Dropping voltage 5V -> 3,3V

Discussion in 'General Electronics Chat' started by durazell, Dec 30, 2012.

  1. durazell

    Thread Starter New Member

    Dec 30, 2012
    Okay first of all I'm completely new to all electronics.
    I have a circuit which uses 3,3 volts and takes it's power from usb port (customized one). I wanted to use PC to power the device from normal usb port and issue custom commands to it. I did voltage drop with two diodes (it reads 3,37V) and it works allright, but i'd like to know if there is a way to measure current in the circuit. I don't have any data sheets/schematics as circuit is proprietary (=consumer electronics).

    Reason to do this, is that I wanted to use regulator but didnt know which one to use because they have current limits (50mA for example) and i have no idea how much current this circuit uses. can i measure resistance between +5v and gnd and use ohm's law to find current or how to find it out?

    Last edited: Dec 30, 2012
  2. panic mode

    Senior Member

    Oct 10, 2011
    you can use multimeter and measure it directly but this means introducing a cut somewhere in a circuit (maybe sacrificial USB cable). USB devices draw up to 500mA so if you are looking for LDO voltage regulator, just pick one that is rated for more (800mA or 1A for example).
    durazell likes this.
  3. kubeek


    Sep 20, 2005
    Measuring resistance won´t help you, because ohm meters are typically designed not to turn on PN junctions, so you would measure some totally unrelated number.

    You can either use the current mode on your multimeter and put it in series with the 5V wire, or put a small resistor like 0.1ohm in series with the 5V wire and measure voltage across that. Then use ohms law to calculate the current, this method is usually more accurate than using a current meter.

    I usually use lm1117 3.3V regulator for this purpose.
    durazell likes this.
  4. durazell

    Thread Starter New Member

    Dec 30, 2012
    lm1117 is good for my needs, thanks for tip!
  5. Externet

    AAC Fanatic!

    Nov 29, 2005

    Three 0.6 Vf silicon diodes in series brings the voltage close enough to needs.
  6. #12


    Nov 30, 2010
    Just to put some predictability into this discussion, here is the graph of forward voltage for the 1N400X series diodes.

    With USB capabilities of 500 ma and more in the 2.0 and 3.0 versions, this could be significant.
    panic mode likes this.