Reverse battery protection with low voltage drop

It is often a good idea to incorporate a diode in series with the power leads to a circuit operating off an external battery. The diode will prevent damage in case of a reversed battery connection but it does have a drawback in that it will produce a voltage drop of typically around 0.7V in the case of an ordinary silicon diode or 0.4V in the case of a Schottky diode.

This voltage drop is generally undesirable because it reduces both the voltage and power available to the circuit. The voltage drop can be reduced significantly by using an enhancement mode Mosfet transistor in place of a diode as shown in the accompanying diagrams. This idea is based on the same principle as that of a "synchronous rectifier" as used in some switching inverters to improve efficiency. Initial operation depends on an internal diode in the Mosfet.

These diagrams show how to use p-channel and n-channel Mosfets to provide reverse battery protection.

Assuming that the battery connection is correct, the diode initially conducts, establishing a forward bias to the gate of the Mosfet. The Mosfet then turns on and transistor action takes over. Current now flows through the Mosfet itself rather than through the diode because of a smaller voltage drop across the Mosfet. In the case of a reversed battery connection, the diode is reverse biased and so is the gate of the Mosfet, so the Mosfet is thereby turned off, protecting the circuit from damage.

The top circuit shows a version using a p-channel Mosfet in the positive power lead to a load. The bottom circuit is a version using an n-channel Mosfet in the negative power lead.

Using this circuit I measured about 0.1V drop across the Mosfet at 2A load current and a 12V input. This compares with about 0.7V under the same conditions using an ordinary diode. This represents an 85% reduction in power dissipation in the Mosfet compared with that in the diode and a 10% increase in power available to a resistive load.

Note that this circuit will not work in battery charging circuits.

Backup battery for AC-powered cordless phones While the article on a backup battery for cordless phones in the October 1999 issue tried to cover all possibilities, inevitably it didn’t. Some cordless phones are powered by AC plugpacks such as 13.5V AC and 10VAC. If you have a 13.5VAC model there is no easy solution but if your cordless phone is powered from a 10VAC plugpack, the solution is shown in the accompanying circuit. The cordless phone itself will have an internal bridge rectifier so that it can be powered with AC or DC. Therefore we can use a modification of the October 1999 circuit whereby the 10VAC plugpack is connected to a bridge rectifier to keep the 12V SLA battery on trickle charge via the 100W resistor. The phone itself is then permanently powered from the unfiltered DC from the bridge rectifier (D1-D4) or from the battery during blackouts via diode D5. SILICON CHIP.

Power-on muting for PC FM tuner

This circuit shows the modifications required to provide a power-on reset.

Here's how to modify the PC board.

On some computers, the PC FM Tuner card (published June 1998) may produce noise while the machine is booting. The solution to this is to provide a power-on reset to the 74LS273 octal D flipflop. The 74LS273 will then mute the 4053 multiplexer and no signal will be fed to the LM386 power amplifier.

The power-on reset is provided by the 1kW resistor feeding pin 1 of IC1 from the +5V line and this is bypassed by the 100mF capacitor. When power is first applied, the 100mF capacitor will hold pin 1 low to reset the chip. The capacitor will then charge via the 1kW resistor and normal operation will then take place.

The modification entails braking tracks on both sides of the PC board as shown on the accompanying diagram and then connecting the 1kW resistor and 100mF capacitor as shown. The +5V connection to IC1 must then be made via a link from pin 14 of IC7.

Note that if your PC FM tuner card does not experience the noise problem while booting, there is no need to do this modification.