Spark Quencher Frequently Asked Questions
What is a spark quencher?
A spark quencher (SPQ) is a network which consists of a capacitor and resistor in a series circuit. Both elements are designed to withstand the high energy surges which are present in their application.
Why use spark quenchers?
The SPQ is used in two types of electrical applications. The first is to suppress the back EMF, which occurs when an inductive load de-energizes. The inductive load referenced may be a motor, a relay, a contactor, a solenoid or in some cases even a heating element. Each of these types of devices has within its construction an inductive element. When the power is removed from an inductive element, a large(up to1500 volt) spike of energy is induced onto the power line. The SPQ is designed to absorb this spike of energy and slowly dissipate it at a lower energy level over an extended period of time. This energy absorption helps protect other components from potential damage due to these energy spike, and aides in prolonging the life of electrical products.
Secondly the SPQ can be used in the protection of solid state switches such as triacs, SCRs and FETs. Many electrical applications have shifted from mechanical switches and relays to these solid state devices for longer and more reliable service. These solid state switches also require protection to allow their proper function. The SPQ has been designed to be used for their protection.
What are the common uses for spark quenchers?
SPQs are placed across motors, relay coils and relay contacts, motor starter contacs to suppress back EMF spikes and EMI/RFI noise. They are used across relay and Motor starter contacts to suppress the arcing that occurs when the contacts are opened, thus prolonging the contact life. They are used in parallel with Solid state switches as snubber circuits.
Why are safety agency approvals important for SPQs?
Like the X&Y capacitor, safety agency approvals assure that the SPQ is designed to meet the severe voltage and current surges that they may be exposed to.
How do I select a SPQ for my application?
The choice of which SPQ to use is typically an empirical decision. The selection of the resistor and capacitor value should be sized to solve the noise problem, but not so large as to cause other concerns such as size or cost. A good rule of thumb is to start with a resistor value equal to the rating of the circuit voltage. Then chose a capacitor sized to the current level of the inductance. For current values to 1 ampere a 0.1mFd capacitor is a good starting point. For larger current values the capacitance should be increased. Okaya SPQs have capacitance values as large 0.5mFd.
Therefore, a 120VAC, 0.4 Ampere inductive load would use an SPQ with a resistor value of 120 ohms and a capacitance value of 01.mFd. If this value does not solve the noise problem, the first step should be to increase the capacitance value followed by decreasing the resistance value. For example a 120ohm/0.1mFd SPQ and a 47ohm/0.2mFd SPQ have the same electrical time constant for energy absorption, but the later combination may work better in a particular application then the former because of the smaller resistance value. Generally there is no one SPQ which is exactly correct for a given application, but rather a series of combinations that will work.
In the case of 3-phase applications, multiple RC networks are combined together in a single package to protect all three power lines.