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I'm about to commence building a project that's basically a USB-controlled dimmer. I think the main components will be:

  • Velleman K8064 voltage-controlled dimmer
  • Phidget 1018 usb interface board
  • Australian mains socket (into which I'll plug a lamp)
  • IEC mains socket (into which I'll plug a power supply lead)
  • a pretty plastic enclosure

I'm not so much concerned about the project working, but as this is my first project involving mains power I'd rather not build something that electrocutes me, or burns down my house.

Could someone with some experience in the field please recommend some best practices, and / or some online resources, suitable for project builders new to mains power?

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  • \$\begingroup\$ Beginner is a meta tag. \$\endgroup\$ – Dean Jun 10 '11 at 9:43
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The Phidget board that you refer to has digital outputs only.

The voltage-controlled dimmer has an optically-isolated analog input, which takes 0-10V on the non-mains side and generates an analog signal on the hot-side (the side connected to the mains) which is read by the dimmer board micro and used to control the dimmer.

The main point is that your USB board (and whatever circuitry you intend to use to control the optoisolator current) must only be connected to the photodiode (non-hot side of the optocoupler). If you try and interface it directly to the dimmer micro, plug the circuit into the mains then plug a computer into the USB port, you're going to have a catastrophic failure on your hands (since you'll be introducing an earth connection to the primary circuitry) or worse, end up electrocuting someone.

You must make sure that inside your plastic box, no part of your Phidget USB board is located near the mains areas of the dimmer board. In North America, depending on the voltages involved and the exact standard, 4 to 8mm spacing is fairly common.

Use wires with 600V insulation rating to connect between the Phidget and the optoisolator, in case they happen to come near any of the mains circuitry along their travels.

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  • \$\begingroup\$ Ah, you're right - analog input only on that phidget board. Time to shopping I think. \$\endgroup\$ – Duncan Bayne Jun 12 '11 at 1:09
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Not sure what the electrical standards are in Australia.

But the best practice is to always have a ground-isoloter-circuit to prevent leakage. This has become law in many countries because it prevents 70% of fires and death by electric shock.

So always ground your pretty box-like a metal shield inside it around yor syste,- Ground both sockets with a continuous ground.

Other than that I would test your product in a secure environment of a high ambient temperature for 72 hours near MAX rating to see if anything could potentially overheat and explode

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My advice as someone who isn't qualified - have a qualified professional check it before you plug it in and use an RCD circuit breaker.

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Make really sure there is no connection between the "hot" part of the circuit (that connected to what you call mains) and the low voltage part that is connected to the USB. Solid state relays, for example, are usually opto-isolated internally so would be fine here.

In addition, it's a really good idea to make sure there is some minimum physical distance between the hot side and low voltage side. 5mm is a good value for this. Again, solid state relays will come with more separation than this between the control pins and the hot pins.

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