Automatic battery switch, also suitable for switching DC power supplies, allows you to choose between two batteries, the one to be connected to the load according to the voltage measured at their ends. It consists of two very low RdsON power MOSFETs (CSD17501Q5A) from Texas Instruments and two LM5050 integrated circuits. The MOSFETs are able to withstand a Vds of 30V and an Id of 100A; the resistance between drain and source in ON conditions is only 2.4 Mohm. The control logic for switching, which can alternately connect one generator or the other with the only load, is entrusted to the LM5050 integrated circuits that measure the voltage at the source and drain terminals of the relative MOSFET and polarize the gate if the potential of the source exceeds that of the drain of more than 0.1 V. In this way the battery with higher voltage will be connected to the load.

The circuit is also suitable to parallel two power supplies for redundancy (as is done on servers and other equipment whose operation and power supply must be guaranteed as much as possible). Nothing forbids using it to power a user with a battery connected to the terminals CONN1 and CONN2 and a power supply on CONN3 and CONN4: in this way when the power supply will be switched on it will supply the energy based on network availability, otherwise, it will provide the battery. Warning! however, you will need to provide separately to recharge the battery.

For use, remember that the circuit can manage power sources whose supply voltage must not exceed 26V (continuous) and switch currents in the order of thirty to forty amps. For reasons of simplicity, in the circuit no protections of any kind have been foreseen, neither from overcurrent nor from the polarity reversal, so be careful and review and recheck the connections before putting the circuit under tension.

• Input voltages: 3.6 ÷ 26 Vcc
• Switchable current: 40 A
• Switching threshold: <0.1 V
•  Dimensions (mm): 36x47 printed circuit (except for the protrusions of the male faston connectors)