unknown schematic symbol

I know the symbol at the bottom is ground, but I can't get what the other symbol is. It can be a single symbol of two arrows pointing at each other, or two symbols each one a arrow.

I also have the PCB for this schematic. It is nothing but two traces (1mm long) with a little gap in between. Is it a RF filter of some sort?


2 Answers 2


This looks like a spark gap.
(You could also look up gas discharge tube (GDT), which is similar to spark gap.)

The likely purpose of this component in your circuit is to protect the rest of the circuit from lightning strike and/or ESD. Spark gaps are often the first line of defense against high voltage.

Advantages of spark gaps: Once the arc is established, a spark gap acts as a crowbar, and it can dissipate a lot of energy. Because of that, spark gaps are used for protection against high voltage high energy threats (lightning, defibrillator). The parasitic capacitance of a spark gap is low, so it doesn't affect the signal. A spark gap can be formed as a PCB feature for free; it doesn't require an additional component in a BOM.

Weaknesses of spark gaps: They fire at a high voltage, hundreds of volts, and the firing voltage is not well reproducible or predictable. To address this weakness, there's usually another overvoltage protection device (such as TVS) in parallel with a spark gap. This additional device clamps at a lower voltage.

What is this component and what is its use?
EEVblog #678 - What is a PCB Spark Gap?

  • \$\begingroup\$ Exactly my thought. \$\endgroup\$ Commented Aug 15, 2014 at 3:46

It is apparently a symbol for a Spark Gap, thus the SG1. However, I generally see the two arrows enclosed in a rectangular box.


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