A Relay driver IC is an electro-magnetic switch that will be used whenever we want to use a low voltage circuit to switch a light bulb ON and OFF which is connected to 220V mains supply. The required current to run the relay coil is more than can be supplied by various integrated circuits like Op-Amp, etc.Relays have unique properties and are replaced with solid state switches that are strong than solid-state devices. High current capacities, capability to stand ESD and drive circuit isolation are the unique properties of Relays. There are various ways to drive relays.Some of the Relay Driver ICs are as below.
- High side toggle switch driver
- Low side toggle switch driver
- Bipolar NPN transistor driver
- N-Channel MOSFET driver and
- Darlington transistor driver
- ULN2003 driver
Relay Driver IC Circuit
Relays are components that permit a low-power circuit to control signals or to switch high current ON and OFF which should be electrically isolated from controlling circuit.
The Required Components
- Zener Diode
- 6-9V Relay
- 9V Battery or DC Power Supply
- 2N2222 Transistor
- 1K Ohm Resistor
- Second Input Voltage Source
In order to drive the relay, we use transistor and only less power can be possibly used to get the relay driven. Since, transistor is an amplifier so the base lead receives sufficient current to make more current flow from Emitter of Transistor to Collector. If the base once gets power that is sufficient, then the transistor conduct from Emitter to Collector and power the relay.
The Transistor’s emitter-to-collector channel will be opened even though no input current or voltage is applied to Base lead of Transistor. Therefore, blocking current flows through relay coil.
The emitter-to-collector channel will be opened and allows current to flow through relay’s coil if enough current or voltage is applied as input to the base lead. AC or DC Current can be used to power the relay and circuit.Relays are electromagnetic devices which allow low-power circuit to switch a high current ON and OFF switching devices with the help of an armature that is moved by an electromagnet.
Driver Circuit is used to boost or amplify signals from micro-controllers to control power switches in semi-conductor devices. Driver circuits take functions that include isolating the control circuit and the power circuit, detecting malfunctions, storing and reporting failures to the control system, serving as a precaution against failure, analyzing sensor signals and creating auxiliary voltages.
A typical digital logic output pin supplies only tens of MA of current. External devices such as high-power LEDs, motors, speakers, light bulbs, buzzers, solenoids and relays can require hundreds of MA and they need same voltages. In order to control small devices which use DC, a transistor-based driver circuit is used to amplify current to the required levels. If the voltage and current levels are in perfect range, the transistor acts like a high-current switch controlled by the lower current digital logic signal. A discrete BJT is used at times in place of MOSFET transistor especially on older or low voltage circuits as shown below.
Basic Driver Circuit using a BJT Transistor
PNP, NPN, or MOS transistors are also be used. Transistor provides current gain. The resistor used on the base of the transistor is 1K ohm. On inductive loads (i.e., motors, solenoids, relays), a diode is often connected backwards across the load to suppress the voltage spikes (back EMF) generated when turning devices OFF.
Inductor V = L* di/dt
A negative voltage spike is produced when turning the device OFF. A diode is also connected across the transistor instead of the load sometimes in order to protect the transistor. The 2N3904 shown below is a small discrete BJT transistor is used for a driver circuit that required less than 200MA. In this circuit with BJTs, Vcc – higher voltage supply than the logic power supply and 6 or 12V DC is required for motors or relays.
The load is directly connected to battery power and cannot passed through the voltage regulator in battery operated devices. Many devices such as motors have more inflow current spike when they are first turned ON. Be cautious on maximum current ratings.
Advantages of Low Side Driver
More interface options are available which includes popular ULN2003 driver.
- Easy to interface to low voltage logic circuitry.
- Fewer components are used.
- Less expensive NPN drive transistors.
- Relay power reduces load on voltage regulator.
- It uses more commonly obtained NPN drive transistors.
- It is easier to interface relay.
- It is economic.
- Uses Industry standard technique.
The ULN2003 has internal clamp diodes. While these work OK in non-critical applications and it leads to rise of glitches.
The clamp, free-wheeling or commutation diode provides a path for the inductive discharge current to flow when the driver switch is opened. If not provided, it will generate an arc in the switch—while the arc will not generally damage a switch contact, it will cause contact degradation over time—and yes, it will destroy transistors—been there, done that. The diode requirements are non-critical and a 1N4148 signal diode will generally work OK in low power applications.
Avoid emitter follower drivers. If the relay is switched to OFF in 4007 diode eliminates back e.m.f and safe guards the transistor. ON status of the relay is indicated by LED.
