I had several issues with car usb adapters (dc-dc step-down regulators), and I'd like to make one myself.


  • Nokia cell phone refusing to charge
  • Motorola phone power supply circuitry fried
  • Insufficient current

The first thing I'd like to know is whether it's fine to use a switching regulator.

  • My understanding is that the output will be a square wave signal, am I right?
  • Will the cell phones like that?
  • Will the ripples and resonance in the signal will a further problem?
  • If I employ a regulator such as the TSR 1-2450, should I add other components to filter the input and clean the output or is it fine to use it just by itself?
  • Am I better off with a linear regulator?

I tried using a LM2576T-5 with the standard configuration:

lm2576t-5 typical application

It worked great! Then I added the suppression circuitry suggested by @tcrosley, but with a couple changes:

my attempt at suppression circuitry how I wired everything up

  • I did NOT double the 100 uF capacitor at the beginning of the first circuit and that at the end of the second.
  • Since I couldn't find a 1.5KE18A, I used a 1.5KE15A, which works at 12.8V, so I suppose it should be fine.
  • The 1N4001 wasn't good for me, since it limits current to just 1A. I used one of the 1N5822 I already had, comforted by what I found here.

This resulted in inverted polarity (!!) on the output of the LM2576, which ceased to work shortly afterwards (I didn't cut the power, because it seemed to work somehow, so I was measuring voltages when it died). I triple checked my soldering work, but all seems in order (well, quite ugly actually!, but correct). I really can't figure out what's the problem. Maybe I'll just get another LM2576 and remove the suppression circuitry.

The power supply for the testing was a 12V, switching (common PC PSU).

The IC is still good and the circuit is finally working. Turns out that the inverted polarity was my fault. I connected 2 usb sockets to the leads, but I had the pin-out upside down! Since the usb socket casing is shortened to pin 4 (ground), and I thought that was pin 1 (Vcc), I ended up this circuit: wrong circuit Now I fixed it, and I will try again with the suppression circuit. I'll use at least the TVS diode. board

It seems like I had this all wrong from the start. I added the suppression circuitry, checked that all was working and mounted the thing on the car. And surprise surprise, it started smelling after a few minutes! While in the LM2576 datasheet it is written that there's no need for a heat sink, the reality is that it gets really, really hot after just a few minutes of phone charging :(
Note how it melted the inductance nearby!
Now, in order to fit a heat sink, I'll have to do a new board...
circuit mounted on the car central tunnel

take 3, finally working properly. thank you all! enter image description here

  • \$\begingroup\$ Have you done any research to find out what the cellphone manufacturers say i.e. what their DC power specifications are? \$\endgroup\$
    – Andy aka
    May 10, 2015 at 19:30
  • 4
    \$\begingroup\$ "the output will be a square wave signal" no way, except maybe when the converter is operating for beyond its design parameters. The output is DC with some ripple. \$\endgroup\$ May 10, 2015 at 19:33
  • \$\begingroup\$ @Andy: I didn't think of that ^^" But for the moment I haven't found anything useful. They usually just state the maximum current (why? shouldn't the device draw just the current it needs? I don't suppose my cellphone will draw 10A if the PS provided that current). All these cellphones I tried charged via USB. So I suppose the charger must meet the USB standards. And yet different phones behave differently. \$\endgroup\$
    – Valentino
    May 10, 2015 at 20:27
  • \$\begingroup\$ thanks to the moderator for reorganizing my post in a more schematic way. \$\endgroup\$
    – Valentino
    May 12, 2015 at 11:13
  • \$\begingroup\$ I once made a charger from TPS54540. Was cool even while charging three cell phones. \$\endgroup\$
    – user76844
    May 30, 2015 at 21:24

2 Answers 2


Yes, it's okay to use a switcher, that's what the majority (if not almost all) of cell phone chargers use. For example one of the Apple chargers is a 1" cube:

enter image description here

To get that small, it has to be a switcher. Of course it goes from 110 VAC to 5v DC. To that, it first rectifies the AC to DC, chops it to a very high frequency (100's of kHz) using the switching regulator, and then it is rectified again. It is much easier to rectify a high-frequency than a much lower one.

