PolySwitch Product FAQ

In comparison to bimetal circuit breakers, the main difference is latching behavior. Although both the bimetal and the PolySwitch are resettable devices, many bi-metal circuit breakers will reset themselves even when the fault is still present. This can lead to large electro magnetic interference (EMI) spikes on resetting The PolySwitch device, however, latches in the high-resistance state until the fault is cleared and power to the circuit is recycled.

PolySwitch devices differ from ceramic PTC devices in their size, initial resistance, and time to react to fault events. Both are resettable devices but the PolySwitch device, compared to a ceramic PTC device of the same hold current, will typically react (trip) much faster because the PolySwitch device is smaller and has a lower resistance.

The PolySwitch resettable device is a polymer positive temperature coefficient thermistor (PPTC). 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.

PPTC circuit protection devices are made from a composite of semi-crystalline polymer and conductive particles. At normal temperature, the conductive particles form low-resistance networks in the polymer. However, if the temperature rises above the device’s switching temperature (T Sw) either from high current through the part or from an increase in the ambient temperature, the crystallites in the polymer melt and become amorphous. The increase in volume during melting of the crystalline phase separates the conductive particles resulting in a large non-linear increase in the resistance of the device.

The resistance typically increases by three or more orders of magnitude. This increased resistance helps protect the equipment in the circuit by reducing the amount of current that can flow under the fault condition to a low, steady state level. The device remains in its latched (high resistance) position until the fault is cleared and power to the circuit is cycled – at which time the conductive composite cools and re-crystallizes, restoring the PPTC to a low resistance state in the circuit and the affected equipment to normal operating conditions.

Because PPTC devices transition to their high impedance state based on the influence of temperature, they help provide protection for two fault conditions – overcurrent and overtemperature. Overcurrent protection is provided when the PPTC device is heated internally due to I 2R power dissipated within the device. High current levels through the PPTC device heat it internally to its switching temperature causing it to “trip” and go into a high impedance state.

Most PolySwitch devices are labeled with the manufacturer's mark. Standard product marking is outlined in each product section of the catalog under each product category. However, we also manufacture many custom parts that can only be identified by a knowledgeable factory representative.

The circulation of substandard knock-off components, sold with counterfeit packaging and labeling is a growing concern for electronics OEMs, and Tyco Electronics recommends that manufacturers exercise care and vigilance in the procurement process.

To provide safe and reliable performance, the company advocates purchasing all Raychem Circuit Protection products only from authorized electronics distributors. For your local authorized distributor go to Find a Local Sales Representative or contact us.

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 excessive humidity and temperature extremes; but under normal storage conditions for electronic components the device’s shelf life is indefinite.

The device resets once the fault is cleared and power to the circuit is removed. This allows the conductive composite to cool and recrystalize, thus lowering the resistance in the circuit and restoring the affected equipment to normal operating conditions.

By combining the PolySwitch device in parallel with another PTC device such as a light bulb it is possible to design a circuit using a PolySwitch device that may reset without powering off the device. (Refer to the speaker application note for an example.)

There are certain conditions in which a PolySwitch device will self-reset when the fault is removed and the power is still on.

Automatic reset conditions
Under certain conditions a PolySwitch device will automatically reset and return to normal operation. Automatic resetting can be very useful for applications where the voltage can be varied during operation.

When the following condition is met, the device will automatically reset:

(V² / 4R_L ) < P_D

V = Operating voltage of the circuit.
RL = Load resistance.
PD = Power dissipated by the PolySwitch device.

Many common solvents used for electrical components are acceptable for cleaning the PolySwitch device. However some solvents can adversely affect device performance. See the product catalog for detailed information pertaining to a specific device or product family.

Yes, the main benefit of this technique is a higher hold current with lower resistance. Refer to the technical overview of PPTC devices in the product catalog for more information.

Devices in Parallel
When two identical PolySwitch devices are placed in parallel, the hold current of the devices will increase and the combined resistance should be half the resistance of one of the devices. The magnitude of the hold current increase is dependent on the configuration of the devices and the consequent impact on the power dissipation. If the power dissipation doubles, the hold current will roughly double as well. If the power dissipation increases by less than a factor of two, then the hold current for the two devices will be less than twice that of a single component. Two examples illustrate this:

1. Two devices are placed in parallel and are soldered to individual traces that are thermally isolated from each other (this can be done by placing the traces far away from each other). By doing this, the power dissipation will be double that of a single part. The resistance will decrease by half and the hold current will double.

2. Two devices are placed in parallel and are soldered within close proximity, perhaps on a single trace. In this case, depending on the trace width, the power dissipation ranges from that of a single device to double that of a single device. If the power dissipation is the same as a single device, then the hold current will increase by roughly 40%. If the power dissipation is somewhere in between, then the hold current can be approximated using the following equation:

I_Hp = √ 2 I_Hs x ( √ P_DP / √ P_DS )

I_Hp = Hold current for parallel devices.
I_Hs = Hold current for a single device.
P_DP = Power dissipation for a parallel device.
P_DS = Power dissipation for a single device.

For most applications there is no benefit to this approach, since installing two PolySwitch devices in series will not double the voltage rating. Because one PolySwitch device will always trip first, the other device will provide no additional protection for the circuit.