DC Relay Driver IC Circuit
Let us see construction of relay driver circuit for relays that are operated from DC power. In order to drive a DC relay, DC voltage is needed in required quantity to rate a relay and a zener diode.Voltage is required for the relay to operate and to open or close its switch in a circuit. Relays exist with a voltage rating. This is known as relay’s datasheet to rate its coil voltage. For the function of relay, it must receive this voltage at its coil terminals. Thus, if a relay has a rated voltage of 9VDC, it should get 9 volts of DC voltage for its working. In order to eliminate voltage spikes from a relay circuit, a diode is required for its proper functioning. The coil of a relay acts an Inductor.
The inductors are electronic components which withstand changes in current and also the inductors are coils of wires wrapped around a conductive core. Voltage spikes damages all components in a circuit and also damages relay’s switch contacts. To prevent these voltage spikes, a diode is kept reverse biased in parallel with the relay which acts as a transient (spike) suppressor eliminates voltage spikes by going into conduction before voltage is formed across the coil. A transient suppressor suppresses these spikes. A diode conducts reverse bias current if voltage reaches a certain threshold. The diode functions to shunt excess power to ground, and the diodes conduct if the voltage reaches breakdown voltage.
The Required Components
- DC Relay
- Zener Diode
- DC Voltage Source or a DC power supply.
The zener diode is placed reverse biased in parallel to the relay.
The Relay used in the above is rated for 9Volts. In this a 9V DC Voltage source feeds the resistor. A Zener diode reverse biased is placed in order to suppress the transients caused by opening and closing the relay. This shunts all excess power to ground if it reaches a particular threshold. This is the process to operate a relay. Driving the loads which were connected to the output taking required power the relay will be closed.
AC Relay Driver IC Circuit
This AC Relay driver IC circuit is a relay that runs with AC power and cannot be run with DC power. In order to run an AC relay, enough AC voltage is required tp rate the relay and transient suppressor. In AC relay circuit we cannot use a diode to remove voltage spikes. This diode conducts an alternate half-cycle with AC power. We use an RC series network by placing across coil in parallel to form a working transient voltage suppressor with an AC circuit. Capacitor absorbs charge which comes excessively and resistor helps to control overflow.Components required to form the circuit is as follows
- AC Relay
- 100 Ohm Resistor
- 0.05 Micro Farad Capacitor
- AC Voltage Source
NOTE: AC voltage source may come out from plug that is inserted into US wall outlet.
Be careful with AC Power that comes out directly from wall outlet as it causes Shock. Consult a Professional before taking power from plug into wall outlet.
When we use a relay with rated voltage 110VAC, we should feed it with 110V from an AC power source. To suppress voltage spikes, resistor and capacitor connected in series acts as transient voltage suppressor.
Relay Driver IC ULN2003
The relay driver uln2003 ic is a high voltage and current darlington array ic, it comprises of 7-open collector darlington pairs with common emitters. A pair of darlington is an arrangement of two bipolar transistors. This IC belongs to the family of ULN200x ICs and various types of this family interface to various logic families. This ULN2003 IC is for 5V TTL and CMOS logic devices. These ICs are used as relay drivers as well as to drive a wide range of loads, line drivers, display drivers etc. This IC is also normally used while driving Stepper Motors. The pairs of darlington in ULN2003 is esteemed at 500mA and can withstand peak current of 600mA.In the pin layout, the i/ps & o/ps are provided reverse to each other. Each driver also has a suppression diode to dissipate voltage spikes while driving inductive loads
This project is designed for a three-phase-solid-state relay system. It incorporates three single-phase units wherein each phase is controlled individually by power TRIAC with RC snubber network for a zero-voltage switching (ZVS).
Opto-isolators are used in each phase to receive switching signals from a microcontroller of the 8051 family, loads are connected in series with a set of TRIACS driven by an opto-isolator. The microcontroller is designed to generate output pulses after zero voltage pulse to ensure that the load gets switched on at zero cross of the supply waveform.
The zero crossing feature of the TRIAC driver, (an opto-isolator) ensures low noise generation thus avoiding sudden inrush of current on resistive and inductive loads. In this project, two push buttons are used for generating the output pulses from the microcontroller randomly, away from the ZVS ie not coinciding with zero voltage supply voltage of the waveform.
Thus the lamp which is used as a load is forced to switch on and off at non ZVS points but the final switch happens at the next ZVS only. Using CRO or DSO we can see the waveform of voltage supplied to load for verifying the switching ON/ OFF the load at zero voltage point.
Furthermore, this project can be enhanced by using 2 back-to-back SCRs, in each phase for heavy load switching as used in industries. Over load and short circuit protection can also be incorporated for higher reliability.
This is all about relay driver ic working and its application project.Hope you might have got a better concept of this project.Furthermore any queries regarding this article or electrical and electronics projects, please visit the site www. edgefxkits.com.
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