The 21st century belongs to portable devices that run on batteries. From smartphones and laptops to smart home appliances and office appliances, new electronic devices are compact in size, more energy efficient, loaded with multiple features and run on power supplied by batteries. These electronic devices usually have components like diodes, transistors, capacitors or ICs with embedded components that are polarized in nature. Therefore, the electronic circuitry of these devices must essentially receive DC power with a specific polarity.
Any battery has two terminals – anode and cathode and current always flows from anode to cathode. In fact, electrons flow from the cathode to the anode. But to keep the definition of current independent of charge carriers, the conventional current direction is always taken from the anode or positive terminal to the cathode or negative terminal.
Many devices, due to the power supply requirement of a specific polarity, have a mechanical mounting or battery design such that the battery can only be connected in a specific polarity. But this is not the case for all devices. There are many devices that run on general-purpose batteries, and the mechanical assembly of the electronic device only has indicators or instructions inscribed on it to secure the battery in a specific way. Still, the battery can be connected to the circuit anyway due to human error.
If the battery is connected with reverse polarity to a device, this could cause serious damage to the battery and also to the electronic device itself. This is not uncommon. Due to reverse connection, the polarized components start getting shocked due to the reverse voltage between them and the device may get permanently damaged. Reverse polarity can also affect the battery and reverse connection may explode the battery or it may be possible that after connecting to a circuit with reverse polarity the battery can no longer hold a charge.
To save the life of the battery as well as electronic devices, it is generally advisable to use a reverse battery protection circuit after the battery or before the internal circuit of any electronic device. A reverse battery protection circuit may also be incorporated into the power input circuit of a device. The reverse battery protection circuit also saves the electronic circuit from any battery backcurrent.
A reverse battery protection circuit can be constructed using a diode, MOSFET or BJT. In this tutorial, the reverse battery protection circuit for each of these components will be designed and tested for energy efficiency with different loads. Instead of considering real circuits as load, different resistances are considered as load in the experiment. The voltage drop in the protection circuit and the current consumed in the load are measured to test the energy efficiency of the protection circuits.
The protection circuit also consumes battery power, which results in wasted energy. Therefore, the protection circuit must consume the minimum energy so that maximum power is emitted into the load. The power supplied to a load is proportional to the voltage available in the load circuit. This is the remaining voltage after the voltage drop in the protection circuit, so the voltage drop in the protection circuit will be measured. The voltage drop across the protection circuit must be minimal. Secondly, the current through the charging circuit will be measured, which will indicate the actual power available to the charging circuit. The greater the current consumed by the charging circuit, the greater the energy consumed by it.
Required components
Fig. 1: List of components required for battery reverse protection
These are the following methods to design the battery protection circuit –
1. DIODE –
The simplest way to design a battery protection circuit is using a diode. A diode conducts current in only one direction and is open circuit for reverse polarity. Therefore, if a diode is connected in series between the battery and the charging circuit, it will only allow current to be conducted for one polarity. The diode will be forward biased and will allow current to flow in the charging circuit only when the battery anode is connected to the diode anode. If the cathode of the battery is connected to the anode of the diode, the diode will become reverse biased and stop conducting current in the charging circuit. This will save the charge or any device connected to the battery. Therefore, the diode must be connected in such a way that the cathode of the diode is connected to the charging circuit and the battery connector is connected to the anode of the diode. The 1N4007 diode can be used for battery reverse protection. The 1N4007 diode has a voltage drop of around 0.7 V and a maximum direct current of 1A.
Fig. 2: IN4007 based battery reverse protection circuit diagram
During the experiment a 3.7 V lithium-ion battery is used which can provide 3.3 V supply voltage. A 1N4007 diode is connected in series to the battery so that the anode of the battery is connected to the anode of the diode . Different load resistances are connected to the battery and diode circuit through switches and the circuit connections are completed by connecting the common ground to the battery cathode.
Fig. 3: Diode-based reverse polarity protection prototype
Therefore, input voltage, Vin = 3.3 V, on measuring the voltage drop across the diode and the current in the load resistors individually, the following results are found –
Fig. 4: Table listing voltage drop across the 1N4007 diode and load current for different loads
From the results above, it can be analyzed that the diode suffers more voltage drop as the current demand in the output load increases. To reduce the voltage drop, a Schottky diode can be used which has lower forward voltage drop compared to the 1N4007 diode.
Fig. 5: 1N5819-based battery reverse protection circuit diagram
If 1N4007 diode is replaced with 1N5819 Schottky diode in the circuit, the following results will be obtained –
Input voltage, Vin = 3.3V
Fig. 6: Table listing voltage drop across the 1N5819 diode and load current for different loads
From the above result, it can be analyzed that the 1N5819 diode will experience more voltage drop as the current demand increases in the output load. But the forward voltage drop of Schottky diode is less compared to 1N4007 diode.
Disadvantages of using diode circuit
• A diode has a voltage drop, so the overall power consumption increases. It can be said that part of the power is wasted by the diode.
• The use of a diode limits the maximum output current that can be consumed by the load. For example, 1N4007 and 1N5819 allow a maximum forward current of just 1A.
Solution
• Schottky diodes with lower forward voltage drop can also be used in place of normal diodes. The diode can be selected according to the maximum current required by the load. Instead of diode, transistor can be used as transistor as it can also be used for switching applications and has less voltage drop and can also withstand high loads.
