shows the test setup. High voltage pulses with a duration of about 100
ns were applied from 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 electrical fast transients,
EFT, found on power lines. The wave shape is similar to ESD but without
the leading peak.
Carbon composition resistors are the resistors of choice for pulsed
applications but are becoming difficult to find, especially in
electronic stores. I need a supply of resistors that will not be
damaged by ESD or EFT waveforms so I decided to test several resistors
from a local electronic store.
Several metal and carbon film resistors ranging in value from 51 Ohms
(one Watt) to 4700 Ohms (1/2 Watt) were tested. 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, 470 Ohm 1/2 Watt "film"
resistor, a 1000 Ohm 1/2 Watt "film" resistor, and 4700 Ohm 1/2 Watt
"film" resistor. The
TG-EFT generator was used up to 2000 Volts followed by the Mini-Zap at
51 Ohm Film Resistor Stressed by ESD Simulator
Figure 3 shows three
of the resistors tested including a 51 Ohm 1 Watt Metal Film
Resistor, a 4700 Ohm 1/2 Watt "Film" Resistor, and a 51 Ohm 1
Watt Carbon Film Resistor.
The 51 Ohm resistors carry more peak current from the Mini-Zap or
TG-EFT, but the 4700 Ohm resistor sees more voltage stress. It was not
clear before the testing started which ones would be more susceptible
to damage. In the past, I have been able to destroy 1 megohm resistors
used in ESD control by using an ESD simulator set to voltages on the
order of 10 kV.
Figure 3. Test Resistors
(from left to right: 1W-51 Ohm Metal Film Resistor, 1/2W-4700 Ohm "Film" Resistor, and 1W-51 Ohm Carbon Film Resistor)
Surprisingly enough, all the resistors survived many tens of pulses with no
change in resistance as measured on an accurate digital multimeter
capable of detecting a change of 1 Ohm out of 1000 Ohms. This is likely
due to the fact that these were all 5% resistors and as such were not
likely laser trimmed. Laser trimmed 1% resistors often do not survive
high peak value pulses because the current concentrates around the end
of the laser cut and the resistance exhibits a cumulative change over many
pulses often ending in an open circuit or occasionally even a short
circuit. The result would also have likely been different if simulated
6 kV lightning pulses were used as these pulses are measured in tens
and hundreds of microseconds. Longer length pulses, such as lightning
pulses, are much more likely to damage resistors.