Simple Load Protection Circuit

7/30/2019 Simple Load Protection Circuit

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Contents

Introduction .................................................................................................................................................. 2

Load Protection Circuit ................................................................................................................................. 4

Features .................................................................................................................................................... 4

Construction .............................................................................................................................................. 5

Sensing element ........................................................................................................................................ 5

Isolator ...................................................................................................................................................... 6

Tripping current adjustor .......................................................................................................................... 6

MOSFET driver .......................................................................................................................................... 7

Relay .......................................................................................................................................................... 8

Circuit Components ...................................................................................................................................... 9

Sensor resistor R5 ..................................................................................................................................... 9

Optocoupler U2......................................................................................................................................... 9

Resistor R3 ................................................................................................................................................ 9

Variable RV2 .............................................................................................................................................. 9

Transistor Q2 ........................................................................................................................................... 10

Resistor R2 .............................................................................................................................................. 10

Push button ............................................................................................................................................. 10

Resistor R4 .............................................................................................................................................. 10

555 timer ................................................................................................................................................. 10

Transistor Q1 ........................................................................................................................................... 11

Resistor R1 & Diode D1 ........................................................................................................................... 11

Relay RL2 ................................................................................................................................................. 11

Diode D3.................................................................................................................................................. 11

Observation ................................................................................................................................................. 12

Conclusion ................................................................................................................................................... 12

Reference .................................................................................................................................................... 13


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Introduction

Protection of power electronics devices require a fast acting mechanism to cut off the power

supply instantaneously in case of an overload or over voltage. An overload can be caused by a

short circuit casing fault current to flow through the circuit or by increased load impedance.

Over voltage can occur due to surging of the power supply or generator instability.

For a long time now, fuses have been used as protective devices to protect both the load and

the power supply. Fuses interrupt excessive currents when the current overheats the fuse

element causing to melt. This mechanism has not been very effective as the power electronics

devices still fail in case of overloading even with a rated fuse in series with the power supply.

This is because of the following disadvantages:

The speed at which the fuse blows depends on how much current flows through it. Theoperating time is not fixed but decreases as the current increases.

The fuse element relies on melting for current interruption. This is affected by theambient temperature of the surrounding environment. This will cause a slow response

to current interruption in cold environments.

While there are different fuse ratings, a fuse will still carry a higher current as theelement starts heating before melting. This higher current flow is enough to destroy

sensitive circuit devices.


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Due to these disadvantages, a need arises for a more effective way to protect power electronics

devices. The desired protection system should:

Be able to interrupt fault currents at a pre set limit disregarding the percentage

overload.

Not be affected by the ambient temperature. Fast acting to ensure protection of sensitive circuit devices. The user should be able to adjust the current limits of the protection system. Able to protect the load against both over voltage and over current. Cheap and easy to implement.


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Load Protection Circuit

The aim of this project is to provide a load protection system that is effective than the fuse in

the speed of operation, fault current interruption and the feature of adjusting the desired

tripping current.

Features

Fast operation Over voltage and over current protection Adjustable tripping current Reset button after tripping LED indicating the protection circuits state.


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Construction

Unlike the fuse, this project is an active system that will require its own power supply.

The main components are:

Sensing element Isolator Trip current adjuster MOSFET driver MOSFET Relay

Sensing element

This project utilizes a series resistor as the sensing element. Current flowing through the load

causes a voltage drop across the resistor. The voltage drop is proportional to the current

flowing through the load. This resistor is critical to the accuracy of the protection circuit, its

resistance should remain constant in the load operating current range and should not be

affected by temperature changes. The resistor value should be chosen such that it dissipates

the least amount of power possible without compromising its sensitivity.


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Alternatively, a current transformer maybe used for an AC circuit. The primary of the current

transformer is connected in series with the load. The secondary is connected to a rectifier and

load resistor connected across the DC output. The voltage appearing across the load resistor

will be proportional to the load current.

In this second method, there is reduced power dissipated but is more complicated and more

expensive than the resistor sensor.

Isolator

The isolator isolates the protection circuit from the protected circuit. This is because the

protected circuit maybe of a much higher voltage than the protection circuit voltage.

An optocoupler is used as the isolator in this project.

Tripping current adjustor

To adjust the tripping current, the magnitude of the sensed voltage is varied before feeding it

to the MOSFET driver. This is achieved by using an NPN transistor in common emitter as a signal

amplifier and a linear potentiometer to provide a variable base voltage.

Adjusting the trip current in this project makes it possible to apply this protection system in a

wide variety of circuits.


