I've been working on a simple power bank project that's been built using Li-ion batteries, the batteries are rated 3.3 V.

I've implemented a charge controller which can efficiently charge the power bank batteries from USB charger without any issues.

The problem here is that I want to create dual output 5 Volts slots 1 and 2 amperes respectively for charging external devices. How can I do that efficiently ? I've surfed the internet and found articles and answers about current limiting using resistors and/or transistors but i'm so confused now.

Thanks so much.

  • \$\begingroup\$ Technically, you don't need to make any special means to limit supply current. It would be nice, but is not necessary. The current is defined by attached device. Your job is to provide proper "charging port signature", so the attached device can determine the limit. \$\endgroup\$ Commented Dec 25, 2018 at 1:01
  • \$\begingroup\$ @Ale..chenski well, after more research i found that i only need what is called a "charging port controller" which is able to provide the charging device signature... also i'll be limiting the current for each port for few Amps for safety, in case if some device draws more current, it won't be harmful. but thank you a lot for your answer, you emphasized my concern. \$\endgroup\$ Commented Dec 26, 2018 at 15:26

1 Answer 1


First you need to boost the 3.3V to 5V, that can be done with a boost converter (Wikipedia). After that you'll have a 5V rail.

Now build 2 current limiting circuits one at 1A and the other at 2A. Building a current limiter is pretty simple with a linear voltage regulator and a few surrounding components. There's an example on how to do it with a LM317. Figure 23 in LM317 datasheet shows an example of how to make a adjustable one. The LM317 is not the most efficient regulator but it will give some ideas. You'll have to do some research to find the most suitable for your application.

  • \$\begingroup\$ Building boost converters from scratch is not easy, you have to handle with EMC especially EMF. And the selection of suitable components is not easy for a layman. \$\endgroup\$ Commented Dec 23, 2018 at 12:38
  • \$\begingroup\$ Who said he has to do it himself, I just suggested he could use a boost converter. Texas Instruments have the simple switchers series which doesn't require a lot from you. \$\endgroup\$
    – JeppeSRC
    Commented Dec 23, 2018 at 12:41
  • \$\begingroup\$ well, thanks, I know that building boost converters that can handle varying input voltage is difficult, also for accurate and steady outputs i should buy a boost converter dedicated IC ... But the issue here is with the current limiting circuits, do you have any reference of how to build such one, using the Linear voltage regulator ? \$\endgroup\$ Commented Dec 23, 2018 at 14:37
  • \$\begingroup\$ Yes I updated the answer \$\endgroup\$
    – JeppeSRC
    Commented Dec 23, 2018 at 14:49
  • \$\begingroup\$ Alright, thanks Jeppe. I really appreciate your help. \$\endgroup\$ Commented Dec 23, 2018 at 14:53

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