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I'm trying to reverse engineer this Chinese power supply and I came across this component:

Here it is on the circuit board:

Component on the PCB

And here's a close-up:

the component

It has a glass packaging, similar to a diode but it isn't polarised. It also has a red strip on the middle.

At first I thought it was a thermistor, then I tried measuring it's resistance while applying heat but it didn't change (always open loop).

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1 Answers1

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It is most likely a low-voltage low-current glass gas discharge (GGD) tube. It will not conduct at all until a certain voltage is reached, then the gas ionizes and it become effectively a short circuit until the current is removed. This small size is to suppress back-EMF, that is short-term low-current pulses. These devices can have capacitance values as low as 1.5 pf, making them useful in RF and microwave circuits to about 2 GHZ (in an smd package). For higher currents Transzorbs or MOV's would be used.

EDIT 1: Glass gas discharge (GGD) tubes are sensitive to fast rising or falling voltages. Rise and/or fall times less than 10uS can cause the gas in the tube to ionize quickly, at a voltage much lower than expected. It is a great idea to read all the fine details of a GGD before using them. In some cases such as back-EMF the fast rise time is an advantage, as the GGD will clamp it at a lower voltage than its normal slow rise-time clamp voltage.

EDIT 2: This could also be a tiny Transzorb. They are much like back-to-back zener diodes but can only handle brief burst of low currents. Transzorbs are not meant to be voltage regulators.

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    The G1 and G2 reference designators on the board sure are a big hint as to a gas discharge device. – Michael Karas Jul 14 '16 at 00:36
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    @MichaelKaras. Agreed. I took those as a clue myself, and saw nothing to contradict that assumption. The "Tranzorb" comment was just to 'cover all the bases'. –  Jul 14 '16 at 00:48
  • Transzorbs is news for me.... Used to know TVS for years but not transzorbs. – soosai steven Jul 17 '16 at 00:42
  • @soosaisteven. Thanks for pointing out the typo-and I worked with them for 15 years... –  Jul 17 '16 at 00:47