What voltage-drop values are expected?
This depends on the part 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, or a typical voltage drop you can use either 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 Rps is the resistance of the PolySwitch device (either R1max, (Rmax or Rmin + R1max)/2) then the voltage drop in the circuit can be calculated as:

Vdrop = Iop x Rps

Can PolySwitch devices be used in parallel?
Yes. The main benefit is a higher hold current with lower resistance.

Can PolySwitch devices be used in series?
This is not practical. Since one PolySwitch device will always trip first, the other device will provide no protection for the circuit.

What is the performance of PolySwitch devices in a space environment?
PolySwitch devices have not been internally qualified for a space environment. If the devices are used in a vacuum environment, the thermal transfer component and the electrical performance of the device will be affected.

How quickly do PolySwitch devices reset?
The time it takes a device to reset to the low-resistance state depends on a variety of factors:

  * Which device it is
  * How it is mounted or fixtured
  * Ambient temperature
  * Nature and duration of the trip event

In general, most devices under expected conditions will reset within a couple of minutes, although many will reset within seconds.

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.

Does aging or resistance of a PolySwitch device change in the untripped state?
Raychem has detected no aging that occurs under normal known application environments in the untripped state that allows the device to exceed the original or known specification values.

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 fuses from some designs due to the low activation temperature of the VTP product family.

What does -2 stand for in a PolySwitch device part number?
The -2 indicates that the part is supplied in a tape-and-reel packaging format.

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 devices for use in telecommunications applications such as the TC, TR250, TR600 and TS product families.

Can PolySwitch devices be bent?
Care should be exercised when forming the leads of any electronic device. With RUE, RXE, RGE and RUSB products the leads can be successfully formed without affecting device performance. With the battery strap products (SRP, LTP, VTP, LR4) this is not a recommended practice unless the lead forming is performed at some distance from the PTC chip so that potential shorts or chip damage cannot form.

What are the effects of potting a PolySwitch device?
In general, potting is not a recommended practice. Although some customers have successfully potted our components, care must be exercised with the material selected for potting as well as with the means of curing the potting. If the potting material is too rigid it will not allow the PPTC device to expand as designed and therefore will prevent the device from operating as designed. 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.

What are the effects of pressure on a PolySwitch device?
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 the device will fail to function as specified.

How can I identify which PolySwitch device I have from a sample or description?
Most PolySwitch devices are labeled with the Raychem symbol and an identifying mark or code. Standard product marking is outlined in each product section of the databook. However, Raychem manufactures many custom parts that can only be identified by a knowledgeable factory representative.

Are PolySwitch devices available for line voltage (120V - 240V) applications?
The highest rated standard product is rated for 60V/40A maximum voltage and current rating. For primary side protection, the PolySwitch product group does not offer a device suitable for the application. However, for the secondary side of many applications, many of our products can be used, provided that the maximum current and voltage ratings are not exceeded.

What is the maximum ambient temperature where PolySwitch devices can be used?
For the operational state this depends on the product family in question. For most of our products the usable range extends up to 85℃ ambient. For some product families (certain TD, chip, and RHE devices) this can be as high as 125℃ and for some others (VTP) this is as low as 70℃. For a non-operational state some of our devices (SMD, miniSMD, TS) will withstand solder reflow temperatures for a short duration. Use of the device above the derating temperatures may cause the device to nuisance trip.

Is the PolySwitch device self-resetting? How? How fast?
Yes, the device is self-resetting once the fault is cleared and the device has an opportunity to cool down. The cooling causes the device to contract and reconnects the carbon black molecules, thus lowering the resistance. The normal way to cool down the device is to power off the equipment that is being protected by the PolySwitch device, cutting the maintain or trickle current and allowing the device to cool. This should not be confused with a bi-metallic device which will also self-reset. A typical bi-metallic device will reset even if the fault is not removed, thus causing a cycling on and off between the fault event and a protected state which may damage the equipment. The PolySwitch device will latch at the high resistance state until the fault is removed.

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 will reset without powering off the device. (See the speaker application note for an example.)

The time it takes a device to reset to the low resistance state depends on a variety of factors:

  * Which device it is
  * How it is mounted or fixtured
  * Ambient temperature
  * Nature and duration of the trip event

In general most devices under expected conditions will reset within a couple of minutes although many will reset within seconds.

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 difference between Rmin, Rmax, and R1max?
Rmin is the lowest specified resistance that any device supplied by Raychem will have. Rmin will determine the specified minimum trip current of the device. Rmax is the highest specified resistance that any device supplied by Raychem will have. R1max is the highest resistance that a device should attain as a result of customer installation or normal use. This value determines the maximum hold current for a device. When a device is tripped by a customer, or installed using temperatures greater than the rated temperature of the device, the resistance of the device as supplied by Raychem (greater than or equal to Rmin and less than or equal to Rmax) may increase to a value less than or equal to R1max.

What are the UL, CSA or TUV file numbers for PolySwitch devices?
  UL File # E74889
  CSA #CA78165C
  TUV #R9477354

See the PolySwitch databook for file numbers for each product family.

What is the relationship between IH and IT? Why the gap?
IH is defined as the maximum current at which a device will not trip at temperature (20℃ to 25℃ depending on product family) in still air. In other words, this is the expected maximum operational current at room temperature.

IT is defined as the minimum current at which the device will always trip at temperature (20℃ to 25℃) depending on product family) in still air. In other words, this is the expected minimum fault condition that is expected at room temperature.

