I'm doing a project which consists of 2 big parts: a control part (a microcontroller with I/O pins expanded, USART-to-USB, a RTC, an EEPROM, a LCD) working @ 5V and an actuators part (optocouplers for interfacing with the microcontroller's I/O pins, MOSFETs driving DC motors working @ 24V and TRIACs driving AC motors @ 230V~). Both parts of the project are supplied by a DC 24V power supply. So, am I achieving galvanic isolation between the control part and the actuators part using optocouplers, even though they have the same power supply, consequently the same GND signal?. Should I bother to separate the control part's GND from the actuators part's GND?

project diagram

  • \$\begingroup\$ How do you control the triacs? \$\endgroup\$
    – jpc
    Commented Aug 13, 2011 at 9:02
  • \$\begingroup\$ I control the TRIACs through MOC3051 optoisolated triac drivers \$\endgroup\$
    – m.Alin
    Commented Aug 13, 2011 at 9:13

1 Answer 1


You'll have galvanic separation between the mains and AC motors on one side, and the 24V/5V on the other side, because of the mains transformer. You also may have separation between 24V DC and 5V DC, but most regulators don't provide that. You'll need an isolated DC/DC converter, where the transformer takes care of the isolation.
If you have that you have 3 mutually isolated parts:

  1. Mains and the AC motors
  2. 24V supply and DC motors
  3. control part

If you don't the 24V motors aren't separated from the control part, despite the optocouplers.

  • \$\begingroup\$ So if I only have a voltage regulator (L7805) on the control part, I guess there's no sense in using the optocouplers to drive the MOSFETs which drive the DC motors, isn't it? Don't they offer any degree of protection, even if the GND is the same? \$\endgroup\$
    – m.Alin
    Commented Aug 11, 2011 at 17:39
  • \$\begingroup\$ Can you recommend any DC/DC converter (24V to 5V), suitable for isolation, that can provide 2A and can be PCB mounted? \$\endgroup\$
    – m.Alin
    Commented Aug 11, 2011 at 17:44
  • 1
    \$\begingroup\$ Yes, they do protect; whatever voltages the optocoupler's transistor sees, they won't be seen on the LED's side. You may keep the optocouplers for that reason. It's just like you say that they share a common ground, so they're not isolated. \$\endgroup\$
    – stevenvh
    Commented Aug 11, 2011 at 17:44
  • \$\begingroup\$ @m.Alin - this one can work off 24V and can supply up to 2.5A. \$\endgroup\$
    – stevenvh
    Commented Aug 11, 2011 at 17:50

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