In my car I have power converter for the cigarette lighter. This power converter has some 3 USB ports and 2 cigarette lighter ports. I have devices plugged into the cigarette lighter ports. One of which is to power my phone (is left in car overnight).

If the phone has enough power it will power the other devices plugged into the power converter, when the car is turned off. Power is not coming from the car. I know this for certain as the other devices will immediately turn off when i remove the charged phone.

I assume the power from the phone is leaking through the USB cable and into the other devices. This is a problem as I don't want the phone to discharge over-night.

I think putting in a diode on the 5V wire for the USB cable for the phone will do the trick. What requirements do I need in a diode to ensure that I don't cause damage to my phone and allow it charge as normal as possible?

  • \$\begingroup\$ What brand is your phone? I want to make sure never to buy such a poorly designed product. \$\endgroup\$ Jan 3, 2014 at 18:13
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    \$\begingroup\$ Just adding a diode in series might not work. It will introduce a voltage drop of about 0.7V which may be enough to prevent your phone from charging reliably. \$\endgroup\$ Jan 3, 2014 at 18:14
  • \$\begingroup\$ @jwygralak67: it's a Motorola Droid 3 \$\endgroup\$
    – TruthOf42
    Jan 3, 2014 at 18:17
  • \$\begingroup\$ @jwygralak67: I was made aware of the 0.7V drop but I was also told that they make diodes that have less drop like 0.2V. Either way I'm willing to take the risk. It's my old phone and if it bricks... oh well. \$\endgroup\$
    – TruthOf42
    Jan 3, 2014 at 18:19
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    \$\begingroup\$ I can't see how it would brick the phone. Worst case, it won't charge. \$\endgroup\$ Jan 3, 2014 at 19:18

1 Answer 1


The main properties you want to work around are the maximum allowed reverse voltage, the forward voltage drop, the maximum forward current and the form factor.

The maximum reverse voltage should be comfortably greater than the USB voltage (5V) - give it at least 6V. Practically, you'll probably find a much higher reverse voltage limit in the diode you select, so don't worry about this too much.

The forward voltage drop should be very low at the current you expect to see when the phone is charging. Depending on your phone it should certainly have a charge rate of less than 2A. You'll have to examine the V-I curve for the diode to determine this. Lower is definitely better here - try for .5V or less at 1A or lower.

The maximum forward current is chosen based on the charging voltage of your phone - again, 2A is generally the maximum charging current you'll see from any USB device (and it's generally pads that draw this, not phones) and you'll want a margin of error, so go 3A or above.

For the form factor you'll probably want a through-hole diode for ease of install. I recommend some heat-shrink around the whole thing to give it some mechanical stability when you're done soldering.


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