2. Using N-channel MOSFET – BS170
The third way to design a protection circuit is by using N-channel MOSFET. The NMOS conducts current when there is a positive voltage at its Gate terminal. Otherwise, the NMOS remains in an open circuit condition. In the MOSFET there is an intrinsic body diode that conducts when forward biased. Therefore, NMOS can be used as a switching transistor to make a reverse battery protection circuit. NMOS generally have less ON resistance (rDS). Due to this, it presents a lower voltage drop in the state of full conduction. N-MOSFET can also handle high loads compared to diode or BJT.
Note : Schematics can be found under the “Circuit Diagram” tab.
Therefore, when the battery is connected correctly, the MOSFET turns on. When inverting the battery, the gate terminal is low, which turns off the MOSFET and the load is disconnected from the battery.
Fig. 7: Prototype reverse polarity protection circuit using N MOSFET on breadboard
During the experiment, a 3.7 V lithium-ion battery is used that can provide 3.3 V supply voltage. An NMOS BS170 is used for reverse protection of the battery. The load resistors are connected through switches between the Gate terminal and the Drain terminal of the NMOS. The battery is connected to the Gate terminal and Source terminal of the NMOS. The NMOS conducts only when the battery anode is connected to the base of the NMOS. If the battery cathode is connected to the base of the NMOS, the NMOS is turned off, cutting off the supply voltage to the load.
Therefore, input voltage, Vin = 3.3 V, on measuring the voltage drop across the transistor and the current in the load resistors individually, the following results are found –
Fig. 8: Table listing Vds and Load Current for different loads
From the results above, it can be seen that the BS170 suffers more voltage drop as the current demand increases at the output. But the voltage drop in NMOS is very less compared to diode.
Disadvantage of using nMOSFET
• MOSFET requires gate voltage above a threshold level to turn on. This means that they will only work with batteries that can supply voltage above the limit. For example, the BS170 requires a minimum of 0.8V at the Gate to turn on.
Solution
MOSFETs with lower threshold gate voltage can be used for low capacity batteries.
3. Using NPN BJT (Bipolar Junction Transistor) – BC547
Another way to design a reverse polarity protection circuit is by using BJT transistors. A BJT can be used as a switching transistor in the circuit for reverse battery protection. The NPN BJT has higher Beta (current gain), so it can operate with low base current. This reduces energy loss. Additionally, they have less voltage drop.
Note :
The schematics can be found in the “Circuit Diagram 2” tab.
During the experiment, BC547 is used for battery reverse protection. The transistor is connected in the circuit in such a way that the charging circuit is connected between the base and collector of the transistor and the battery is attached to the base and emitter of the transistor. A pull-up resistor is used at the base of the transistor so that the base can be biased properly. When the battery is connected in such a way that the anode of the battery is connected to the base of the transistor, the forward voltage at the base switches the transistor to the ON condition and current starts flowing from the collector to the emitter.
This completes the circuit and the load receives incoming power. When the cathode of the battery is connected to the base of the transistor, the base of the transistor is not polarized and the transistor switches to the OFF condition. No current flow remains between the collector and emitter of the transistor and the load circuit is open. This will save the load/device from reverse current.
Fig. 9: Prototype of reverse polarity protection circuit using BJT on breadboard
During the experiment, a 3.7 V lithium-ion battery is used that can provide a supply voltage of 3.3 V. A BC547 transistor is connected in such a way that the load resistances are connected between the base and collector of the transistor and the battery connectors are connected between the base and emitter of the transistor.
Therefore, input voltage, Vin = 3.3 V, on measuring the voltage drop across the transistor and the current in the load resistors individually, the following results are found –
Fig. 10: Table listing Vce and load current for different loads
From the results above, it can be seen that the BC547 suffers more voltage drops as the current demand increases at the output. But the voltage drop in BJT is very less compared to diode and MOSFET. Therefore, BJT works better than MOSFET and diode as a reverse battery protection circuit.
Disadvantages of using BC547
• The circuit must be designed to maintain a base current such that it can drive a high load with minimum power loss. This is due to the fact that the collector current depends on the base current.
• The BC547 allows a maximum current of 100mA through the collector. This limits the maximum current that can be drawn by the load.
Solution
• In some cases, BJT like 2N2222A can be used to solve the current limit problem. The 2N2222A allows maximum current of 1A.
• MOSFET can be used in place of BJT as MOSFET has lower resistance compared to BJT and can withstand high loads. But with the use of MOSFET, it is necessary to compromise on power loss as MOSFET has higher power loss than BJT.
Conclusion -
When comparing the use of diode, BJT and MOSFET as reverse battery protection circuit, the derived results are summarized in the following table –
Fig. 11: Table listing battery reverse protection characteristics using diode, NPN BJT and N-MOSFET
Thus, it can be concluded that when using diode, NMOS and BJT for reverse battery protection, the use of BJT is the most energy efficient, but has current limitation. Alternatively, NMOS can be used, but it presents a threshold voltage problem. Therefore, for load circuits with low current demand, using BJT is better. If the load circuit has high current demand and operates at high power, the use of NMOS is recommended. For low-cost circuits in which voltage drop or current demand is not an issue, a diode can be used.
Circuit diagrams
| Circuit Diagram-NPN-BJT-Based-Reverse-Battery-Protection |
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