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I have a "smart" lightbulb that requires 110V AC input and uses 7 watts. Due to stormy weather here in Florida I have frequent but minor power outages that last seconds at a time. This smartbulb doesn't have non-volatile memory, so I have to reprogram it every time the power goes out.

I'm wondering how involved of a schematic I would need to ensure that the bulb receives power for at least a few seconds after the power goes out.

I really don't know anything about hardware electronics (I'm a programmer), but maybe I could get away with using some big fat capacitor? I'm not even sure where to begin.

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    \$\begingroup\$ You should add more information about the Smart Bulb you are referring to. A link to a data sheet would be good. \$\endgroup\$ – FiddyOhm Jul 23 '16 at 9:34
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    \$\begingroup\$ How does the bulb remember when it is switched off normally? i.e., What's the difference between a power cut and switching off? \$\endgroup\$ – Transistor Jul 23 '16 at 10:02
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    \$\begingroup\$ Possibly it uses bluetooth or wifi to be controlled, so inside you have some dc dc thingy, so he can possibly crack it open and slap a biggish cap on the micro power supply... \$\endgroup\$ – Vladimir Cravero Jul 23 '16 at 10:48
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    \$\begingroup\$ We can offer only broad (and mostly useless to you) speculation since we know nothing about this "smart lightbulb" you are referring to. A proper solution depends on more detailed information about the internals of this "smart lightbulb" and how it is deployed in your installation. \$\endgroup\$ – Richard Crowley Jul 23 '16 at 14:22
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    \$\begingroup\$ This sounds like a good opportunity to make a device using a microcontroller board which detects power outages and re-programs the bulb for you. \$\endgroup\$ – Andrew Morton Jul 23 '16 at 14:32
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Since the current consumption of your smart bulb is only 7 watts, I am assuming it must be an LED bulb. Internally, most LED bulbs use a power supply design consisting of a "dropper" capacitor, full bridge rectifier, and a smoothing capacitor.

The "dropper" capacitor is wired inline with the hot from the mains. The voltage from the mains is 120 VAC at 60 Hz. The voltage is higher than what is required. This capacitor is used to "drop" the voltage down, since the capacitor is an impedance in the AC circuit. This increases the component cost, but is very efficient since the capacitor has very little loss at 60 Hz.

The full bridge rectifier creates DC current from the AC current. I won't elaborate on this since there is plenty of reference material on the internet.

The smoothing capacitor causes the DC current side to have less ripple. The ripple would be 100% of the peak voltage without this capacitor. On most bulbs this is just there so the LEDs do not visibly blink to the human eye. Since this is a smart bulb, the smoothing capacitor is probably better than average to allow the electronics to function correctly.

Such a power supply configuration is non-isolated and potentially lethal. But since it is contained in the package of the bulb, it is relatively safe.

In order to act as a very brief UPS, you could modify the bulb to use a much higher capacity smoothing capacitor. The capacitor's energy storage is measured in farads, you would want a larger capacitor. But a capacitor is a poor UPS. The voltage drops linearly with respect to charge, unlike a battery's voltage. So there a few problems:

  1. By the time you select a large enough capacitor, it will no longer fit inside the housing of the bulb.
  2. When the bulb is first powered on, the capacitor must be charged. The full bridge is responsible for carrying the current for this. The capacitor draws an enormous amount of current initially. This could damage the full bridge rectifier by exceeding it's current rating.

If you own your home and can afford it, a much better option would be to have the lighting in your home powered by a separate electrical circuit. This circuit can then include backup power for your lighting. The equipment for this is commonplace in commercial settings due to fire codes. It would not require any modifications to your bulbs. This actually makes your home significantly safer as well.

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if if bulb requires AC to operate it's not simple (easiest to buy an off-the shelf UPS), if it will run of 170V DC it's simple but potentially exciting.

So have a bridge-rectifier feeding a big fat capacitor via a resistor. you could maybe scavenge the bits from an old PC powersupply.

the result will be a lamp socket with DC on it (and both terminals live) so assuming Edison screw base for the lamp potentially exciting.

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