October 9, 2019
What is Relay Suppression?
There are many specifications to consider when selecting the right relay for your circuit. One of the most important decisions is suppression type - unsuppressed, resistor, or diode. Understanding the function of a relay is essential to this decision.
How do relays work?
Relays are small electrical components that act as switches. They use a small input current to open or close a switch which is typically controlling a higher current circuit. In order to operate, relays use a coil of wire, which creates a magnetic field when supplied with current. This field will pull/push an armature with a contact on its end so that it either completes a circuit by touching another contact, or turns a circuit off separating them. The diagram below gives a visual of the wiring of a relay.
Rationale for Suppression
A coil of wire acts as an inductor, meaning when it’s supplied with power it builds up to its maximum current, but when power is cut off it takes time for the current to completely dissipate. This means that when a “hot” coil in a relay with no suppression is turned off, its inductive properties cause it to want to keep current flowing, but there is nowhere for it to go.
This causes a voltage transient spike, a momentary voltage spike anywhere from 100V to 800V in a 12V or 24V automotive relay. After extended use, voltage transient spikes can damage components which control the state of the relay. For example, the spike could cause a microcontroller that is hooked up to the relay, to malfunction. A solution is to add a suppression component (diode/resistor) in parallel with the coil to give the current somewhere to go after power to the coil is turned off.
However, when we add suppression to the relay, it extends the amount of time the coil has current flowing, which in turn causes the magnetic field to dissipate slower. Therefore, the contacts will separate at a lower initial speed. This proves to be problematic because whenever the contacts separate, an arc of electricity forms which causes the transfer of a miniscule amount of melted material between contacts (a.k.a., tack welding). Eventually, after MANY activations, this leads to electrical failure of the relay. Slowing down the armature during separation of the contacts, causes an extended drop-out time of the arc and leads to a shorter life of the relay.
Things to Consider When Considering Suppression Type
Type of suppression is dependent on the application and what conditions your relays will endure. Some factors to consider are relay life, heat dissipation, control current, cost, transient voltage spikes and EMI. The below chart and suppression descriptions may assist you in making your decision.
As mentioned previously, no suppression causes the largest transient voltage spike, but it also comes with minimal tack welding. This makes unsuppressed relays have the longest lifetime of the three options, and is usually the type used when relays are tested for life expectancy. Not using a resistor or diode in the relay also means it is the cheapest option of the three. The drawback is unsuppressed relays are not common, so they may have availability issues.
A resistor adds minor cost and only a slight increase in the time it takes for the armature to open after deactivation. Having a resistor in parallel with the coil causes an increase in the controlling current and in heat dissipation, because it draws current whenever the relay is activated. However, it reduces the transient voltage by 50% and it is widely used across the industry.
The final suppression type is the diode. It is placed in parallel with the coil so that when it is connected to power all current flows through the coil, but when it is disconnected it allows the coil to send current through the diode until it dissipates - almost completely eliminating transient voltage spikes. However, diode suppression has the largest drop-out time which causes the life of the relay to be cut in half. Also, diode suppression may cause short circuit issues if plugged-in or wired incorrectly.