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 variance in resistance as well as the change of resistance after a trip event or high temperature installation will determine the closeness in value of IH to IT. For most of our products this makes the 2:1 ratio the most practical specification value.

What happens if the current exceeds IH but does not reach IT?
Depending on a variety of factors, such as:

  * The rise time of the current
  * The length of time the device is exposed to this current
  * The ambient temperature

the device may exhibit a variety of behaviors. It may:

  * Stay in the low resistance state
  * Transition to the high resistance state quickly
  * Transition to the high resistance state after a long time

The range of current values between IH and IT represent 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 it may be a sufficient current to cause the device to switch to the high resistance state. IH defined as the maximum amount of current the device will hold at rated temperature (either 20℃ or 25℃) without tripping. The probability that the device will trip increases as the current approaches IT, defined as the minimum amount of current necessary to cause the device to trip at rated temperature. The device however, if operated above IH, but below IT, could take a long time to trip.

When will a PolySwitch device reset?
The reset condition is a function of the current and voltage as well as the temperature. The device will usually begin to reset when its temperature decreases below 90℃ (We can say that a device below 80℃ has essentially reset). When the following condition is reached: V2/(4*Ri)
The time it takes a device to reset to the low resistance state depends on a variety of factors:

  * Which device it is
  * How it is mounted or fixtured
  The ambient temperature
  * The nature and duration of the trip event

In general, most devices under expected conditions will reset within a couple of minutes although many will reset within seconds, if the current is completely removed, to allow the device to cool.

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:

  * The device used
  * The voltage across the device
  * The power dissipation of the device

The value of this resistance can be found using the following formula: Rt = V2/Pd.

How many times can you trip a PolySwitch device at the maximum voltage and interrupt currents?
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 PTC characteristics. For the telecom devices TR, TS, TC, they have a rating for maximum surge voltage for specific fault events that can occur in telecom applications. This may be as few as ten times or as many as several hundred times with the device still meeting the original specification values. 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.

What is the failure mode of a PolySwitch device?
The typical failure mode of the device is to fail in a high resistance state. This means that the device does not return to its original low resistance value such that it can maintain the original specified hold current. In order to achieve UL recognition, the device must be tripped 6,000 times and still exhibit PTC behavior and stay in a tripped state for over 1000 hours while exhibiting PTC behavior. If a device is subjected to fault events that exceed its rated voltage and current, the device can fail in a fashion that can exhibit arcing and rupture into flame. Abuse beyond the expected use, as outlined by such things as the UL test requirements, can also cause this failure mode to occur as these devices are not recommended for intrinsically unsafe environments.

What is the composition of the coating on PolySwitch devices?
For R-line and TR parts this is a flame retardant epoxy. For strap devices it is a polyester tape. This coating meets either UL94V-0 or IEC695-2-2 requirements.

What is the composition of the solder holding the parts together?
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.

What are the basic differences between a PolySwitch device and a fuse or other circuit protection device? How does a PolySwitch device work with overvoltage devices to provide protection?
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. The typical performance of a PolySwitch device is similar to that of a time delay fuse. Both devices need to have the thermal derating of the device taken into account, but the PolySwitch device does not need to have an I2t derating since it does not degrade as a fuse does under start-up conditions.

When comparing a PolySwitch device to a bi-metallic circuit breaker the main difference is latching not resettability. Both devices are resettable, but the bi-metallic circuit breaker can reset itself even when the fault is still present. This can lead to large EMI spikes on resetting and when tripping and potentially reconnecting a fault condition that could damage equipment and be unsafe. The PolySwitch device will latch in the high-resistance state until the fault is cleared and the power is cycled off and on.

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.

The most common application where PolySwitch devices are used in combination with overvoltage devices are 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 other overcurrent events.

How does the polymer PTC 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 by I2R effect. 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. When the internal temperature of the device reaches 125℃, the change in resistance increases dramatically. 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 sufficient to maintain the temperature of the device and keep the PolySwitch device at the high resistance level. When the power is cycled off and the fault removed, the PolySwitch device is allowed to cool. As the device cools, it contracts to its original shape and reconnects the carbon particles thus lowering the resistance of the device to a level where it can hold the current as specified for the device. This cycled can be repeated multiple times.

What are some of the major applications for PolySwitch devices?
PolySwitch devices are used in many major name brand computers, mobile phone battery packs, telecommunications equipment, televisions, power supplies, industrial controls, automobiles and a host of other applications.

How do the miniSMDC devices differ from standard SMD devices and other miniSMD products?
The miniSMDC family has improved inspectability of the solder joints. The devices are both reworkable and wave solderable.

R-line devices expand a "few" percent when tripped. Will they go back to their original state? Does this happen to other PolySwitch devices?
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.

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 may change if they are exposed to excess humidity and temperature, but under normal storage conditions the shelf life is indefinite.

What are some of the industry standard methods and solvents for cleaning PolySwitch devices?
Water and Freon are both acceptable methods for cleaning PolySwitch devices. Other solvent testing has been performed on PolySwitch devices. See the databook for details.

Does TR600-XXX meet the UL94V flammability testing?
It has been tested to IEC695-2-2 Category D, Needle Flame Test.

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:

  * UL1459
  * FCC Part 68
  * ITU K20 and K21
  * Bellcore 1089
  * FCC Part 68

What does -1 at the end of a part number mean?
The -1 at the end of a part number designates a one-inch-long lead on an radial-leaded part.

What is the maximum temperature the body of the PolySwitch device will reach?
The practical limit is 125℃, but most devices will not exceed a surface temperature of 110℃.

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.