Context: A digital phase-controlled light dimmer system.

  • 120V AC mains
  • I use an isolating transformer (and rectifying circuit) to generate +12V and +5V DC rails.

I want to use reverse phase control (I've read that this produces less EM noise), so I'm looking to switch the AC mains using 2 Power MOSFETS (e.g. FDB14N30) with the source pins connected together.

In order to put the MOSFETs into the "on" state, I need to raise Vgs to 10V-30V, but in order to do this, I think I need to hook up the MOSFET source pins to my isolated ground rail, and the gate pins to my +12V switching signal.

By doing this, have I defeated the whole purpose of an isolated power supply, in terms of danger of death and damage to components?

  • \$\begingroup\$ I've always used a triac to switch mains voltage... I didn't realise a power MOSFET would do the same job... what are the differences? \$\endgroup\$
    – BG100
    Commented Jan 8, 2011 at 1:34
  • \$\begingroup\$ The main difference I'm going with is that Power MOSFETs can be switched off part of the way through a cycle, enabling reverse phase control (epanorama.net/documents/lights/lightdimmer.html). Apparently Gate Turn-Off Triacs will do the same thing, but I was having difficulty finding a vendor for GTO's. \$\endgroup\$ Commented Jan 8, 2011 at 1:55
  • \$\begingroup\$ Okay, I've read up on reverse phase control, is noise really a problem for you? Could you just use an input LC filter to eliminate the necessity of it? The extra cost of the filter will probably be made up for the savings in other components. \$\endgroup\$
    – Thomas O
    Commented Jan 8, 2011 at 2:33
  • \$\begingroup\$ It's possible that adding an LC filter will do the trick in terms of reducing noise. However, I don't have the experience to know instinctively if this is the case. (See electronics.stackexchange.com/questions/8733/…) \$\endgroup\$ Commented Jan 23, 2011 at 0:08

3 Answers 3


If you want to maintain isolation (I assume you have some legitimate reason for it beyond it being "good", i.e. user-accessible low-voltage electronics) and still drive your floating MOSFETs, you could generate a gate voltage with a full-wave rectifier and a shunt regulator, then switch it using a standard optoisolator. The drive current will be incredibly low if you're only switching at 60 Hz, so your regulator's Iq can also be quite low.

  • \$\begingroup\$ So: we have 2 separate DC regulators: one to give us +5V for microcontroller logic, and between +12V and +30V to drive the MOSFETs. The optoisolator protects the microcontroller circuit from the +12V non-isolated supply. \$\endgroup\$ Commented Jan 23, 2011 at 0:06

You're switching AC. Use a TRIAC. For isolation, use an opto-isolator with zero crossing detection - for example, many of these would be suitable.

  • \$\begingroup\$ He said that he "want[s] to use reverse phase control", which is impossible with TRIACs. \$\endgroup\$
    – Nick T
    Commented Jan 8, 2011 at 2:24
  • \$\begingroup\$ Thomas: if you could point me to a Gate Turn-Off thyristor or triac I could use with 120V AC mains, that would be helpful. \$\endgroup\$ Commented Jan 8, 2011 at 2:50
  • \$\begingroup\$ @Isaac Sutherland, I think the problem is that GTOs are high current devices. I couldn't find any rated below 400A continuous conduction current. There just isn't a market for them for fully switchable AC, it seems. \$\endgroup\$
    – Thomas O
    Commented Jan 8, 2011 at 3:38

You can use an optoisolater for this... Search for MOC3020


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