Thick film power resistors are used in a wide range of applications. These include power supplies, motor control, load banks, braking control and ESD management.
Common circuit applications include surge and pulse suppression, measurement circuits, snubber circuits and filters. In this post, we discuss the main issues to consider when choosing a high power thick film resistor.
Inrush current limiting
An input current surge can occur at the initial switch-on of electrical equipment. The surge is generally high power but short duration.
When choosing a current limiting resistor, the first step is to define the surge event. This includes pulse width and applied voltage/power.
As inrush surges are short duration, the resistor will not have time to dissipate pulse energy as heat. This should be considered when making any calculations. But those calculations can be complex and making a selection is often based on general rules and experimentation.
Pulse protection
Compared to a surge event, a pulse tends to be a lower amplitude but longer duration. The purpose of a pulse resistor is to suppress the pulse and protect electrical equipment.
As discussed above, the most important consideration when choosing a resistor is the nature of the pulse (amplitude, shape and duration). Then choose a resistor based on manufacturers data or consultation with their engineering team. It may be necessary to approximate (or convert) the shape of the pulse to match those shown on manufacturers datasheets.
Load banks
The purpose of resistor load banks is to test electrical equipment including motors, UPS systems and solar inverters. The resistance value is usually variable to simulate various loads.
Select resistance values to give the required resistance value (or values) with the minimum number of resistor components. The key issue is the dissipation of the significant heat generated by the current flow through the load. Cooling can be via forced air or immersion in de-ionised water or oil. Load banks use high power resistor devices.
Snubber resistor
Snubber resistors reduce the impact of voltage spikes that could damage sensitive components. They also reduce the impact of EMI interference.
A snubber circuit generally uses a resistor and capacitor connected in parallel. The resistor should be capable of carrying the current generated by the capacitor discharge. It should also be capable of dissipating the heat generated during the relatively short discharge period.
The resistor value should be as low as possible to minimise power loss. Low inductance is also an important consideration.
Current sense resistor
The current through a resistor causes a voltage drop. That voltage drop can then be measured and appropriate action taken based on the result.
There are three key issues to consider when choosing a current sense resistor – noise, resistor tolerance and TCR. If the voltage drop is low, noise can have more of a (percentage) impact on the result. But increasing the voltage drop increases the resistance value and increases the power loss.
A high resistor tolerance value will cause a high variance in the resistor value and, therefore, the voltage drop. A high TCR will cause a high variance in resistor value with temperature. Both will cause measurement errors.
Harmonic filters
Harmonic filters suppress harmonic frequencies that would otherwise compromise the efficiency of an electrical system.
Choosing a resistor for a harmonic filter is a complex task. It requires an understanding of the entire electrical system, the range of harmonics and their potential impact.
Grounding resistor
Grounding resistors protect equipment in the event of a fault to earth. A resistor is inserted between neutral and earth to limit current to a safe level.
A grounding resistor must absorb high currents, and this is the key consideration when choosing the resistor. Thick film resistors are common in low to medium current applications.
Braking resistor
Braking resistors slow down or stop motors while minimising losses.
There are a wide variety of loads and therefore a variety of resistor technologies used in braking applications. Thick film resistors are used for smaller loads.
Key considerations when choosing a braking resistor are safety, power dissipation and heat management.
The resistor should be fail-safe. It should withstand high current flow and surge/pulse events and be able to dissipate the heat generated by the high current.
An extensive range of resistor technologies and devices are available from a wide range of manufacturers. When choosing a power resistor for a particular application, it is useful to use the services of the manufacturers’ application and sales support teams. This is particularly important where safety is an issue.
If a standard device is not adequate application-specific thick film power resistor manufacturers can produce a custom device.