Definitions

snubber

shock absorber

Device for controlling unwanted motion of a spring-mounted vehicle. On an automobile, the springs act as a cushion between the axles and the body and reduce the shocks produced by a rough road surface. Since some combinations of road surface and car speed may result in excessive up-and-down motion of the car body, shock absorbers—which today are hydraulic devices that oppose both compression and stretching of the springs—slow down and reduce the magnitude of these vibratory motions. Seealso damping.

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A snubber is a device used to suppress ("snub") voltage transients in electrical systems, pressure transients in fluid systems, or excess force or rapid movement in mechanical systems.

Fluid systems

Mechanical systems

A snubber can be a mechanical device that limits pressure or velocity surges on measurement devices. These connect between the process and the measurement device allowing a relatively slow change in pressure or velocity to limit damage to the hardware. Such devices are common in industrial environments.

Electrical systems

Snubbers are frequently used in electrical systems with an inductive load where the sudden interruption of current flow would lead to a sharp rise in voltage across the device creating the interruption. This sharp rise in voltage might lead to a transient or permanent failure of the controlling device. A spark is likely to be generated (arcing) and to cause electromagnetic interference. The snubber prevents this undesired voltage by conducting transient current around the device.

RC snubbers

A simple snubber comprising a small resistor (R) in series with a small capacitor (C) is often used. This combination can be used to suppress the rapid rise in voltage across a thyristor, preventing the erroneous turn-on of the thyristor; it does this by limiting the rate of rise in voltage (dV/dt) across the thyristor to a value which will not trigger it. Snubbers are also often used to prevent arcing across the contacts of relays and switches and the electrical interference and welding/sticking of the contacts that can occur. An appropriately-designed RC snubber can be used with either dc or ac loads. This sort of snubber is commonly used with inductive loads such as electric motors. The voltage across a capacitor cannot change instantaneously, so a decreasing transient current will flow through it for a small fraction of a second, allowing the voltage across the switch to increase more slowly. While the values can be optimised for the application, a 100 ohm resistor in series with a 100 nanofarad capacitor of appropriate voltage rating is usually effective. This type of snubber is often manufactured as a single component.

Diode snubbers

When the current flowing is DC, a simple rectifier diode is often employed as another form of snubber. The snubber diode is wired in parallel with an inductive load (such as a relay coil or electric motor). The diode is installed so that it does not conduct under normal conditions. When current to the inductive load is rapidly interrupted, a large voltage spike would be produced in the reverse direction (as the inductor attempts to keep current flowing in the circuit). This spike is known as an "inductive kick". Placing the snubber diode in inverse parallel with the inductive load allows the current from the inductor to flow through the diode rather than through the switching element, dissipating the energy stored in the inductive load over the series resistance of the inductor and the (usually much smaller) resistance of the diode (over-voltage protection). One disadvantage of simple rectifier diode used as a snubber is that the diode allows current to continue flowing, which may cause the relay to remain actuated for slightly longer; some circuit designs must account for this delay in the dropping-out of the relay.

More-sophisticated solid-state snubbers

In some dc circuits, a varistor or two inverse-series Zener diodes (collectively called a transorb) may be used instead of the simple diode. Because these devices dissipate significant power, the relay may drop-out faster than it would with a simple rectifier diode. An advantage to using a transorb over just one diode however, is that it will protect against both over and under voltage if connected to ground, forcing the voltage to stay between the confines of the breakdown voltages of the Zener diodes. Just one Zener diode connected to ground will only protect against positive transients.

In ac circuits a rectifier diode snubber cannot be used; if a simple RC snubber is not adequate a more complex bidirectional snubber design must be used.

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