I'm trying to reverse engineer this Chinese power supply and I came across this component:
Here it is on the circuit board:
And here's a close-up:
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).
Best Answer
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.