Which diodes to use for flyback?

Discussion in 'General Electronics Chat' started by AgeingHippy, Jun 28, 2011.

  1. AgeingHippy

    Thread Starter New Member

    Nov 28, 2010
    Hello All

    I am building a simple motor driver using the SN754410 quad half-h bridge driver.

    Basically it can carry 1A per motor with the supply voltage between 4.5 - 36 volts.

    Initially I will be using a 3v motor, but would like to be able to use any motor provided the 1A per motor is sufficient.

    My question is which diodes should I use for this motor driver and why.

    Additionally, I have the following in stock so if any of them are adequate I would rather use them.
    1N4001 Rectifier diode 1A 50v
    1N4002 Rectifier diode 1A 100v
    1N4148 Signal diode
    1N914 Signal diode 75v
    SB160 Schottky 60v 1A
    N5821 Schottky 30v 3A (seem very chunky so probably overkill)

    Thanks for any assistance.
  2. tom66

    Senior Member

    May 9, 2009
    I would use a 1A schottky or a fast diode. SB160 looks fine.
  3. SgtWookie


    Jul 17, 2007
    The 1N400x diodes have the right current rating, but are too slow to use in an H-bridge.
    The 1N4148/1N914 are fast, but only rated for 100mA current and would burn up.

    Keep in mind the Voh and Vol specifications of that 754410 IC, and the power dissipation that will occur at high currents.

    If it is sourcing 1A current (Ioh=-1) then the output voltage will be Vcc2 -2v; so that is 2 Watts of power dissipation; if kept at room temperature it might be as low as a 1.4v drop for 1.4 Watts of power dissipation.

    If the other side of the H-bridge is then also sinking 1A, Vol at Iol=1a will be up to 2V for another 2 Watts of power dissipation; but if kept at room temperature it might be as low as 1.2v.

    So anywhere from 1.2v+1.4v=2.6v drop for the minimum @1A current and 26 Watts, and up to 4v and 4 Watts for the maximum.

    Also note that in free air @ 25°C, the maximum continuous total IC power dissipation is 2075mW, or 2.075 Watts. That is nearly 1/2 of what your power dissipation might be. Without extra cooling provisions, ~500mA when running at room temp will be about the most you'll get out of it without overheating.

    So if you're running a 3v motor, you will have to give Vcc2 at least 2.6v more than that, or 5.6v, and possibly as much as 3v+4v=7v. Note also that you will not be able to operate it at 1A continuously unless you somehow provide heat sinking/adequate cooling for the IC.
  4. AgeingHippy

    Thread Starter New Member

    Nov 28, 2010

    Thanks Guys.