I'm no electrical engineer, but I do know my way around electronics and other electrical projects fairly well.

Just depends on what your asking.

OK, bear with me here, i am in no way very electrically adept but I'm always willing to try something once.

I need help in designing a switch that when a device ceases to receive electrical current from a standard 110 wall plug (AC yes?) that device then switches to running off of an internal battery source (DC right?). I know these exist obviously, laptops for example, but if this switch type already has a name I would love to know what it is.

Also, this is for a small scale build. Say utilizing a board about 1" square.

Any questions to clarify what I'm aiming for please ask! Though I may only divulge only so much info as to elude to what I am designing.


Funny that I ask for an electrical engineer when really most anyone with any electrical experience could answer my question and any others that follow.  

rfw2003 wrote on 07/11/12 at 22:42:44:


So this switch could be engineered to the scale of say...the size of a 1" cubed space? How so? maybe a little bigger at a 2" cubed space?

I guess my biggest question would be what components are involved?

The easiest way to do this to make sure the power supply is truly uninterrupted is to get a power-supply that's the same voltage as what you want and then get a battery that's the same voltage. When you unplug the power-supply, the battery just keeps powering everything.

Laptops don't "switch" from AC power to DC power, they're always DC power. Laptops just will dim the screen and slow the speed of the processors and even the hard-drives when they're unplugged to conserve battery power. They're always running off the same circuit the battery is on.

Uninterruptible power supplies don't switch from AC to DC either. They have power inverters that run off DC and the AC line supplies a power supply that powers that inverter and charges the batteries. When the AC gets cut, nothing happens except that the batteries are no longer getting charged.

It's pretty simple really, it's no different than the charging system in a car except instead of an engine spinning an alternator, it's a 120v AC line powering a DC power supply. Turn off the engine, the lights don't turn off. No switches flick to change from alternator to battery power. It's just up to the battery to keep the lights on, the alternator can't do it anymore.

Is the 1 inch gadget operating from AC or DC; i.e., is there an onboard power supply?   If it is a DC gadget then it's pretty easy to use one or more diodes to isolate the battery and power supply DC voltage.  The diodes prevent the DC supply from trying to back charge the battery (assuming non rechargeable), and/or the battery from trying to dump current into the DC supply when it goes nonop.   If the battery is rechargeable, then there are other ways to wire the diodes so that charge the battery where there is input power.   There's no need to engineer anything as this is well understood technology and there are lots of examples of this on the web.   For instance, automatic emergency lighting that lights up when power is removed have the circuit you might be looking for.  You can build or buy these units that plug into individual AC sockets.

BTW, I'm an MSEE.

ZAR wrote on 07/12/12 at 10:40:16:


Funny you should ask, because I might be a little bit.   I used to work for a company that manufactured "fill height detectors" that we installed in breweries throughout the world.   These machines were used on can lines to detect under filled sealed cans of beer.   They used an Americium 241 alpha emitter source and scintillation detectors (Geiger counters).   There were many occasions where I had to stick my hand in the machine while it was running to clear a toppled can of beer and probably got a dose of alpha radiation (we wore dosimeters, tho).

Well, you're mostly right.   Usually it depends on how much power the device requires and/or how long it is expected to operate without power (outage)?   For instance, emergency lighting may require substantial power, but may only need to run for 20 minutes until everyone has exited.  If your device is a tiny nano power type gadget that runs for years on a 2032 coin type power cell, then there's no need to rig a charging system.   On the other hand, if it's a lighting, noise making, or motorized device then it probably requires larger AAA or AA cell power, and rechargeables might make sense.

Most engineering projects go thru 4 distinct phases...

1) a clear problem definition (don't underestimate this; it sets your starting point for the rest of your solution)

2) a proposed solution with goals it will solve & achieve (i.e., a gadget)

3) a REQUIREMENTS phase where you decide the physical dimensions, power requirements, color, noise level, etc.

4) the DESIGN phase where you build one or more prototypes that meet or exceed the REQUIREMENTS you have defined.  These units are used for testing.  If they fail, then you may to restart step 4 or even go back to step 3 and rethink the project from a different approach.   Note, this is called empirical scientific method.

Keep in mind if you're applying AC directly to this gadget (rather than using a DC wall PS) it could be LETHAL!  Most power supplies use a small transformer which steps down the voltage, but also electrically isolates the circuit from the AC line.   You can apply direct AC (rectified, of course) to a circuit, but it can be tricky and DANGEROUS if you don't have a good understanding of AC/DC principles.