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Flyback Protection — Parallax Forums

Flyback Protection

bennettdanbennettdan Posts: 614
edited 2011-05-06 00:10 in Propeller 1
I have a project that I want to drive some solenoids with mosfets and I wanted to know what everyone has used to keep the Prop and mosfet drivers safe from flyback currents. A schottky diode is what I was hinking of using. The solenoid is a 12vdc about 1amp by the way.

Comments

  • Mike GreenMike Green Posts: 23,101
    edited 2011-05-01 18:50
    Any ordinary silicon diode will do. Something like the 1N914 or the 1N4001 will do just fine. The reverse voltage of the diode should be greater than the supply voltage. The current rating doesn't matter much because the purpose of the diode is to prevent the build-up of the flyback voltage as the magnetic field collapses, not to bypass it at its peak. You don't need a Schottky diode although you can use one if that's what you have on hand.
  • bennettdanbennettdan Posts: 614
    edited 2011-05-01 19:34
    Thanks Mike for the tip.
  • Toby SeckshundToby Seckshund Posts: 2,027
    edited 2011-05-01 23:09
    I have seen people worry about the time it takes for the diode to start its conduction of the reverse spike and put a resistor across the winding as well.

    If the solenoid is being used for something that brings it in, and it stays in for a reasonable time, then at the expence of a few more components it could be good to have a two stage cct, as well, that gets the core shifting at full current and then after closure only the holding current is maintained, although if the useage is continuous rapid movements then this would be a waste of time.
  • ElectricAyeElectricAye Posts: 4,561
    edited 2011-05-02 03:43
    Toby Seckshund wrote: »
    ... it could be good to have a two stage cct, as well,....

    At the risk of sounding stupider, I have to ask: what's a cct?
  • Toby SeckshundToby Seckshund Posts: 2,027
    edited 2011-05-02 04:41
    Just me being too lazy to type "circuit". Probably just an English abreviation.
  • Mark_TMark_T Posts: 1,981
    edited 2011-05-04 16:51
    Ordinary diodes are usually fine - so long as the peak current of the diode is enough for the 1A the solenoid takes it can handle it. However there is one possible problem - switching speed. By using a diode like this you slow down the switch-off process in the relay - not that speed often matters in a relay circuit. If switching speed is important the solution is adding a zener in series with the regular diode but cathode-to-cathode so that the reverse voltage across the coil is much larger (diode forward drop plus zener breakdown) causing the current to drop much faster.

    Using a schottky diode rather than a regular one would exacerbate the slowness of switching BTW.

    The flyback currents aren't the problem its the _voltage_ that does the damage.
  • AleAle Posts: 2,363
    edited 2011-05-06 00:10
    I have this circuit where a MOSFET drives a trafo at high frequency. Due to the high current involved the MOSFET overheated and short-circuited the gate and the drain, the result were 12V at the driver's output... and destroyed :-(. I'm thinking that a pulse-trafo may be a good idea for the next try... and some other driver.
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