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Need to reduce 5V 500ma to 3V 30ma

Discussion in 'General Electronics Discussion' started by Amitp, Oct 16, 2016.

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  1. Amitp


    Oct 16, 2016

    I've near zero knowledge of electronics. My question may be incorrect or very simple. But here it is :)

    I need to reduce 5V 500ma (usb port output) to 3V 30ma.

    I've a Bluetooth keyboard and it has some problem that it completely drains new pair of AAA batteries in about 2-3 hours (even if I'm not using the keyboard). In about 2-3 batteries are dead with some leakage from the battery terminals.

    Keyboard power requirements: 3V 30mah ( 2 AAA batteries )

    I'm trying to power the keyboard using the USB port of my computer. I cut off one end of a spare USB cable and tried to connect with the terminals in the battery compartment. +ve with +ve and -ve with -ve (just to be clear).

    After connecting a battery indicator in the keyboard blinks rapidly and keyboard doesn't work.

    What is required to reduce the input 5V 500ma to 3V 30ma. I tried using a resistor rated 560 ohm(figured the resistor rating after some googling and understanding color coding). But my multimeter show 5V with / without resistor being attached to +ve line (may be resistor is faulty).

    I've a few old non working electronics. I can pull out components from them.

    Can anyone suggest what resistor capacity will be required to achieve 3V 30ma ?
  2. davenn

    davenn Moderator

    Sep 5, 2009
    hi there
    welcome to EP

    no, you just need a 3V PSU capable of at least 30 mA ... the keyboard will draw only what it needs

    there are 3V regulator chips out there ( actually around 3.3V is the standard)
    sure to be many selling 3.3V fixed regulators, do a search on ebay

    Last edited: Oct 16, 2016
    Terry01 and Amitp like this.
  3. Amitp


    Oct 16, 2016
    Hi Dave,

    Thanks for your quick response.

    By using a 3V PSU it would consume a power socket. I can go for that if nothing else works.

    Just out of curiosity, reducing 5V to 3V is complex that buying a 3V PSU is better option ?

  4. davenn

    davenn Moderator

    Sep 5, 2009
    there may well be 3V plug packs out there I haven't looked the best way to go

    but either way you are loosing the advantage of the Bluetooth and it is no longer cable free
    the batteries in my BT keyboard last best part of a year
    So maybe that keyboard is faulty
  5. davenn

    davenn Moderator

    Sep 5, 2009
    I assume you are referring to a computer KB ?
  6. Amitp


    Oct 16, 2016
    Yes, it is a computer KB. I'm ok to lose wireless feature. Instead of buying a 3V PSU I would prefer buying a new wireless keyboard.

    It was just that I wanted to try if I can power this one using USB port. Just to add KB works fine for 2-3 hours after installing new batteries, until it kill them.

    Anyways, thanks for your help. I'd try somewhere else.

    Last edited: Oct 16, 2016
  7. davenn

    davenn Moderator

    Sep 5, 2009
    you surely can see how that is pointless .... you mite as well just buy a USB KB

    I still suggest that you existing KB is faulty and causing the batteries to drain quickly, powering it externally isn't really going to solve the problem if there is a fault
  8. Alec_t


    Jul 7, 2015
    The AAA battery has a quoted capacity of ~ 1000mAh, so if your keyboard is draining it completely in 3 hours it must be drawing over 300mA !! Conclusion: the keyboard is faulty.
  9. Audioguru


    Sep 24, 2016
    Energizer AAA alkaline batteries can supply 30mA for 25 to 30 hours. Cheap Chinese "Super Heavy Duty" AAA batteries will drop dead in 2-3 hours.
    Cirkit and bushtech like this.
  10. Engineer_Paul


    Sep 5, 2017
    Easiest way is use a NPN emitter follower, and connect a small zener diode to the base 3+.7=3.7 volt zener and resistor between the base and collector 5-3.7= 1.3v assuming beta =40 then you will need 1ma bias current. 1.3/1ma =1.3k or 1k to be on the safe side. That is only 3 components, and it is always a good idea to put a small ceramic capacitor or other capacitor with a low series equivalent resistance across the output of any power supply to prevent problems with random signals due to feedback. .1uF is usually a good choice between emitter and ground. Zener is base to ground.
  11. kellys_eye


    Jun 25, 2010
    A couple (or three) series wired silicon diodes will drop enough to make it work. I doubt the keyboard will worry too much about the voltage being 10-20% either side of the 'rated' voltage, them being batteries that can vary in full terminal voltage depending on the type fitted.
    Terry01 likes this.
  12. Externet


    Aug 24, 2009
    As above, 3 diodes 1N4001 in series :

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