Portable Desk Fan

I have a project that can be run off of one wall socket (via a 150mA, 12V DC wall transformer), or integrated into the batteries. I know there are many products out there where you can just plug into the wall outlet, even when the batteries are in, and neither the wall adapter or batteries in short-circuit them. How do I connect the power supply circuit so that it works as a. .. ie, commercial portable desk fan (you know, the ones that can be run out of "D" batteries or a wall transformer). I need to make a complex with transistors and relays, or is it just a matter of using LEDsa Live for ever who can relate to that!

If you do not use rechargeable batteries, it is quite simple, 2 diodes will. One of the positive output of the adapter to a point (new) common output, the chain battery positive to the point of common output. Attach the negative connections together. The exit point of new municipalities is where you attach the load.

If you want to charge the batteries, you need a more complicated circuit.

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If you wire power diodes (rectifiers) in series with the battery, it will act as a switch to stop the current return. A diode must submit a diode voltage across it (0.6 V). If your power transformer wall has more voltage than the battery voltage + drop diode, it will back bias the diode reverse current flow.

You should only need a diode for what you do because you do not want to stop the battery drain current when the transformer is plugged in.