Snubber
From Wikipedia, the free encyclopedia
A snubber is a simple electrical circuit used to suppress ("snub") electrical transients. Snubbers are frequently used 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.
In a similar fashion, a snubber can also be a mechanical device that limits pressure surges on measurement devices. These connect between the process and the measurement device allowing a relatively slow change in pressure to limit damage to the hardware. Such devices are common in industrial environments.
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[edit] RC snubbers
Frequently, a snubber can consist of just a small resistor (R) in series with a small capacitor (C). 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 the subsequent 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.
[edit] 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. This may result in the relay remaining picked-up slightly longer; some circuit designs must account for this delay in the dropping-out of the relay.
[edit] 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 1 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 1 zener diode connected to ground will only protect against positive transients.
In ac circuits, a rectifier diode cannot be used and one of the more complex (and bidirectional) snubber designs may be used instead.
