Low voltage (12–48VDC) circuits are used in a wide variety of applications, including, low voltage surge protection device (SPD), automation control systems (PLCs), industrial control contact relays, low voltage power supply unit (PSU), security alarm system and many other system types. Overvoltage transients can damage integrated circuits (ICs) or other sensitive devices, as well as cause interruptions in service or loss of data. They may even pose a threat to equipment users. Transients on these lines are often the result of lightning interference, inductive spikes from power switching, or fast transients from induced power line fluctuations. For example, a relay switching on/off can cause a magnetic transient in the coil inductance, which produces a high voltage spike. To suppress these transients, circuit designers typically incorporate a shunt element that acts as a clamp, such as a varistor.
When compared with other voltage suppression technologies, varistors offer one of the most cost-effective ways to protect against high energy surges. Varistors are voltage-dependent, nonlinear devices with electrical behavior similar to that of back-to-back zener diodes, providing excellent transient suppression performance. One type, the metal oxide varistor (MOV), can dissipate very high levels of transient energy across the entire bulk of the device, so they are often used to suppress high-energy transients found in industrial or AC line applications.
To extend the life of the relays used in industrial automation, and motor control circuits, circuit designers often use, MOVs on the secondary side. The MOVs clamp down, transient voltages and prevent arcing during the switching of the relay contacts. The MOV absorbs the arcing energy from the energy released from the magnetic fields of the relay.
Low voltage surge protective device (SPD) modules (Figure 2) are often used in industrial applications to provide modulebased surge protection of complete systems. These SPDs are typically DIN rail modules. The high surge handling density that the latest MOV designs make possible inside these SPDs can offer significant protection advantages.
Telecom Power Supply Units (PSU) typically range from 36VDC to 72VDC on the high end voltage range. The LV UltraMOV™ can be used for applications where the voltage is less than 56VDC.
The desire to save energy drives the adoption of LED lighting. LED light bulbs with 24V power lines are widely used for home and commercial applications. The use of an MOV at the input circuit will enhance the surge capability and hence the life of the LED light.
Solenoids are widely used in the gas and water valves for the process control applications. The MOV is connected to help dissipate the energy stored in the magnetic field when the solenoid is activated. This will help protect the power transistors that are used in the circuit.
Higher surge rating for the same size disc with Littelfuse LV UltraMOV
|Diameter mm||VRMS (V)||VDC(V)||LittelfuseLV UltraMOV Series P/N (Max.Op.Temp.85°C)||LittelfuseLV UltraMOV Series P/N (Max.Op.Temp.125°C)||LittelfuseLV UltraMOV Series Imax (8/20)(A)||LittelfuseLV UltraMOV Series Wmax (2ms)(J)||Supplier 1 Imax (8/20)(A)||Supplier 1 Wmax (2ms)(J)||Supplier 2 Imax (8/20)(A)||Supplier 2 Wmax (2ms)(J)|
HOW TO SPECIFY A LOW VOLTAGE DC MOV
Example of MOV selection process for surge protection:
Circuit conditions and requirements:
- 24VDC circuit
- Current waveform for surge is 8×20μs; voltage is 1.2×50μs
- Peak current during the surge is 4,000A (single 8/20us surge)
- Requirement is to survive 500A 100 surges (repetitive 8/20us surge)
- Other components (control IC, etc.) are rated to withstand 100V maximum.
Approach to finding a solution:
To find the voltage rating of the MOV, allow for 20% headroom to account for voltage swell and power supply tolerances.
- 24VDC x 1.2 = 28.8VDC
- So look at 31VDC rated MOVs
- Determine which MOV disc size to use – identify those that minimally meet the 1,000A 8/20 single surge requirement.
→ Use the Repetitive Surge Capability Chart in the LV UltraMOV™ Series datasheet to determine repetitive surge capabilities of each series per the 100 pulses @ 500A requirement
→ Use V-I Curve in the datasheet of the selected MOV to verify that the peak voltage will be below the 100V ceiling.
Determine the LV UltraMOV™ Varistor disc size needed by confirming
the surge rating will meet the application requirement. In the following
table, we have selected a 14mm MOV with a 31VDC max continuous
voltage rating as a possible solution to meet our need. Then, we
will use the Repetitive Surge Capability Chart and V-I curves to verify
that the selected MOV p/n can meet the requirements.
LV UltraMOV™ Varistors
For these types of applications and many others, the Littelfuse LV UltraMOV™ Series of low voltage, high surge current, radial leaded varistors (Figure 3) offers superior surge current ratings, energy handling capabilities, and ability to withstand multiple strikes. Its patented, proprietary formulation provides far greater surge withstanding capability than other technologies on the market today. For example, a 10mm LV UltraMOV Series Varistor offers a maximum surge current rating that’s four times higher than either a standard or other suppliers MOV of the same size to protect against high peak surges, including lightning strike interference, electrical fast transients on power lines, and inductive spikes in industrial applications. The higher surge rating also provides for longer life and reliability because there’s less degradation of the MOV over its lifetime.
Available in disc sizes from 5mm to 20mm, LV UltraMOV Series Varistors are just half the size of standard MOVs with the same surge capability. For example, circuit designers could replace an ordinary 10mm MOV with a 500A surge rating with a 5mm LV UltraMOV Varistor with the same surge rating. Circuit designers who use LV UltraMOV Varistors can also reduce their part counts based on the total surge level. These devices’ compact design reduces both PCB space requirements and component height, making more room available for higher value functional components. The use of a smaller disc also reduces the weight and cost of the final product.
High operating temperatures are common in industrial settings. When specified with the optional phenolic coating, these varistors can be operated in environments up to 125°C, so they’re well suited for use in severe conditions.