SD card 5v to 3.3v

Discussion in 'General Electronics Chat' started by Razor Concepts, Sep 21, 2009.

  1. Razor Concepts

    Thread Starter Active Member

    Oct 7, 2008
    I am reading/writing to a SD card using a microcontroller that operates on the 5v logic level.. I have successfully used this circuit:


    Is that a "proper" circuit? Is it better to use one of those conversion ICs (MAX something?)?

    Also, can I use a voltage divider for the 3.3 supply to the SD card since it draws minimal power?

  2. SgtWookie


    Jul 17, 2007
    Well, your Arduino to the SD card should work OK and represent a light load (1mA), but your data transfer reliability from the SD card to the Arduino would be marginal at best, non-functional at worst - unless you were using an ADC or comparator to interpret anything over 2.2v as a "1" and anything under 1.1v as a "0".

    If your Arduino has Schmitt-trigger inputs based on 1/3 and 2/3 Vdd/Vcc, the 2/3 threshold will be 3.33v, which is 0.03v higher than your SD can output.

    However, if you connect the SD data output to the Arduino input using a 1.2k resistor, and use a 3.9k resistor from that Arduino's pin to +5v, you'll have about a 2.5v peak to peak signal that will cross both the 1/3 and 2/3 thresholds by a fair margin (>300mV), and still have a current draw of <=1mA from the SD output. That is, of course, assuming that your SD output can source or sink 1mA without losing much voltage, and you aren't looking for blazing speed.

    If you want high speed and better reliability, you'll need to look at level translator ICs.
  3. davebee

    Well-Known Member

    Oct 22, 2008
    I've heard that SD cards can draw a pretty hefty burst of current at times, during writes, I think, and it is recommended that they be powered with a regulator.
  4. kubeek

    AAC Fanatic!

    Sep 20, 2005
    For data the resistor divider for tx and simple connection for rx is ok. But use a proper regulator chip to provide 3.3V for the SD card.

    Or maybe think about shifting to 3.3V for the microprocessor as well.
  5. iamnaseer

    New Member

    Nov 30, 2010
    whr i can find the all components detail of this circuit ?
  6. tom66

    Senior Member

    May 9, 2009
    Putting a 3.3k resistor or so in series works well, so long as the other device has ESD (most do, esp. an SD card which is frequently handled.), and it allows bidirectional comms.