I'm working on a project to use PIR sensor to save on electricity bill in a university, almost a fresh graduate in Electronics Eng.

Load is Fluorescent tubes 44w each via ballast,there will be between 16-36 lamps available per relay, the idea is to control them using:

  • Micro-controller
  • inexpensive PIR sensor
  • Relay
  • 5v Power supply,

Few problems occurred: First, there will be a lot of wirings and bigger chance of something going wrong in the setup, especially since each room can have a separate power supply, or multiple rooms sharing a power supply.

Second, Fluorescent lamps as I read, can take up to 100 times the rated current at start, our relay for now is a 30A relay SLA-5vdc-SL-A module, current for some rooms is calculated to be 7A, I've read about SSR, it can solve the problem, but are they safe (I mean the ones made in China), what could go wrong?

Third, for the power supply, many options are available, but the place this will be installed is critical (a university), so safety is a huge concern, system running 24/7 but at low current, maximum per room 500mA, PS can be connected to up to four rooms together, or one per room.

Is the project on the right path at least? any suggestions?

  • \$\begingroup\$ Inductors draw maximum startup current if turned on a zero voltage. Better turned on at Vmax!. Consider the capacitor equivalent with everything phase shifted). || 100x surge current is "unlikely" but quite a few times more is feasible. | | Nothing from China can be trusted if you do not know the bona fides of the seller or maker and that either or both never sell junk. A good step on that journey is to buy Chinese product from a known fully trustable reseller who has done the checking for you. Digikey is one example but there are others. \$\endgroup\$
    – Russell McMahon
    Commented May 6, 2016 at 12:15

1 Answer 1


I've never heard of fluoros drawing 100x rated power at start up (seeing as they tend to use inductive balasts, this seems unusual). Incandescents can draw a lot more for a mS or so as their cold resistance is much lower than their hot resistance. Now not to discourage you, but in many places it's illegat to do any mains wiring without an electricians licence (I know, I'm an EE student myself). But, that doesn't mean you can't do this as part of ypur project (my final year project had mains voltages as well), you just need to get it checked by a qualified electrician, they can probably give ypu some good advice on what to and not to do, more than we here can as we can't be there with you in person. I'd avoid cheap chinese electronics (expensive chinese electronics tends to be quote good). You get what you [ay for and at mains voltages, you can't afford to skimp (you can sometines get away with it at fiddle friendly voltages though - i.e. less than 50V.

  • \$\begingroup\$ thank you, the person doing the mains wiring is a professional, not me \$\endgroup\$
    – tronics
    Commented May 9, 2016 at 3:34

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