In this case, the decreasing rate-of-change of inductor current will be governed by the inductor winding resistance (ignoring the forward voltage drop in the diode). The kickback voltage creates a spark that degrades the contact surface, and over time, this sparking can cause contact failure.Ī backwards diode connected across the relay contacts will switch into conduction when kickback voltage is applied. The polarity of the kickback voltage is opposite to that of the original voltage that was used to energize the inductor.
But since the voltage source no longer exists - we have an open circuit - there is nothing (other than the inductor’s winding resistance) to limit the magnitude of the voltage induced, and this kickback voltage will therefore appear across the relay contacts that are in the process of opening - contacts that have, in fact, barely moved from their closed position.
Now consider what happens when the relay contacts open: The magnetic field in the inductor immediately begins to collapse, and as it does so, it will attempt to drive current back into the voltage source. At some point, the inductor will saturate - i.e., it cannot be magnetized further - whence the limiting current will then be governed by the winding resistance. As time advances, and dependent upon the load inductance, the current through the inductor will increase at a constant rate determined by the ratio V/L. The minus sign indicates that the inductor resists a change in current by opposing the applied voltage.Ĭonsider a voltage applied across an inductor through a set of relay contacts: At the instant of contact closure, load current is zero. It states that the rate-of-change of current (amperes per second) in an inductor is proportional to the voltage applied across the inductor and inversely proportional to its inductance (henries). Examples of inductive loads include motors, solenoids, or other relay coils.Ĭurrent and voltage in an inductive circuit are related by Faraday’s Law, one form of which is V = -L di/dt. This opening can occur intentionally, or can occur during contact closure due to contact bounce. Relay contacts in DC circuits are subject to sparking as they open when they control an inductive load. The diode is called a “snubber” and it serves to protect the relay contacts from overvoltage in inductive circuits. NOTE that this technique is applicable only in DC circuits. Abend asks about diodes placed backwards across relay contacts. I looked this up to find more for you, check out these web pages: This is a good explanation: Once these were installed the RFI noise was removed, or at least reduced enough, so that the computer would not lose its place in the program and all worked fine from then on.Īside: A flyback transformer where this flyback voltage is put to good use, see: Until I realised I was not using any flyback diode on all the relays. But, it was always glitching, the program would get lost and hang up. This diode has a number of names, but I like calling it the flyback diode.Įxample: I once built a 8080 based microcomputer and programed it to turn lights on and off in the house when we were on vacation. If high enough, this can cause damage, but even if it’s a low voltage, it can cause RFI and cause a circuit to misbehave. The energy stored in the magnetic field of the relay coil is now released and the voltage can get quite high and case a spark. It is used to protect the components of the circuit from burning out from a release of the induced voltage when the relay is turned off.
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