Help please about the very simple power failure light project

[mamand]

Hi,

I am a student from Phillippines  and
i have seen the project about the simple power failure light.
One of the items is the capacitor right? Ahmm i'm just confuse about the specs of the capacitor.
In your link http://www.electronics-lab.com/projects/motor_light/024/index.html they just put there C1 = 470uF
I'm confuse what would be the voltage of the capacitor. Please help me.. It is useful to our school..
Rep ASAP..

Thanxxx..

 

[audioguru2]

The 470uF capacitor has a 1N4742A zener diode across it. I looked up the datasheet for the zener diode on www.datasheetarchive.com and found out that it limits the voltage to 12V.
I would use a 25V capacitor for it to last a long time.

The 1.2k resistor does not have a power rating. It will have an average voltage of 204V across it and its current will be (204V/1200 ohms =) 170mA which will destroy the little zener diode. The resistor will heat with 35W.

So DO NOT power the circuit from 240VAC.

 

[Hero999]

This is a totally silly idea.

Why hash a dangerous transformerless power supply together with a 12V relay when it would be cheaper, safer and more reliable to use a relay with a 240VAC coil in the first place?

9V is too lower voltage to power three white LEDs connected in series properly.

It isn't hard to design a circuit to trickle charge rechargeable the batteries when the mains power is on. The propper way of doing this is to use a transformer to charge the batteries via a constant current source (Tr1, R1, R2, D1 & D2) and use a transistor to switch on the LED when the mains power fails (Tr2, R3 & R4). A toggle switch can be added to disconnect the batteries when the device is not in use.

View attachment 41405

 

[audioguru2]

Why are garbage and dangerous projects like this one in our projects section??

 

[mamand]

Ahmm. Thanx 4 your idea. I didnt know that it is a dangerous project.
Is your circuit tested? If it is, I would try to build your
circuit and use it rather than the other one.

It would be better if you indicate the specs of the parts of your circuit.

Thanx 4 your help, If there would be a better project that you will suggest, cheap and easy to build
I would also accept it..

Thanks very much...
ASAP Repz...

 

[Hero999]

My design has not been tested but it consists of sub-circuits which have been tried and tested.

The mains transformer is just a standard 9V >100mA or >1.2VA unit. Just make sure it's reasonable quality and has short circuit protection or is fitted with a thermal fuse. If you're worried about the hazards associated with mains, simply replace the transformer and BR1 with a 9VDC wall brick mains adaptor, rated to above 100mA.

Tr1 is generic NPN transistor, BC338 will do, Tr2 is a general purpose PNP transistor, B328 will do.

The resistors can all be 0.25W 5% carbon film but higher-specification 0.5W 1% metal film is fine though overkill.

It's possible to simplify the circuit by replacing the constant current source circuit with a resistor but the battery won't charge as well.

The circuit can also be scaled up by using a 12V transformer, a power transistor constant current source to charge the batteries, six AA cells in series and a switched mode constant current source to power a high power 300mA LED on a heat sink.