Tap 5V power from single Live wire Light switch  - Page 1
Forum section: electronics / projects

author: stan001
author_rank: Contributor
timestamp: April 02, 2013, 01:34:39 am
content: Hi,Not sure what is the correct term for this, how do I tap 5/12V D/C from a single A/C live wire light switch into a softtouch switch with electronics inside without turning on the light itself ?Pls refer to the 2nd diagram from the URL below...http://www.malaysiaswitches.com/technical-info/Thanks

author: ejeffrey
author_rank: Super Contributor
timestamp: April 02, 2013, 04:48:21 am
content: It isn't possible without letting some current flow through the lamp even when off. A typical approach would be to make a low power DC-DC converter to drop the 120 VAC down to 12 VDC. The current draw is small (a few mA) so the voltage drop across the lamp is low enough to be negligible. You would then use a rechargeable battery or supercap to power the circuit while the lamp is 'on'. If you need to be able to operate indefinitely with the lamp on, you would need something more complicated.Edit: I should point out: incandescent lamps will definitely work fine when used in a circuit like this. Their resistance when off is a few 10s of ohms (it goes up substantially when hot). More modern lighting like LEDs or CFLs have internal power supplies that may not work well if you try this.

author: C
author_rank: Super Contributor
timestamp: April 02, 2013, 09:39:08 am
content: The simple answer Use a battery.if you want to recharge the battery from the line, You have two options, while the switch is off, while the switch is on.while it's off:ejeffrey's answer, and as stated partially turns on load what the switch is connected. For a 100 watt light bulb the light might see a few volts as stated. As that 100 watt light bulb gets smaller towards 1 watt, the few volts increase. A lot of things can be damaged trying to work with low voltage.While it on;just as bad, Instead of full voltage to the load you try to steal a little. A normal switch might supply 115 volts to the load, if you steal 12 volts then the load sees 103 volts. For most loads this is no problem, but for some bad. The while on, adds and additional problem, stealing that power causes the soft switch to heat. This may be at an ok level for the 100 watt light, but could cause a fire with a toaster (1200 watts).I would say most do the while offSo Both could damage the load connected to the switch, one might cause a fire.If you buy/build something you need to play by it;s rules. And. Someone will probably forget the rules.Change the battery or use solar charger to charge battery.C

author: amyk
author_rank: Super Contributor
timestamp: April 02, 2013, 10:38:39 am
content: CFLs will tend to flash at periodic intervals and have a greatly reduced life as a result. Basically the leakage current will charge the input cap up to the point where it fires the tube, then charges again and it repeats.

author: NiHaoMike
author_rank: Super Contributor
timestamp: April 03, 2013, 04:23:17 am
content: You can also tap power between hot and ground, but you'll need to use Y rated caps and the amount of current is very small.

author: stan001
author_rank: Contributor
timestamp: April 03, 2013, 07:57:51 am
content: Thanks for all the replies...I was just wondering how the product in the listed URL does it and gotten local safety approval too... but it does mentioned that it does not work with CFL...