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Effect of High Voltage Pulses on Resistors – ESD and EFT

       Carbon composite resistors perform well under pulsed conditions, but are becoming increasingly difficult to obtain. This article presents the results of testing several modern thin film resistors using high voltage pulses. In this case, the resistor performed well and showed no damage. Figure 1 shows the test setup. A high voltage pulse of approximately 100 ns was applied using a Mini-Zap ESD simulator in contact discharge mode and a Fischer Custom Communications TG-EFT high voltage pulse generator. The TG-EFT waveform simulates the fast transient EFT in a power line. The waveform is similar to ESD, but without the leading edge.
       Carbon composite resistors are the resistors of choice for switching applications, but they are becoming increasingly difficult to find, especially at electronics stores. I needed resistors that would not be damaged by ESD or electrical stress, so I decided to test a few resistors at a local electronics store.
       Several metal and carbon film resistors were tested, ranging in value from 51 ohms (1 watt) to 4700 ohms (1/2 watt). Figure 2 shows a 51 ohm, 1 watt carbon film resistor connected to the Mini-Zap for testing. Also tested were a 51 ohm, 1 watt metal film resistor, a 470 ohm, 1/2 watt thin film resistor, a 1000 ohm, 1/2 watt thin film resistor, and a 4700 ohm, 1/2 watt thin film resistor. The TG-EFT has a maximum voltage of 2000 volts, followed by the Mini-Zap’s 10,000 volts.
       Figure 3 shows three test resistors, including a 1-watt, 51-ohm metal film resistor, a 1/2-watt, 4,700-ohm thin film resistor, and a 1-watt, 51-ohm carbon film resistor. The 51-ohm resistor passes more peak current from the Mini-Zap or TG-EFT, but the 4,700-ohm resistor experiences more voltage. It was not clear before testing which resistors were more susceptible to damage. In the past, I have used an ESD simulator set to about 10 kV to destroy 1-megaohm resistors to test for ESD. Amazingly, all of the resistors withstood dozens of pulses with no change in resistance, as measured by an accurate digital multimeter capable of detecting a 1-ohm change in 1,000 ohms. This is probably because these are 5% resistors and therefore less likely to be laser trimmed. 1% laser trimmed resistors generally cannot withstand high peak pulses because the current is concentrated at the end of the laser cut and the resistance shows cumulative changes over several pulses, often resulting in open circuits and sometimes even short circuits. Results may also be different if simulated 6 kV lightning pulses are used because these pulses are measured in tens and hundreds of microseconds. Longer pulses, such as lightning pulses, are more likely to damage the resistors.
       Fig. 3. Test resistors (from left to right: 1W, 51 ohm metal film resistor, 1/2W, 4700 ohm thin film resistor, and 1W, 51 ohm carbon film resistor)
       All tested resistors withstood dozens of 10,000 V surges from the Mini-Zap ESD simulator. This shows that modern thin film resistors with resistances up to 4700 ohms and power levels as low as 1/2 watt without laser trimming are suitable for applications subject to ESD and EFT stress. For safety, you should use an ESD simulator to test any resistors you plan to use in this way, as regular resistors are not designed for this type of use.
       Equipment related to this tech tidbit: Thermo Scientific Mini-Zap ESD Simulator Fischer Custom Communications TG-EFT High Voltage Pulse Generator
       Mr. Smith has presented presentations on high-frequency measurements, circuit design, electrostatic discharge, and electromagnetic compatibility. He is the author of the book, High-Frequency Measurements and Noise in Electronic Circuits. His website, http://emcesd.com (www.dsmith.org), is extremely popular, with thousands of visitors each month viewing more than 150 technical articles and other topics.
       He also provides consulting services on general design, EMC, transient immunity (e.g. ESD and EFT) and switching power supply noise. His specialty is solving puzzles quickly, often within a few days. His work covers digital and analog circuits in everything from large diesel machines to chip-level circuits. His client base is extensive and includes many well-known large electronics and industrial companies, as well as mid-sized companies and startups.
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Post time: Dec-09-2024