as you point out, it is a really cheap transistor, can't expect too much. I actually ended up screwing it into the contact side of an old CPU heat sink from my parts bucket. Overkill perhaps, but it works like a charm and I get a rock solid 5V now.

[julienrl]

good to ehar a heatsink helped!

[julienrl]

ps. sorry for the 12 day delay!

[josephthorpwausau]

here is what i have 470uf 16v x3, 470uf 25v x2, 1000uf 16v, 47uf 200v will any of these work

[julienrl]

anything around 200-500uf and 14-30v is a very safe choice. In fact almost any capacitor could work just fine.

[josephthorpwausau]

i did find a 220uf 25v maybe that will work if you could let me know that would be greatly appreciated

[josephthorpwausau]

is there a different Capacitor i could use maybe a 220uf 35v Capacitor

[mmeeker]

I mad one of these for my house the tricky part was coming from 120V to 12V then to 5V but with your instructions it wasn't to bad. You're very detailing instructions helped me out tons

[julienrl]

what did you do to go form 120AC to 12DC?

[mmeeker]

there are plenty of ways, i went the easy route and found an old 120Vac to 12Vdc and took it apart and put it insdie my enclosure then i connected the + coming from the adapter to the 5v transistor then to whatever i hooked it up to i wired mine up to power hard drives and cd drive

[julienrl]

okok, that's usually what I do (use an existing AC to DC transformer). I was just wondering if you had made that by yourself too.

[mmeeker]

i'm sure it wouldn't be to hard to do its just what i had on hand lol my first project actually. it got me back into electronics its kinda like being a kid all over again haha

[mole6999]

Do you think a 2 amp fuse would be ok?

[julienrl]

You can if your transistor can take it or if you never plug in more than 2 maybe 3 devices at most ( or verify that your devices don't draw more current than your transistor can handle).

[mole6999]

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.

[Phil B]

It is good to find someone else who uses hot glue as a "project case."  It is very helpful that you identified the pins in the USB socket for power connections.  Thank you.

[Hycro]

Sometimes I use cloth duct tape, when it's something where it just needs to be protected from shorts, and a little protection from the elements, and does not heat up much, if at all...

[julienrl]

glad i could be of help