In this post we elaborately discuss regarding the many kinds of snubber circuits using resistor/capacitor, diodes, varistors, and also learn which of these topologies is the most efficient when it comes to protecting relay contacts from sparking and fusing?
Ideally, a diode snubber is generally preferred for relays controlling DC load, while the second best are the ones that work with resistor and capacitor networks or using RC components. That said, in an AC circuit, a varistor or RC network happens to be the most effective.
Types of Surge Suppressors:
CR type Snubber
This type applies to both DC and AC circuits. However, the load impedance has to be lower than the RC circuit impedance when utilizing the relay for supplying the AC voltage. The moment the contacts open, current will flow to the inductive load through the snubber.
Let’ say the load if a motor or a Solenoid, the release time of the relay contacts will be increased.
How to Select the Resistor and Capacitor Values
You can choose a resistance value of 0.5 to 1 Ω per 1 V of contact voltage. As these values rely on many factors including the load properties and deviations in characteristics.
You can verify the values for R and C by experimenting. As the capacitor suppresses the discharge when the contacts are opened, the resistor arrests the current once the contacts are closed. So, it is recommended to choose a capacitor with dielectric strength between 200 to 300 V.
For an AC circuit, select a capacitor that has no polarity.
In case there are uncertainties regarding the ability of the chosen capacity to cut off arcing at the contact when using high DC voltages, then connect the snubber across the contacts rather than the load.
Diode type Snubber
The electromagnetic energy that is within the inductive load is channeled as current through the diode which is parallelly connected to the induced load. The current is then dissipated as Joule heat by the resistance from the load. Circuits of this type elevates the contact release time.
How to Select the Diode
A diode is placed parallel to the induced load for this type of suppressor. The electromagnetic energy stored in the inductive load will revert and dissipate itself as heat because of the diode.
This kind of suppressor increases the release time more than the RC network type.
The reason is an optimum diode has a reverse breakdown voltage of more than 10 times the circuit voltage, and a forward current larger than the rated load current.
Thus, it protects the contacts from excessive arcing when they are opening.
Diode +Zener diode type Snubber
When a Zener diode is connected in series with a diode type suppressor, the circuit will reduce the relay contact release time. Commonly, this happens in scenarios where the release time of a diode is very slow.
This type of suppressor is also not suitable for an AC circuit but good for a DC network. In terms of the breakdown voltage of the Zener diode, it is around the same value of the supply voltage.
Varistor type Snubber
This kind of suppressor is suitable for use with both AC and DC circuits. The varistor circuit avoids a high voltage from being channelled across the contacts by employing the constant-voltage properties of the varistor.
Furthermore, this type of circuit also relatively increases the release time. You can make the varistor work efficiently when you connect it across the load at a supply voltage of 28 to 48 V.
In case you have a supply voltage of 100 to 240 V, you need to connect the varistor across the contacts for effective suppression.
How to Select the Varistor Type
To select a good varistor, first, you must make sure the cut-off voltage VC is multiplied with √2. Then, the value must be checked if it is more than 1.5 times the supply voltage.
Keep in mind that setting the VC too high is not good as its efficiency will be reduced thus fails to cut off high voltages.
Do not use surge suppressors in the following combinations.
If only a capacitor is connected across the relay contacts, the setup is extremely efficient to reduce arcing. However, because of the huge electrical charge stored in the capacitor when the contacts are open, the current flows to the contacts again when they are closed. Over time, this will cause contact welding.
In another setup, the capacitor can be connected across the loads and in parallel. This configuration is very capable of reducing relay contact arcing while they are opening. On the flip side, because the charging current to C drives into the contacts when they are closed, instant contact welding may eventually occur.
Wrapping up
Normally, switching a DC inductive load is considered more difficult than switching a resistive load. But with a suitable contact protection circuit, the same level of efficiencies can be achieved with both types of load.