Power Resistors Put the Brakes on Medical Motors

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Feedback loops are the bread and butter of control systems. To prevent something from spiraling into larger and larger oscillations or spiked values, it’s critical to bring them back into normal ranges. 

Not only is that the case for electrical circuitry, but for physical systems. Any moving device under the power of a motor of any sort typically needs a braking system to control speed and bring something to a complete halt. 

In the medical field, that is a surprisingly wide set of equipment and devices. Hospital beds have motors, as do CT, MRI, and X-Ray machines. Physical therapy equipment, booms, and even robotic surgical systems are motorized. Medical pumps, lab automation, biosafety cabinets, and ventilators and many more medical applications may require braking systems.  

Medical devices that use motors, whether rotor, linear, or any other form, need to control the movement activity. Forego a braking function and the result is disastrous, with malfunctions and errors on the lighter side and injuries and death potential outcomes. 

Some form of electrical braking, rather than depending on mechanical friction alone, is generally called for, which means eliminating kinetic energy from the system, typically in the form of heat. 

Power resistors, with their ability to handle and dissipate high transient currents are an intrinsic part of dynamic braking systems. The same underlying technologies and component designs that can slow and stop objects as large as trains and elevators can handle the energy loads in medical devices and equipment. 

However, not all power resistors, even though frequently used in dynamic breaking, are necessarily a fit. Medical equipment and its uses provide significant constraints. The space available is often restricted. That not only means a need for smaller parts, but less room for air circulation, and so a requirement for especially efficient heat transfer away from other components. 

Power resistors for breaking functions in medical equipment should be relatively low profile and encased in metal and often compatible with heatsink mounts for increased and faster power dissipation. 

For example, Ohmite’s type 270 have wattage ratings from 12 to 1000 watts with an overload capability of 10 times the rated wattage for five seconds. The 280 is for low resistance values, from 0.1 ohm for a 300-watt unit, with special order units available for up to 1,500 watts and the ability to withstand frequent start-stop cycles. 

HS aluminum housed resistors are designed for direct heatsink mounting with thermal compound with power dissipation from 15- to 300-watts. While the use of proprietary heat sinks with lower thermal resistances is acceptable, uprating is not recommended. For maximum heat transfer it is recommended that a heat sink compound be applied between the resistor base and heat sink chassis mounting surface. It is essential that the maximum hot spot temperature of 200°C is not exceeded, therefore, the resistor must be mounted on a heat sink of correct thermal resistance for the power being dissipated. 

HS series resistors are manufactured in compliance with MIL 18546 and IEC 115 specifications and are also RoHS compliant. Designed for direct heatsink mounting with thermal compound to achieve maximum performance, the HS series offer high power to physical volume ratios and are wound to maximize high pulse capacity. At high ambient temperature dissipation derates linearly to zero at 200°C.  

Models offer values from 5 ohms to 100,000 ohms, depending on wattage. There are also low inductive (NHS) models, indicated by an N before the HS code, for example, NHS50 would be a 50-watt low inductive aluminum housed resistor. Custom design HS series are also available. 

The quality of parts is important in medical equipment designs, which are critical applications in service of human health and life, so work with a vendor who can deliver quality. Ohmite, for example, has decades of experience in the needs and requirements of the medical industry and is well-known for its expertise in power resistors of all types, including those used in braking. 

While descriptions and specs on Ohmite’s website will be helpful, they aren’t the only resource the company offers. Contact Ohmite and communicate with an expert who at no charge can help you specify the best type of filter for your application and requirements.