Simple AC off switch

[Wastedfun]

Might be easy question, but im working on something light an emergency light.

When its plugged into the wall, i want it to turn off the DC current, but still charge the battery pack. When it is unplugged or the power goes out, the LED will get the dc current from the battery pack.

I have looked at transistors and relays and can fig out out to allow dc current with ac power using a relay, but i cant figure out how to make the relay switch go on When no ac power is avail.

I have the batt pack and charger partt figured out, i just need a little help with the switch

Thought?

Thx
WF

 

[crutschow]

Use a double-throw relay with normally-open and normally-closed contacts. You just connect the circuit through the normally-closed contacts (which make contact when the relay is de-energized).

 

[MikeMl]

The solution Crutschow proposes ac holds the relay in, so it goes off when the ac fails, which is wasteful of power because it takes several watts to hold a relay in. It is on for months to years before the ac fails.

A more efficient method is to use a discarded, but small, DC output wall-wart (plug-in charger) for an old cell phone. Use the dc output to hold off a transistor while charging the battery. When the ac power fails, the lack of dc at the output will allow the transistor to turn on, turning on the load.

Using a high-side transistor (either a PNP or PFET) makes this simple.

How much current does it take to light your LED?

What voltage does it take to properly float charge your battery?

 

[MrAl]

Hi,


Here is a quick drawing of two versions. One uses one bipolar transistor and one NMOS for use with higher current LEDs, and the other uses two bipolar transistors for lower power LEDs.

 

[Wastedfun]

Thank you for the replies!!!

I am using and old camera charger and battery for the power pack. The cam charger has a ac plug in and transformer providing 12v to the charging base so i can tap into that line for the dc switch.

Then, the positive feed from the batter is what i want cut off while ac is on, and when ac fails a small series of maybe 6 low power LEDs light up.

It gets better lol, im trying to tie the led strip to also use the light switch controlling the light bulb. So when ac is on, light works as normal. When ac is off, the light switch controls flow of the dc power and turns a dmall led lamp instead. Make sence?

WF

 

[Wastedfun]

MrAl,
Im confused on the schematic you provided (thank you very much again for that), i am a novice at this but i bought some stuff to try and recreate the top schematic.

My confusion is that the starting point where the transformer converts to dc going in is going to ground out when i hook it up. The positive connection goes in, and connects to two different points and uses diodes to direct current upwards. Then, the ground is connected at the bottom but goes right to the same connection through another diode. Does this not short it out? I ask out of pure ignorance because my limited knowledge on dc tells me connecting the pos to the ground without something in between is just going to short it out. What am i missing here?

Thanks
WF

 

[MrAl]

Hi,

Try this then...

 

[Wastedfun]

Okay that looks like it wont short. Is my lack of understanding basic electronic schematics at fault here? If so sorry but thank you again!

 

[MrAl]

Hi,

No i think you did well.

What happened was you had a pre-existing circuit that you wanted to modify, and that circuit already had one transformer line going to ground and i guess you did not want to modify that, so we had to change the detector circuit a little to allow its use with a transformer that already had one line going to ground. That's all it was.
You may want to post the entire circuit so we can get a better idea what you are doing and advise a little better on how to proceed.

 

[Wastedfun]

I actually used the second one Al gave me and it works perfectly. Now i just need to figure out how to get it to use a light switch when on back up mode as i posted in other topic