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MOSFET driver

The MOSFET driver provides the signal for turning on the MOSFET. It is important that the

driver turns on the MOSFET instantaneously when the current exceeds the pre set limit.

The 555 timer in bistable operating mode has been used as the MOSFET driver. In bistable

mode, the 555 timer has two stable states: the HIGH and the LOW states. Taking the reset input

low make the output go low state; taking the trigger input low make the output go into the high

state.

This 555 timer characteristic is the one that has made it possible to use the 555 timer as the

MOSFET driver. The high state turns the MOSFET on while the low state turns the MOSFET off.

555 timer bistable mode operation schematic. (image courtesy of www.555-timer-circuits.com)


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Bistable mode graphs. (image courtesy of www.555-timer-circuits.com)

Relay

The relay is the device that interrupts the fault current when the MOSFET is turned on. The

relay should be rated at a current higher than the circuits fault current to ensure reliable

operation of the relay.

Using a solid state relay instead of an electromechanical relay in this project ensures a much

faster operation than an electromechanical relay. This relay is powered from the same source

that powers the 555 timer IC.


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Circuit Components

Sensor resistor R5

This is resistor is connected in series with the load, the current flowing through the load

presents a voltage drop across this resistor. The voltage drop is directly proportional to the load

current. In case of over current, this voltage will be higher than when there is no over current.

Optocoupler U2

The optocoupler is used to isolate the protection circuit from the protected circuit. This is

because the protected circuit can be of much higher voltage than the protection circuit. It is

important to use the isolator also because the protected circuit can be AC while the protection

circuit is DC. This can be applied in AC motor protection.

Resistor R3

Resistor R3 limits the optocouplers secondary current.

Variable RV2

Variable resistor RV2 is used to adjust the base voltage of NPN transistor Q2. Adjusting this

voltage varies the tripping current of the protection circuit.


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Transistor Q2

This NPN transistor acts as a variable switch to trigger the 555 timer when the base voltage

increases beyond the preset limit.

Resistor R2

Resistor R2 limits transistor Q2 collector current when the transistor turns on.

Push button

Once the 555 timer is triggered, the output remains high even when the trigger pin has gone

high. To bring the output to low again, the reset pin need to go low. This push button brings the

reset pin to low momentarily; this will bring the output to low.

This push button is used after the fault is removed.

Resistor R4

Resistor R4 limits the push button current.

555 timer

The 555 timer acts as a MOSFET driver. It is connected to operate in bistable mode. When it is

triggered by transistor Q2 due to over current, the output goes high switching on the MOSFET.

When the MOSFET is switched on, it activates the relay thus cutting off the supply and

consequently protecting the load and the supply from over current.


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Transistor Q1

Transistor Q1 is an n channel MOSFET. It is used to activate the relay to facilitate the

disconnection of the load from the power supply. The MOSFET needs to have a fast turn on

characteristics for fast operation of the protection circuit.

Resistor R1 & Diode D1

LED D1 lights up when the 555 output goes high. This indicates that the protection circuit has

cut off the load from the power supply. Resistor R1 limits the current flow through LED D1.

Relay RL2

The relay is the actuator in the protection circuit. When the output goes high, the MOSFET

connects the relay between vcc and ground activating it. Activating this relay breaks the

contacts along the load supply line, interrupting the fault current.

An electromechanical relay is shown on the schematic, a solid state relay can be used instead of

the electromechanical relay for a faster operation.

Diode D3

Diode D3 protects the MOSFET against transient emf due to the coil inductance when the

output goes low. This diode is not needed for a solid state relay since it has no inductance.


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Observation

Simulation of the protection circuit using Isis professional produced the expected results.

However, the simulation showed that the protection cuts off the load from the supply when the

simulation is started. Resetting the 555 timer deactivates the relay and power is supplied to the

load.

Conclusion

In conclusion, the protection circuit has been proved to work using simulation. However,

extensive tests need to be done on a working prototype to investigate the circuit parameters

such as the tripping time using non ideal circuit components.

While this circuit is more complicated and more expensive than the fuse, it promises total

protection of power electronic devices against over voltage and over current faults.

Even after this circuit is implemented in equipment protection, the fuse will still be used in the

same circuit as a last resort or a secondary protection device should the protection circuit fail.


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Reference

www.555-timer-circuits.com

en.wikipedia.org
http://www.555-timer-circuits.com/http://www.555-timer-circuits.com/http://www.555-timer-circuits.com/

Documents

Simple Load Protection Circuit