When and how quickly do PolySwitch devices reset?
The reset condition is a function of the current and voltage as well as the temperature. The time it takes a device to reset to the low-resistance state depends on a variety of factors: type of device; mounting; configuration; ambient temperature; and duration of the trip event. In general, most devices will reset within a couple of minutes, although many will reset within seconds.
What is the shelf life for a PolySwitch device?
There is no practical limit to the shelf life of a PolySwitch device if it is stored properly. Some device characteristics such as solderability of leads, may change if the devices are exposed to excess humidity and temperature, but under normal storage conditions for electronic components, the shelf life is indefinite.
Does resistance of a PolySwitch device change in the untripped state?
Resistance of a PolySwitch device can change with time depending on ambient environment. Device specifications take into account expected application environments.
How can I identify which PolySwitch device I have from a sample or description?
Most PolySwitch devices are labeled with the Raychem Circuit Protection symbol and an identifying mark or code. Standard product marking is outlined in each product section of the Databook. However, we also manufacture many custom parts that can only be identified by a knowledgeable factory representative.
How does the polymeric positive temperature coefficient (PPTC) effect work?
A polymer PTC device comprises a polymer matrix that is loaded with carbon black particles to make it conductive. Since it is conductive it will pass a given amount of current. If too much current is passed through the device, the device will begin to heat due to I²R heating As the device heats it will expand. As it expands, the carbon particles will separate and the resistance of the device will increase. This will cause the device to heat faster and expand more, further raising the resistance. This increase in resistance is sufficient to substantially reduce the current in the circuit. A small amount of current will still flow through the device and is sufficient to maintain the temperature of the device and keep it at the high resistance level. When the power and fault are removed, the PolySwitch device will cool. As the device cools, it contracts to its original shape returns to a low resistance level where it can hold the current as specified for the device.
What are the basic differences between a PolySwitch device and a fuse or other circuit protection device?
The most obvious difference between a PolySwitch device and a fuse is the feature of resettability. While both products provide overcurrent protection, a single PolySwitch device can provide this protection multiple times, whereas after the fuse has provided its protection, it must be replaced for the circuit to function properly. Technically a PolySwtich resettable device is a conductive polymer based thermistor. Thermistors are characterized by either negative temperature coefficient behavior (NTC) where the resistance of the device decreases with temperature or positive temperature coefficient behavior (PTC) where the device resistance increases with temperature. A PolySwitch resettable device is a polymer positive temperature coefficient thermistor.
When comparing a PolySwitch device to a bi-metallic circuit breaker the main difference is latching not resettability. Both devices are resettable, but many bi-metallic circuit breakers will reset themselves even when the fault is still present. This can lead to large EMI spikes on resetting, and potentially reconnection of a fault condition that could damage equipment and be unsafe. The PolySwitch device will latch in the high-resistance state until the fault and power are cleared. PolySwitch devices differ from ceramic PTC devices in their initial resistance, time to react to fault events, and size. Both products are resettable but the PolySwitch device, compared to a ceramic PTC device of the same hold current, will typically react (trip) much faster than the CPTC because the PolySwitch device is smaller and has a lower resistance.
What are some of the major applications for PolySwitch devices?
PolySwitch devices are used in computers and peripherals, mobile phone battery packs, telecommunications and networking equipment, power supplies, industrial controls, automobiles, consumer electronics, and a host of other applications.