September 1, 1997

Spark gaps provide ESD protection

Bill Ehlers, PSC Inc, Eugene, OR

Using a spark gap fabricated as part of the conductor pattern of a pc board provides significant protection against ESD damage to an interface circuit. Working with inexpensive passive components, spark gaps can protect interface connector pins against repeated 25-kV discharges. And if you position the spark gap under the connector body, no additional pc-board space is necessary.

You can incorporate the spark gap, nominally 0.008 in., as part of the pc-board mounting pattern of a through-hole connector (Figure 1). The purpose of the gap is to provide a large and robust area for arcs to jump across. A single point-shaped gap also works but is not as durable or repeatable over variations in the pc-board manufacturing process. The pattern with the spark gap should sit under the connector on the side of the board opposite a solder wave or hand-soldered joint. Don't apply the solder mask over the gap.

Tests of a 0.008-in. spark gap etched on a pc board revealed it's effectiveness. A 1000-1 Tektronix (Wilsonville, OR) scope probe measured the voltage across the gap. The KeyTek (Lowell, MA) ESD test system and a discharge network consisting of a 150-pF capacitor and a 300 ohm series resistor created the ESDs. The arc was directed to a wire soldered to the spark-gap pattern to simulate an ESD strike on the interface cable or connector pin. The return path (ground lead) for the ESD connected to the larger side of the spark gap. Noise effects caused some false triggers of the scope, but these false triggers were easy to distinguish from the real waveform captures. With the KeyTek model CIA-V contact-current injector providing a 7-kV discharge to the spark gap, the peak voltage across the gap was 2000V. A 15-kV air discharge to the spark gap resulted in a 2500V peak voltage across the gap.

In addition to the spark gap, RS-232C inputs require a series 500 ohm resistor. Both the MAX202 (Maxim Integrated Products, Sunnyvale, CA) and the ADM232A (Analog Devices, Norwood, MA) can withstand more than 20 discharges at 25 kV. The spark gap arcs over, limiting the voltage on the IC. The series resistor then limits current into the ESD-protection structure of the IC. The series resistor should be size 1206 to better withstand the high voltage (the parts' mounting pads are also spaced farther apart than in a 0805 pattern). The series resistor forms a voltage divider with the resistance of the driving RS-232C transmitter and cable. A weak transmitter and a long cable degrade noise margins. However, because the typical cable in a PSC bar-code-scanner installation is less than 25 ft long and the data rate is low, the resistor has little effect on a typical application.

RS-232C driver outputs cannot take advantage of the spark gap. You can't use any series resistance with these outputs, and the output impedance is low. A Motorola (Phoenix, AZ) MMBZ15VDLT1 Transzorb connected from the drive output to ground protects the IC from dozens of 25-kV discharges. Because the Transzorb clamps at approximately 15V, the spark gap never arcs over. Note that other RS-232C parts have ESD protection to 15 kV, such as some from Maxim and Linear Technology Corp (Milpitas, CA). As with RS-232C outputs, these parts do not benefit from a spark gap because their internal circuits clamp the voltage on the pin before a gap can arc. These parts add approximately 40 cents to the cost of an RS-232C-transceiver design, whereas two resistors and two Transzorbs add only 20 cents.

In addition to the spark gap, 74HC logic inputs require a series 10 kilo ohm resistor. A National Semiconductor (Santa Clara, CA) 74HC14 withstood more than 20 ESDs at 25 kV. The spark gap arcs over, limiting the voltage on the IC. The series resistor then limits current into the built-in clamping diodes of the IC. For the same reasons as in the RS-232C-input case, the series resistor should be size 1206. The series resistor adds a propagation delay of 80 nsec on the rising edge and 60 nsec on the falling edge.

74HC outputs also benefit from the spark gap. Combined with generic silicon clamping diodes to V_CC and ground, a 100 ohm series resistor protects a National 74HC14 output from dozens of 25-kV ESDs. A 47 ohm resistor did not protect the sample part as well, causing a failure after only one or two 25-kV discharges. You need to carefully evaluate the value of the series resistor for each application.

Finally, as with RS-232C drivers, RS-485 drivers cannot take advantage of the spark gap. You must protect these drivers with Transzorbs, one on each line to ground. Two Motorola MMBZ15VVDLT1 Transzorbs protected a National 36C278 driver against dozens of 25-kV discharges. (DI #2072)

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