



Figure 2. Test Setup for Measuring Induced Current in One Meter Cable







Figure 3. Spark Gap EMI source for Experiment







Figure 4. Test Setup for Measuring Induced Current in One Meter Cable [1]







Figure 5. Discharge at 3 m From Cable and Current Probe - #1

(Vertical scale ~ 5 mA/div, Horizontal Scale = 2 ns/div)







Figure 6. Discharge at 3 m From Cable and Current Probe - #2

(Vertical scale ~ 5 mA/div, Horizontal Scale = 2 ns/div)







Figure 7. Discharge at 1 m From Cable and Current Probe - #1

(Vertical scale ~ 10 mA/div, Horizontal Scale = 2 ns/div)







Figure 8. Discharge at 1 m From Cable and Current Probe - #2

(Vertical scale ~ 10 mA/div, Horizontal Scale = 2 ns/div)





Figure 9. Discharge at 30 cm From Cable and Current Probe - #1

(Vertical scale ~ 20 mA/div, Horizontal Scale = 2 ns/div)





Figure 10. Discharge at 30 cm From Cable and Current Probe - #2

(Vertical scale ~ 20 mA/div, Horizontal Scale = 2 ns/div)







Figure 11. Null Experiment Discharge at 30 cm From Current Probe





Figure 11. Null Experiment Discharge at 30 cm From Current Probe - #1

(Vertical scale ~ 5 mA/div, Horizontal Scale = 2 ns/div)







Figure 12. Null Experiment Discharge at 30 cm From Current Probe - #2

(Vertical scale ~ 2 mA/div, Horizontal Scale = 2 ns/div)



There are many devices very sensitive to ESD in use today. They carry an ESD damage threshold rating of Class 0, less than 50 volts Human Body Model. For such devices, especially the very sensitive devices like modern disk drive heads, damage from ESD remote from the device is a real possibility. ESD must be controlled not just at the component, but in the nearby environment as well.





Figure 1 shows the test setup used for the experiment (the first one done in my new Boulder City, NV facility) and Figure 2 shows a close-up of the test table. A one meter Cat 5 Ethernet cable was used and the current in the cable was measured by a Fischer F-65 current probe.The ESD source is shown in Figure 3. It is described inon this site. ESD is generated in the very small gap between the points of the strips of copper foil tape when a charged object is brought close to one of the copper foil strips. This is caused by electrons jumping across the gap due attraction or repulsion by the applied electric field. Figure 4 shows a charged strip of Teflon™being passed back and forth along the plastic backing of the copper foil strips. This generates hundreds of ESD events between the copper foil strips as electrons are chased back and forth across the gap. I estimate the breakdown voltage of the gap to be several hundred volts. The rising edge of the radiated field is on the order of 200 to 300 ps.Three distances were used between the ESD events and the one meter cable/current probe: 3 m, 1 m, and 30 cm. Figure 5 shows the induced current at 3 m. The ringing in the waveform is due to the resonant frequency of the copper foil strips (~500 MHz) and to some extent the lower resonant frequency of the 1 m cable. Note the peak current of about 17 mA. One of the most sensitive devices in use today are the read heads in disk drives. They have an ESD Human Body damage threshold of just a Volt or two (not thousands or even hundreds of volts, a few volts)! [2] It takes just a milliamp or two for a nanosecond to destroy the head. [2] Clearly, if such a head was not in a shielded enclosure and connected to wires (for testing possibly), it could easily be damaged from across the room. If one were to split the 1 m cable in the middle to form a dipole, the two terminals at the middle of the dipole would have enough voltage between them at a 70 Ohm source impedance to damage a disk drive head from a voltage point of view as well.Figure 6 shows another typical current waveform generated showing greater than 20 mA flowing in the 1m cable, an even worse case. This is enough to damage even the older GMR heads of 15 years ago. Those heads were damaged by 20 mA flowing for a nanosecond.Figures 7 and 8 show the measured currents at a distance of one meter from the ESD events. The peak current for this distance was over 40 mA!Figures 9 and 10 show the measured currents at 30 cm. The peak current is greater than 80 mA in Figure 10. The induced current from a remote discharge having more energy could be much larger than this.In order to check for EMI from the ESD directly into the measurement, a null experiment, the 1 m cable was removed from the current probe, leaving the probe alone on the table as shown in Figure 11. ESD was generated as before at about 30 cm and the results are shown in Figure 12 and 13. The peak reading recorded was about 4 mA. This represents less than 5% error compared to the greater than 80 mA displayed in Figure 10, an acceptable amount of error.