optoisolated MCU tacho input

Discussion in 'The Projects Forum' started by Mosaic, Feb 13, 2011.

  1. Mosaic

    Thread Starter Member

    Aug 2, 2010
    Not to revive a not-so-old post but I've been facing a similar challenge.

    Have a look here for details on an optoisolated MCU tacho input:


    About half way down the scroll bar.

    Also , let me suggest the LM2937 LDO vreg for automotive use. But you WILL need a properly rated transorb & perhaps MOV to protect from the load dump.

    I use both...a uni Directional SA30A 600W unit from Jameco backed up by a VZ24za50 MOV plus a ferrite SMD /cap pi filter. You see the SA30A works FAST -about 1ns, while the MOV is slower perhaps 25ns. So the chosen Sa30A clamps at a higher peak V (49Vc) than the MOV 43V Vc...both are below the 60V the LM2937 Vreg can handle. For longer transients the MOV will kick in and clamp it b4 the SA30 dies, thus handling the 'load dump" . Additionally I have a 10Ohm 3W wirewound resistor inline with my MCU circuit to limit the transient current. Thus a 100V transient will max out at 10Amps ...which is handled by the MOV handily.

    Actually since I am only concerned with currents generated by the P.D. of the Vclamp...a 100V transient - a 43Vclamp = 57V. Dropped across a 10Ohm = 5.7Amps....The 20,000Amp MOV won't blink.
  2. bertus


    Apr 5, 2008