How does a PolySwitch device work with overvoltage devices to provide protection?
The most common application where PolySwitch devices are used in combination with overvoltage devices is in the telecom applications. Here overvoltage devices such as thyristors, gas discharge tubes, MOVs, or diodes provide protection against lightning and power cross faults. The PolySwitch device protects the overvoltage protection device in some of these fault events and can also provide protection against overcurrent events.
Do you have any parts that meet standard telecom industry specifications?
Our devices help telecommunications equipment pass various telecommunication specifications worldwide. The following are just some of the specifications we help telecommunications equipment meet: UL1950, FCC Part 68, ITU K20 and K21, and Telcordia 1089. Refer to the Telecom web product selector, slide rule, or Databook for further information.
Can PolySwitch devices be resistance sorted?
Some of our devices are supplied in a resistance sorted variant of the standard product. This capability is primarily intended for the devices that are designed for use in telecommunications applications such as the TR and TS product families.
Are PolySwitch devices available for line voltage (120 - 240V) applications?
LVR series devices are now available for line voltage applications. Contact your local rep for further information.
What voltage-drop values are expected?
This depends on the device in question. Typically the voltage drop of the device can be calculated if you know the resistance and the steady-state current flowing through the device. The value to use for the resistance of the PolySwitch device is the R₁max value to determine maximum voltage drop. A typical voltage drop can be calculated by using the Rmax value or, if that is not supplied, then a value that is the average of Rmin and R₁max. If Iop is the normal operating current and Rp is the resistance of the PolySwitch device then the voltage drop in the circuit can be calculated as: Vdrop = Iop x Rp
Do PolySwitch devices expand when they are tripped?
All PolySwitch devices will expand when in the tripped state. After cooling the device will return to its original size and shape. The device may not return to its original resistance but will return to a value that meets the specification. Care should be taken to avoid mounting the device in a configuration that constrains the expansion.
How long can a PolySwitch device stay in the tripped state without damage?
The UL rated devices must sustain a 1000-hour trip event without losing PTC characteristics. Longer trip events can be sustained with a fault event that is less than the maximum rated voltage and current for the device. The longer the device is held in a tripped state the more likely it is that the device will not recover all of its original resistance value when reset and therefore may not meet the original device specifications. The degree to which each device will suffer this degradation is highly dependent on the fault event and the device in question. Designers should keep in mind that the PolySwitch device is intended to protect against faults and failures and is not intended to be used in applications where it will be expected to be tripped as the normal mode of functioning. As stated in the Databook, these devices left the tripped state can result in device damage, arcing and flame.
Will the PolySwitch device cycle? How does it stay latched?
PolySwitch devices will not cycle between a normal and a tripped state when a fault condition is present. When the PolySwitch device trips it goes from a low resistance to a high resistance state. In the high resistance state a small amount of trickle current is still present. This small trickle current is sufficient to maintain the PolySwitch device in the high resistance state. The PolySwitch device generally requires the power to the circuit to be interrupted, allowing the PolySwitch device to cool before it will return to the normal low resistance state.
What is the composition of the solder used in the devices?
For most of our products, the solder is similar to Sn63 or Sn60 type solder. Some of our product families (such as RHE) may use a higher temperature solder. Work is underway to convert many devices to lead-free solders.
What is the resistance of a PolySwitch device in the tripped state?
The resistance of the device in the tripped state depends on the following: device used; voltage across the device; and power dissipation of the device. The value of this resistance can be found using the following formula: Rt = V²/Pd.
What is the resistance of a PolySwitch device after it has been tripped and allowed to reset for an hour?
It will be less than R₁max for the device.
What is the difference between Rmin, Rmax, and R₁max?
The resistance of a PolySwitch device under specified conditions (eg: at 20℃), before connection into a circuit. Devices of a particular type will be delivered with a range of resistances; therefore, a minimum value (Rmin), and/or a maximum value (Rmax) are often given. The maximum resistance of a PolySwitch device at room temperature one hour after being tripped or after reflow soldering is R₁max.
Can I use a PolySwitch device for overtemperature control?
Although primarily intended as an overcurrent protection device, many devices have been successfully used to prevent overtemperature conditions as well. A good example of this is our VTP product family which allows battery pack designers to eliminate thermal devices from some designs due to the low activation temperature of this product family.
What is the relationship between IH; and IT? Why the gap?
For most of our products there is a 2:1 relationship between IT and IH. For some products this can be as low as 1.7:1 and for others as great as 3:1. The material and manufacturing variances in resistance as well as the change of resistance after a trip event or high temperature installation will determine the ratio of IT to IH. For most of our products this makes the 2:1 ratio the most practical value.
What happens if the current exceeds I-hold (IH) but does not reach I-trip (IT) ?
Hold Current or I-hold (IH) is the largest steady state current that, under specified ambient conditions, can be passed through a PolySwitch device without causing the device to trip into the high resistance state. Trip Current or I-trip (IT) is the smallest steady state current that, if passed through a PolySwitch device, will cause the device to trip, under specified conditions.
The device may exhibit a variety of behaviors depending on a variety of factors such as:current rise time; length of time the device is exposed to this current; and ambient temperature. The device may stay in the low resistance state, transition to the high resistance state quickly, or transition to the high resistance state after a long time.
The range of current values between IH and IT represents a zone where performance of the device with respect to tripping can not be predicted with certainty. Depending on the initial resistance of the device and the ambient and mounting conditions, the device could either maintain a low resistance state and hold this current or switch into a high resistance state if the currect is sufficiently high.