Coordinated circuit protection for relays, transformers and solenoidsWritten by Mike Edwards Wednesday, 07 January 2009 10:31
Consumer electronics and industrial equipment can benefit from a coordinated overcurrent/overvoltage circuit protection scheme that helps improve equipment reliability, reduce component count and meet critical safety agency requirements. Because overcurrent and overvoltage protection are often viewed as two unrelated elements of the circuit protection design process, protection strategies can often result in costly multi-component solutions, and synergies between protective devices can be overlooked.
Relay Design Considerations
Relays are frequently used in consumer electronics and industrial equipment to control high currents and voltages with lower signal levels, or to switch currents that must be isolated from the control circuit.
A relay consists of a pair of one or more contacts and a coil which, when energized, causes the contacts to open or close. Relay damage can result from excessive voltage or current. A common problem can occur when a relay interrupts current to an inductive load and causes a voltage spike. If the voltage spike is severe enough to exceed the relay’s contact voltage rating the contacts can be damaged (V = Ldi/dt). This damage may occur suddenly or slowly, over many years of operation.
Additionally, excessive current through the relay contacts can cause damage when the contacts open and the current is interrupted. Excessive currents and voltages can also damage the relay coil. If a relay coil is designed to be energized for only a short duration in normal operation, normal operating current may eventually harm the coil if it is accidentally energized for an extended period of time.
Protecting Against Overcurrent Damage
A popular relay protection component is the polymeric positive temperature coefficient (PPTC) resettable overcurrent protection device. It is made of a conductive filler, such as carbon black, that provides conductive chains throughout the device. As shown in Figure 1, the PPTC device exhibits low-resistance characteristics under normal operating conditions, but when excessive current flows through the device, its temperature increases and the crystalline polymer changes to an amorphous state.
This transition causes the polymer to expand, breaking the conductive paths inside the conductive polymer. The change causes a dramatic increase in the device’s resistance. This increase in resistance reduces the amount of current that can flow through the device to minimal levels.
The PPTC device will remain in this state until the fault is removed. Once this occurs, the device cools, the carbon chains reconnect and the device returns to a low-resistance state. A traditional fuse can be used instead of a PPTC device, but a fuse must be replaced after each trip event. The major advantage of the PPTC device is its resettable functionality, which helps reduce costly service calls and warranty returns.
Protecting Against Overvoltage Damage
A number of overvoltage devices can be used for relay protection. One of the most cost-effective solutions is the MOV (metal oxide varistor) or MLV (multi-layer varistor). A varistor is a variable resistor, or a voltage-dependent, nonlinear device, with a resistance that decreases as the voltage applied across the device increases.
MOVs are made of zinc oxide powder, sintered with other metal oxide ceramics. The resulting structure is a polycrystalline ceramic that consists of distributed zinc oxide grains separated by metal oxide ceramics. The boundary of two adjacent zinc oxide grains creates a p-n junction-like semiconductor characteristic that blocks current conduction at low-voltage levels, and provides non-linear current conduction at higher voltage levels. This device can be used in protecting power circuits because rather than going to a short circuit when it trips, it decreases in resistance to reduce the voltage across the device. The MLV uses the same technology as the MOV, except that the MLV comes in a small surface mount package with electrodes sandwiched between multiple layers of zinc oxide.
Figure 2 shows a typical relay protection circuit. A PPTC device is placed in series with the relay coil to limit the current to the relay in case of a fault or accidental overload. This figure also shows a PPTC device in series with the relay contacts where the selection process is the same as for the relay coil.
It is important to choose a PPTC device with a voltage rating equal to or greater than the maximum expected voltage. The device must also have a hold current equal to or greater than the maximum steady-state current in normal operation. Additionally, the maximum ambient temperature must be taken into account because the hold current decreases as the ambient temperature increases.
Figure 2 also shows an MOV or MLV in parallel with the relay contacts. These devices are rated according to voltage and maximum surge current. It is important to select a device that will not conduct significant current at the normal peak voltage. MOV specifications include a maximum allowable AC or DC voltage. Each MOV and MLV device also has a maximum surge current rating. The usual standard for rating surge currents uses an 8/20 microsecond wave shape (i.e., an 8-microsecond rise time, and a 20-microsecond delay time to half the peak value). As the varistor size increases, the 8/20 microsecond surge current rating increases. An MOV or MLV can also be used in parallel with the relay coil, as shown in Figure 2.
Protecting Transformers and Solenoids
Transformers can also be exposed to overcurrent and overvoltage transients. Installing a PPTC device in combination with an MOV on the primary side of the AC Mains input can help protect electronic equipment from damage caused by overcurrent and overvoltage faults. Unlike a single-use current fuse, the resettable PPTC device also helps protect against damage resulting from conditions where faults may cause a rise in temperature with only a slight increase in current draw.
Certain overload conditions may cause the MOV device to remain in a clamped state where it will continue to conduct current. This may eventually result in an overtemperature failure of the device. As shown in Figure 3, placing the PPTC device in series with and in close thermal proximity to the MOV can help protect the MOV in extended overload conditions - by transferring heat to the PPTC device. This causes the PPTC device to trip faster, limiting the current through the MOV device.
This technique lets designers leverage the temperature response of the PPTC device and replace other thermal protection elements in the circuit. Not only does the PPTC device perform dual functions in this case, it also provides a fully resettable solution.
Probably the most common cause of solenoid failure is mechanical blockage. This can occur when the solenoid becomes contaminated with dirt or debris that lodges between the armature and the inside of the coil, blocking proper movement. Other problems include misalignment, broken springs or an opposing force - such as an object leaning against a CD-ROM tray ejector button.
Any of these conditions can cause constant current to be applied to the solenoid, increasing coil temperature and ultimately burning the coil insulation and wires. Installing a PPTC device in the circuit, in close proximity to the windings, can help protect against damage caused by both overcurrent and overtemperature conditions.
Coordinated overvoltage/overcurrent circuit protection can help designers reduce component count, provide a safe and reliable product, comply with regulatory agency requirements and reduce warranty and repair costs. PPTC devices offer resettable functionality and low resistance in the circuit. They are rated to 240 Vac, permitting maximum voltages of up to 265 Vac and can be installed in the AC Mains input lines. MOV and MLV devices help manufacturers meet a number of safety agency requirements, and provide high current-handling and energy absorption capability as well as fast response to overvoltage transients.
About the Authors:
Editorial Director: Ryerson Polytechnical Institute electronic engineering technologist with over a decade of manufacturing experience and 20-plus years in technical publishing, is also trained in hydraulics, electro-pneumatics, bearings, mechanical CAD software, sensors, motor drives and electric motors.Website: www.dpncanada.com