12 VDC battery supply with automatic 12 VDC adapter override

Question

I am designing a 12 V circuit to provide 12 V DC (500 mA). I would like to have 12 V battery but also be able to plug in 12 V adapter to automatically override the battery without a switch and without removing the battery.

I saw a 12 VDC with 9 V battery circuit on this site with two diodes but I’m afraid this may not work for me as the voltage will be the same for both diodes so it won’t override? Thanks for your help.

Answer 1

The easiest way is to use a jack with a built-in switch in addition to the diodes, since you cannot predict which will supply current when they are similar voltages.

From the CUI datasheet for their PJ-202A:

The switch would be in the minus side if the adapter, as is customary these days, has the center pin positive.

Answer 2

^{simulate this circuit – Schematic created using CircuitLab}

Figure 1. Using a relay to disconnect battery when mains present.

• BAT1 supplies the load when the mains is off.
• D1 prevents backfeed from the battery to the relay coil (which would disconnect the battery).
• When the mains supply, V1, turns on RLY is energised, BAT1 is disconnected and the load is powered via D1.

Be aware that when the power fails that RLY1 may not switch off until V1 falls to 4 or 5 V. This means that the load will be pretty much unpowered until the relay drops out. There are two solutions:

• Add some big electrolytics in parallel with the load to maintain voltage during the transition.
• Use a 5 or 6 V relay fed via a Zener diode (D3). The relay should drop out at about 3 V which, with the Zener diode, will cause it to drop out when V2 falls to about 9 V.