In your case, you will be doing the same except you don't need the first rectifying step.

The output of a DC-DC converter will be filtered DC, with minimal ripple (about 150 mA or 3% for the TRS1-2450).

You can also make your own regualtor using a "buck" switching regulator IC, like the NR887D, which accepts an input up to 18V and a output up to 14v at 2A (you will set the voltage of 5v using two resistors). It is available from Digi-Key for $1.58 in a 8-pin DIP package.

enter image description here

Here the 22 µF capacitors C4 and C5 remove the ripple.

You need the extra margin on the input since 12v car batteries often are as high as 14v. You will also want to add some suppression circuitry on the input since there are often spikes on the 12v line too. You can use a circuit like this:

enter image description here

This circuit can be used either in front of the DC-DC converter or the discrete switching regulator above.

  • \$\begingroup\$ you were of great help!! thank you! I'll start looking for the components \$\endgroup\$
    – Valentino
    May 12, 2015 at 11:31
  • \$\begingroup\$ I'll use a LM2576T-5. Regarding the suppression circuitry: will the 1.5KE18 diode make a shortcircuit when the voltage at its ends is higher than 15.3V? that part is expensive (relatively to the rest of the project). are there any alternatives? \$\endgroup\$
    – Valentino
    May 16, 2015 at 13:38
  • \$\begingroup\$ @Valentino Yes it will temporarily make a short circuit (for like µs), that's the point, to keep high voltages away from the rest of the circuit. That part is 67¢ in singles, there is an equivalent part SM6T18A that I found for 43¢. That's as low as I could find. \$\endgroup\$
    – tcrosley
    May 16, 2015 at 15:27
  • \$\begingroup\$ lucky you! :) I can't get it for less than 5€ (with shipping) \$\endgroup\$
    – Valentino
    May 16, 2015 at 19:09
  • \$\begingroup\$ Wow. That price was from Digi-Key. Price falls to 15¢ in quantities of 1000 (but shipping not included). \$\endgroup\$
    – tcrosley
    May 16, 2015 at 20:54

One of the problems with charging from USB is that originally it was specified to only supply up to 500 mA (actually only up to 100 mA until the device has communicated with the host controller and said that it wants more). Nowadays there's more or less standard ways to signal that the host is actually a charger and avoid the need to speak the USB protocol – one way that is almost universal is connecting the D+ and D- lines together. This lets the phone know that it is allowed to draw more current. I think Apple devices use a different way though.

You're definitely better off with a switching regulator, and the Traco DC/DC converted module you linked to looks good to me (though it's limited to 1 A, while tablets would probably charge faster if they got 2 A). It should be fine by itself without any extra filtering capacitors, and it's definitely a lot simpler to use than building your own switching regulator from ICs, inductors, etc.

  • \$\begingroup\$ thanks! I'm aware of the limitations of the UBS protocol and how to identify the port as a charging port (as you said, I will short the data pins D+ and D-, but I think it's safer to put a little resistor in between. <200ohm should work) \$\endgroup\$
    – Valentino
    May 12, 2015 at 11:18
  • \$\begingroup\$ I'd rather use a packaged solution like the Traco, as long as it's capable of providing at least 1.5A. If I can't find that, then I'll build one. Thank you very much for your help. \$\endgroup\$
    – Valentino
    May 12, 2015 at 11:20
  • \$\begingroup\$ The most common USB charging standard allows for up to 1.5A of current at 5V +-5% and is signaled by shorting the data pins together. But, supplies also have a current detection circuit that drops the voltage when the current draw exceeds the amount the supply can safely supply. So if it can only supply 1A, it will drop voltage when current draw exceeds that and devices will adjust. I don't know how crucial this is with real devices that charge with USB. As written it seems like without this current detection the device may draw excessive current, or maybe it might just attempt the full 1.5A. \$\endgroup\$
    – tom r.
    Jun 11, 2021 at 11:52

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