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Stackpole Product Notice -Stackpole Product Highlight - UL Recognized Resistors


RALEIGH, NC (July 4, 2011) - UL Recognized Resistors

UL Recognized Resistors

This product highlight will discuss Stackpole’s UL Recognized resistors. Many different types of applications from TV’s and displays, to home appliances require a UL recognized resistor. Now many other types of end products are using the failsafe requirements defined by UL for their designs as well. Stackpole recently received UL 1412 recognition for their SP3A 3 watt failsafe wirewounds. This document will give a brief overview of the requirements of UL1412 specifically, and will discuss Stackpole’s UL recognized products and their performance in relation to those requirements.

UL1412 Requirements

UL 1412 was a spec designed to reduce the risk of fire or electric shock in radios, TV’s, and displays. The principles of this spec are also used in utility meters, high end fire safety equipment, electric hand tools, and home appliances. The key tests required in UL1412 are as follows:

  • positive opening test which ensures that less than 1% of the rated current is conducted after the part opens;
  • opening time calibration test, which requires all tested resistors to fall within a minimum and maximum fusing time specified on the manufacturer data sheet; the SP3A graph is shown below;
  • two separate overload tests which require the resistors to fuse without flame, significant spark, or significant heat rise such that a piece of cheesecloth placed on the resistor does not ignite.

The above tests are not difficult for other types of resistors to pass, such as fusible film resistors. The problem with fusible film resistors is that they are not able to handle surges of any magnitude without fusing or significant resistance shift. This presents the opportunity for a unique fusible wirewound which can withstand thousands of 6KV surges without opening or significantly shifting, but will still fulfill the failsafe requirements of UL. However creating a fusible wirewound that fails in a safe manner without sacrificing its surge handling isn’t a trivial task. Some fusible wirewounds are created by weakening the weld from the element to the end cap on one end, or by mechanically reducing the resistance element diameter in the middle. These methods may create resistors that have inconsistent fusing characteristics and will inhibit their ability to withstand the required voltage surges. Stackpole has a proprietary design which doesn’t reduce the short duration pulse handling of the resistive element, but focuses the energy from a continuous overload on the element wire, causing it to fuse rapidly and consistently without excessive heat.

SP3A 3 Watt UL1412 Recognized Failsafe Wirewound Resistors

The UL recognized SP3A (file no. E344795) provides an answer for customers who have needed a robust resistor to withstand repeated voltage surges yet fuse without causing catastrophic damage to critical components. The fusing characteristic chart above shows the max fusing time at 140 watts is around 10 seconds. 120 VAC across the highest resistance value currently available in the SP3A, 100 ohms, would imply 144 watts of power. This would be the worst case fusing scenario; all other resistance values will fuse faster. This fusing is well under the typical 30 second time to fuse normally specified for fusible resistor types and is what helps set the SP3A apart from other fusible resistors as a failsafe protective device.

For the protection of small electric motors such as those used in washers and dryers, the SP3A is put in series with the motor. In the event of power supply failure, the SP3A fuses and protects the motor. If the motor fails or becomes stuck, the SP3A fuses to protect the power delivery circuitry. In both cases, the SP3A serves to isolate and protect the two sides from each other, which is important because of the relative cost of each. For protection in utility meters, the SP3A is used in series with an MOV, which is then placed in parallel to the meter itself. This allows the SP3A and MOV combination to survive thousands of lightning events without failing, but will fuse quickly and safely under continuous overload, such as when the meter is incorrectly connected to the building.

Although only the 4 Watt, SP3A is UL recognized, Stackpole can provide this type of fusible response in other sizes, resistance values, tolerances, and wattages. This technology can also be applied to surface mount molded wirewound resistors. These custom parameters would mean the part would no longer be UL recognized, but would meet the requirements for UL recognition. Stackpole can also tailor other parameters to fit special customer requirements, such as pulse power handling, pulse voltage handling, and inductance. Not all parameters can be changed at the same time, as there are tradeoffs when adjusting a resistor design to achieve a certain parameter. Stackpole has over 30 years of engineering experience building wirewound resistors and can help guide design engineers into making the best choice based on their desired performance factors.

Other UL Recognized Resistors

Other UL recognized resistors from Stackpole include the RWT series of thermal fusing wirewounds, file no. E255350, and the SPR series discharge path film resistors which are UL1676 recognized. The RWT series is a wirewound resistor with a 142C thermal fuse integrated into the ceramic vertically mounted housing. This also provides a product with excellent surge characteristics and yet very benign failure modes under continuous overloads. The RWT series can support power ratings from 1.5 to 2 watts, with the RWT5 rated at 1.6 watts recognized by UL. The SPR series is a metal film axial leaded resistor series qualified to UL1676. The SPR offers a 0.5 watt and 1 watt power rating in UL recognized resistance values from 1M to 12M ohms. Other sizes and resistance values with the same technology are available as the ASR series, but they are not currently recognized by UL.

Pricing for these three UL recognized devices is as follows:

  • SP3A = $0.18 to $0.25 each.
  • RWT = $0.30 to $0.36 each.
  • SPR = $0.05 to $0.10 each.

More Information and Data Sheets Available At:

For questions on these and any other Stackpole product please contact Stackpole at:

Stackpole Electronics, Inc.
2700 Wycliff Road Suite 410,
Raleigh NC 27607
phone: 919-850-9500

Stackpole Electronics Inc. is a leading global manufacturer of resistors supplying to the worlds largest OEMs, contract manufacturers and distributors. Headquartered in Raleigh, N.C., the privately held company began manufacturing in 1928 as part of Stackpole Carbon Company in St. Mary's, Pennsylvania. Now affiliated with Akahane Electronics, Stackpole has manufacturing facilities in Japan, Taiwan, China and Mexico; warehousing facilities in El Paso, Hong Kong and Japan; and sales offices in Tokyo, Hong Kong and Taiwan.


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