Relays or contractors are often used to switch mains powered inductive loads such as motors, valves or electromagnets. When the device is switched off an arc can form across the relay contacts as they open. This leads to premature relay failure if measures are not taken to suppress the spark. In addition to relay damage, the high voltage spark causes interference and EMC issues. The most common method of suppressing the arc is to connect a snubber in parallel to the contacts. This device is a series resistor and capacitor network. Now when the contacts break, the energy stored in the inductor has a path to dissipate through the snubber to generate a small amount of heat in the resistor. Typical values of the snubber components are R = 1 to 100 Ω and C = 10 to 1000 nF. Relay contacts open at unpredictable times during the mains AC cycle; at the time of maximum current, the induced ‘back-EMF’ and contact arcing problems mentioned above will be most severe.
Semiconductor relays using thyristors or triacs will always turn off when the voltage across them passes through zero, for a load with a highly reactive power component the voltage and current will be phase shifted so that this point corresponds to the instant of maximum current. A snubber will be an essential component in this situation. Snubbers are however not completely unproblematic; a small current will continually flow through the snubber RC network (and load) when the contacts are open. Not only is this wasteful but with a very light load such as a fan, it can often be enough to keep the fan running. Increasing the value of R reduces its effectiveness at suppressing arcs. There is, however, an alternative solution to the problem of switching inductive loads. The method suggested here is really quite simple: make sure that the inductive load can only be switched off when the current waveform (not the voltage) passes through zero. With no current flowing, there will be no energy stored in the load inductance to cause problems. Using this approach it’s possible to dispense with the snubber completely. This was the train of thought that passed through the author’s mind and set him on the path to designing this zero-current switching electronic relay.
So to the solution: The parts of the circuit handling the power are the bridge rectifier B1, the DC base-emitter junction of T3, turning it off and bringing T1 into conduction thereby switching the load on.
To turn the load off the TTL input is brought low to turn off the phototransistor. T3 can now only be turned on when T2 turns off. T2 remains conducting until the load current passes through zero when the voltage across D1 and D2 drops to zero. So after the TTL input goes low, the load remains switched on until load current sinks to diagrams confirmed the circuit function using inductive loads. CH1 shows the voltage at the collector of T1 and CH2 the voltage at the emitter of T1 and the base of T2 which corresponds to the absolute value of current flowing in the load. CH3 shows the control voltage applied to the gate of T1. A phase shift of 20 ° between the voltage and current waveform is used in this simulation. The author confirmed the simulation results with observations of the finished working circuit.
Internet Links
[1] www.elektor.com/100270
[2] www.ni.com/multisim
