I'm currently trying to build a USB charger device for my mobile phone using an old 12V car battery as power source. I purchased different step down modules (see below) to regulate the voltage from approx. 12V down to 5V.

My problem is, that as soon as I connect my mobile phone to the output of the step down regulator, the previously adjusted output voltage (via potentiometer) drops for about 0.7V causing the mobile phone to not charge properly. Example: I configured the step down module to provide 5.2V at the output. Without load, there is no problem. With my mobile phone attached the output of the step down regulator, the voltage drops to approx. 4.5V.

I checked the input voltage of the regulator from the battery. Without Load: 12.85V, with mobile phone connected: 12.74V.

Is the "unstable" input voltage responsible for the massive voltage drop on the step down output? Or is there anything I can do to improve output voltage stability under load? Because I don't want to set the regulator to 5.9V only that it will really deliver 5V when connected to my phone - I think that's not very comftable for the mobile phone?!

Thank you!

I tried the following "ready-to-use" modules:

  1. MP1584EN
  2. LM2596

1 Answer 1


The input must not affect the output, it's a closed loop system, which means that in wide range of input voltage, it will regulate the output to what you configured.

And since i haven't seen such unregulated DC/DC for a long time, i suggest to first understand how do you connect it and what are you measuring. Specifically, you have to measure on the module itself to ensure regulation. The wires must be thick enough to hold charge current. Of course, in the end the measurement should be taken near the phone. If the cable is poor, you may need to raise the DC/DC output voltage.

  • \$\begingroup\$ You are talking about an "unregulated DC/DC". What circuit would you recommend instead? What should I use for my application?Please be a little bit more helpful with your answers. I know that I'm not experienced but just pointing me out to "understand it" doesn't help. \$\endgroup\$
    – John
    Commented Aug 6, 2015 at 6:01
  • \$\begingroup\$ Both modules are regulated. What i am saying, is that probably both are doing well, but the USB cable may be not good enough, or it's connection to the module. Anyway, measure the output on the module itself to make sure. There is that big capacitor near the potentiometer, measure on it. \$\endgroup\$
    – user76844
    Commented Aug 6, 2015 at 6:06
  • \$\begingroup\$ Some modules may require load for regulation, the data sheet for these may be lacking \$\endgroup\$
    – crasic
    Commented Aug 6, 2015 at 7:56

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