Short circuit

Overcurrent or Short Circuit Protection

In a short circuit condition, a large current flows which are never intended. In this case we might damage our power source, our electronics devices and every associated component. It also risk of fire due to excessive heat. To avoid the short circuit, we need to design a circuit and connect it in between our electronics device and power source. The function of circuit should be such that it interrupt the circuit in short circuit condition. Let’s see the functional block diagram of short circuit protection device.

Functional block diagram of short circuit protection

As you can see that a overcurrent protection device is added in between power source and electronics device. In that device there are two blocks. First is latch able relay and other is overcurrent detector. When short circuit or overcurrent occurs then overcurrent detector detects it and turn latch able relay off.

 Here we used latch able relay because as overcurrent occurs and detector turn off the relay there will not any flow of current and relay will turn on again.

We will use a relay in which two contacts are given. One of them should be normally closed and other is normally closed. By using some additional components a show, we will make this relay latch able.

Latch able relay interrupt circuit

As you can see that the coil of relay is connected in series with a bulb and a transistor which acts like a switch. Flyback diode is connected to prevent over voltage at the collector.

At the beginning when we power the circuit and assuming that there is not any overcurrent condition. So, overcurrent detector pulse will be low that means transistor (Q2) will off and transistor (Q1) will on.

Relay is turned ‘ON’, normally closed contact is open and normally open contact is closed. So, device is powered up. As short circuit or overcurrent occurs, overcurrent detector pulse is high. This high pulse turns transistor Q2 ‘ON’. So, current will flow through the resistor R1 and transistor Q2 and transistor will get a low voltage and it is turns ‘OFF’. Relays is turned ‘off’, contacts are back in their initial position.

Then device is turned off and there is no longer overcurrent exists. So, overcurrent detector pulse goes low again. But transistor Q2 is still pulled up to high voltage and is still ‘ON’. So, the relay is still ‘off’.

To turn the circuit ‘on’ again we have to push the normally closed switch and break the circuit and then release the switch. This way we have created the latch able relay.

Overcurrent detector circuit

As we all know that when a current flows through any resistor, it create a voltage drop across that resistor. So, we will use a resistor in series with the load to calculate the amount of current flowing through the load.

But if we use a resistor and current flows through it, it will create power losses. As a solution we can use a power resistor with very low resistance. It will significantly decrease the power loss.

But as the resistance is very low, voltage drop across resistor will also very low. So, we have to amplify that voltage using an op-amp. This amplified voltage then compared with a reference voltage using a comparator.

Amplified voltage is connected with non-inverting input of the comparator and reference voltage is connected with inverting input of comparator. By changing the value of reference voltage we can change the over current limit.

When load changes current draw, the voltage drop across resistor also changes. If this voltage is more than the set reference voltage that’s means load is drawing more than intended current.

Since the load voltage is connected with non-inverting input of comparator and now it is high. So, output of comparator is now high. This way we have created the overcurrent detector circuit.

So, now let’s see the overcurrent detector circuit.

Now let’s combine the both circuit to create the complete overcurrent protection circuit.

Short circuit or overcurrent protection circuit

List of component we need –

  1. One LM358 op-amp IC
  2. Two BC547 NPN transistor
  3. One green LED
  4. One 2-pin connector
  5. 1n4007 diode
  6. 1 two contact relay
  7. Potentiometer (10kΩ)
  8. Resistor (220Ω, 1kΩ, 10kΩ, 20kΩ)
  9. Power resistor R4 (0.1Ω, 5watt)
  10. Normally closed  (NC) switch

As you can see that we have successfully created the circuit according to our block diagram. To this circuit work we have to first open the normally closed (NC) switch. We have our latch able relay which interrupts the circuit according to overcurrent detector pulse. And we have our overcurrent detector circuit which gives signal to latch able relay according to current flowing through load.

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