Pressure on the device will affect the electrical performance of the device. If the pressure is sufficient to restrict the expansion of the device during a trip event then the device will fail to function as specified and could result in device and equipment damage. Care should be taken to avoid mounting the device in such a way as to constrain expansion.

Care should be exercised when forming the leads of any electronic device. With radial leaded products, the leads can be successfully formed without affecting device performance. With battery strap products this is not a recommended practice unless the lead forming is performed at a sufficient distance from the chip in order to minimize potential shorts or chip damage.

In general, potting is not recommended. Although some customers have successfully potted our components, care must be exercised with the material selected for potting, as well as the means of curing the potting material. If the potting material is too rigid it will not allow the PolySwitch device to expand as designed and therefore will prevent the device from operating as intended. Even if the material is a "soft" potting compound, the thermal transfer characteristics of the device will be affected and the device will perform differently than as specified.

The typical failure mode of the PolySwitch device is to fail in a high resistance state. This means that the device does not return to its original low resistance value and will not maintain the original specified hold current. In order to achieve UL recognition, the device must meet two criteria:

1. be tripped 6,000 times and still exhibit PPTC behavior and;
2. remain in a tripped state for over 1,000 hours while exhibiting PPTC behavior.

If a device is subjected to fault events that exceed its rated voltage and current, or is exposed to multiple trip events that exceed the UL test conditions, the device may exhibit arcing and flame.

Each PolySwitch device is rated to handle a specified operating voltage. Each device can withstand a specified interrupt current as a fault event. To obtain UL recognition, the device must be tripped at least 6,000 times and still exhibit PPTC characteristics. Telecom devices are rated for maximum interrupt voltage for specific fault events that generally occur in telecom applications, with the device still meeting the original specification values. This may be as few as ten times or as many as several hundred times, Designers should keep in mind that the PolySwitch device is intended to protect against faults and is not intended to be used in applications where it will be expected to remain in the tripped state as the normal mode . As stated in the product catalog,PolySwitch devices that are left in the tripped state can cause device damage, including arcing and flame.

PolySwitch devices have not been qualified and are therefore not recommended for outer-space environments. The thermal transfer component and the electrical performance of the device is negatively affected by vacuum conditions.

For R-line and TR parts this is a flame-retardant epoxy. For strap devices it is a polyester tape. These materials meet either UL94V-0 or IEC695-2-2 requirements.

For the operational state of most PolySwitch products the usable range extends up to 85°C ambient. For some product families (i.e. PolySwitch TD, Chip, AHR, and RHE devices) this can be as high as 125°C ambient and for others, such as the PolySwitch VTP device, it may be as low as 70°C ambient. In a non-operational state, some devices (i.e. PolySwitch SMD, miniSMD, TS) will withstand solder reflow temperatures for a short duration. Use of a device above its temperature rating may cause it to nuisance trip.

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 remain in the low resistance state, transition to the high resistance state quickly, or transition to the high resistance state after an extended time period.

The range of current values between I_H and I_T 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 current is sufficiently high.

Although primarily intended as an overcurrent protection device, the PolySwitch device can also be caused to trip by thermally linking it to a component or equipment that needs to be protected against overtemperature conditions – such as a motor. If the equipment temperature reaches the PolySwitch device’s switching temperature it will transition to its high impedance state, regardless of the current flowing through it. In this way, the PolySwitch device can be used either to reduce the current to the equipment to very low levels, or as an indicator to the control system that the equipment is overheating. The control system can then determine what action is appropriate to protect equipment and personnel.

Another example of how this technique can be used is with the PolySwitch VTP product family which allows battery pack designers to eliminate thermal devices from some designs due to the low activation temperature of the device.

UL rated devices must sustain a 1000-hour trip event without losing PPTC 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 remain in the tripped state as the normal mode of functioning. As stated in the product catalog, devices left in the tripped state can cause device damage, arcing and flame.

The PolySwitch LVR series devices includes components that are rated for line voltages of 120 VAC and 240 VAC, for up to 2A of operating current at 20°C. They are frequently used for AC Input applications and to help protect electric motors, controllers and transformers used in commercial and home appliances.

Raychem Circuit Protection components have been helping equipment manufacturers comply with telecom specifications worldwide for decades:, including UL1950, FCC Part 68, ITU K20 and K21, and Telcordia GR1089. Refer to the Telecom section on our website or the product catalog for detailed application information. Refer to the telecom PolySwitch devices section or the product catalog for further information.

Coordinated overcurrent and overvoltage protection can help designers comply with safety agency requirements and minimize component count and cost. For instance, coordinating overcurrent and overvoltage protection at the AC input is accomplished by using Raychem Circuit Protection’s ROV series metal oxide varistor in combination with the PolySwitch LVR device to improve equipment reliability in the harsh AC environment, and help fulfill the IEC-61000 test requirements.

In a typical protection system employed by network equipment manufacturers to comply with ITU-T K.20 requirements, SiBar™ thyristors help protect sensitive electronics from fast overvoltage events, including lightning transients and PolySwitch devices provide current limiting during some anomalous operating conditions such as power contact and power induction.

Tyco Electronics offers a broad range of overvoltage devices, including: thyristors, gas discharge tubes, MOVs, and ESD suppression devices that can be used with PolySwitch devices for integrated overcurrent, overtemperature and overvoltage protection.