Relays, including reed relays are very easy to use, however when using them there are a few simple precautions that can be adopted to ensure that the best performance and greatest reliability is obtained.
Understanding some of the circuit techniques required for relays can make a marked difference, especially when interfacing relays to other electronic circuits.
The relay circuit considerations can be split into two main areas: the driver circuits, and the switched circuits
Applying the correct components and protection to both can make a significant difference to the circuit operation and also the reliability of the relay.
Relay control circuits
The relay circuits used for controlling relays often use semiconductor devices. Although the simplest relay circuits would simply involve a switch closing a circuit, the applications of relays often require a small signal, possibly from some form of a microcontroller circuit or other device to actuate the relay.
When driven in this way, it is necessary to some form of semiconductor driver. The simplest is a bipolar transistor, although FETs work equally well.
NPN common emitter relay circuit
The relay is actuated by a coil. This creates the magnetic field that is used to actuate the relay, whether it is a reed relay or an electromechanical relay. The could means that when the semiconductor switch is in its ON state, current will start to flow. It will rise gradually as a result of the inductance and this will mean that there will be a certain time before the relay actuates. However when the switch is suddenly opened a large back EMF will be generated. This could be sufficiently large to damage the driver device.
The level of back EMF generated will be equal to -L di/dt - in other words the higher the rate of change the large the back EMF voltage generated. Even for low values of supply rail, the back EMFs generated can rise to several hundred volts if the switching is fast enough. This is more than enough to destroy a semiconductor device.
To suppress this back EMF a diode is typically placed across the coil. As the back EMF will be in the opposite polarity to the normal voltage across the coil, the diode which is reverse biassed in normal operation, will go into forward conduction and all the current caused but the back EMF to dissipate, thereby suppressing the back EMF. Using the diode protection circuit for the relay driver, it will only be subjected to a maximum of the supply voltage plus the forward conduction voltage of the diode, which silicon is 0.6 or 0.7 volts.
Ideally the clamping diode should be as close tot he relay coil as possible. In the case of reed relay circuits, the code can even be placed inside the mu-metal screen - this helps reduce the level of radio frequency interference generated and can improve the EMC performance.
When this arrangement is used in a typical common emitter driver circuit, which is probably the most usual form, it can be seen that the diode is applied directly across the relay coil.
When a high voltage is applied to the input, this cases current to flow in the base circuit turning the transistor on, causing current to flow through the relay coil and actuating the switch.
In this particular circuit the series base resistor is chosen to be 2k2. This provides sufficient base current to enable the transistor to turn on for the relay. It is required in circuit to limit the base current. The resistor from base to 0V is chosen to be 22k. This needs to be about ten times that of the series resistor and it is required to ensure the base returns to zero volts if the base is open circuit or the drive voltage removed.
The values should be chosen for the particular conditions of the circuit, the transistor gain, operating current for the relay, etc.
It is also possible to use PNP transistors rather than the NPN version shown. When doing this, obviously the supplies need to be reversed, but also the diode polarity needs to be reversed.
NPN emitter follower relay circuit
Whilst the common emitter relay circuit will be the most popular, it is sometimes useful to use a common collector or emitter follower configuration for the relay circuit.
This relay circuit just replaces the emitter resistor with the relay coil. Again the diode is incorporated into the relay circuit to prevent damage from the back EMF induced at turn-off.
The base resistor is placed in the circuit to limit the base current, although in many instances this may not be required.
Like the common emitter circuit, this one too can use a PNP transistor, but with the diode polarity and supply reversed.
In some circumstances higher levels of current gain may be required. This issue can be solved using a Darlington transistor. However be aware that the base emitter voltage drop is twice that of a single transistor, i.e. 1.2 volts instead of 0.6 volts for a silicon transistor and Darlington.
Relay switched circuits
Whilst it is important to design the circuits to drive the relays correctly, there are also points to note about the circuits that are being switched by the relays as well. This is particularly important for reed relays where the contacts are more prone to damage.
One of the key areas of importance is the current experience once the contacts are closed. Even when driving what may be thought to be low current circuits, the in-rush current caused by capacitors used for decoupling, etc can result in huge current spikes are switch-over. This can significantly reduce the life of the reed relay because the inrush current can exceed the rated maximum current by many times.
Even comparatively small capacitors can use current spikes on many amps and this can significantly reduce the lifetime of the relay contacts, especially those of reed relays.
The fact can be reduced by balancing the amount of decoupling and selecting the minimum value consistent with applying good decoupling on the voltage rails or lines that are switched. It is also possible to use small series resistors to reduce the surge. Here the voltage drop across the series resistor needs to be calculated and if any current is carried, this needs to be retained within he acceptable limits.
There are many different circuits that can be used with relays. The actual relay circuit that is best will depend upon many factors and will often arise from the surrounding circuits add the general requirements.
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