I found this in CRT TV on the CRT tube driver PCB.

enter image description here

enter image description here

What are they and what is their schematic symbol?

  • 4
    \$\begingroup\$ An overvoltage spark gap perhaps? \$\endgroup\$
    – Dzarda
    May 23, 2014 at 10:02
  • \$\begingroup\$ Was is directly connected to the pins on the CRT? If yes its almost certainly a spark gap. \$\endgroup\$ May 23, 2014 at 10:04
  • \$\begingroup\$ What is that cloudy-white plastic material around it? Is it rubbery, flimsy, solid, etc.. \$\endgroup\$
    – Funkyguy
    Jul 31, 2014 at 18:22
  • \$\begingroup\$ @Funkyguy it is solid, not at all rubbery. \$\endgroup\$
    – Cornelius
    Jul 31, 2014 at 18:23
  • \$\begingroup\$ IS there an actual gap between the wires in side the part labelled with "air gap" from the pictures it looks like a nick in the wires but there could be a small gap in there too. \$\endgroup\$ Jul 31, 2014 at 18:43

1 Answer 1


As noted in comments, the part shown in the photograph is a spark gap. This page lists several such spark gaps and similar / related parts.

Below is an inert-gas filled spark gap, closest in functionality to the part in the question, yet safer due to being glass encapsulated:


A more common modern version of this device is offered by Littelfuse and others, and comes in several different packages:

Littelfuse Spark Gap

The through-hole versions are available on eBay for around $1 to $2 in single units, if you would like to experiment with one.

Various schematic symbols for spark gaps are documented on WikiMedia:

Spark Gap schematic symbols

Of these, symbol #2 is most commonly supported in the schematic software I have used.

  • \$\begingroup\$ To me, #1 is a surge arrester (i.e. metal oxide varistor.) #4 is a generic symbol for variable resistance, i.e. a temperature measuring thermistor. I would use #2 for a spark gap. \$\endgroup\$ Aug 1, 2014 at 4:26
  • 2
    \$\begingroup\$ For info, the Littelfuse style gas-filled spark-gaps are used extensively in telecomms for lightning protection. In fact, give or take every UK phone line has one across the end where it enters the telephone exchange. \$\endgroup\$
    – John U
    Aug 1, 2014 at 9:18

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