Trying to identify a component in a switching power supply?

Question

Have an old PC switching power supply with a component labeled DA1 in its circuit board. Can anyone tell me what it is? It looks like a glass enclosed, small signal diode but with only a black band around its middle, not at one end such as a diode would have. I assume it's not a diode as elsewhere on the board there are other components I see as diodes which have a marking band around only one end, they are labeled D1, D2, D3, etc., and the board markings indicate which way these parts should be oriented on the board. The markings for DA1 do not indicate any such orientation, as would be necessary if it were some kind of diode. Might anyone know what this part is?

The switching supply seems to work only every umpteenth time power is applied to it, and the DA1 component's location in the circuit appears related to the main switching circuit's initial starting drive for before the main regulation pulse width feedback loop gets going. Identifying its function seems important.

Answer 1

Device Adapter switch!!!!!!1

Answer 2

Zero ohm resistors (used to do things like connect the analog ground to the digital ground at one specific location, etc.) typically have a single black band around the center, but I've never seen one made of glass! Or possibly it is a pair of anti-parallel diodes in one package? If so, should have never more than 0.6V drop across it in either direction. Other than those 2, I can't think of what it might be...

Good Luck -
GH, a fellow tech/eng

PS - for what it's worth, the vast majority of switcher problems I 've seen over many years is, second, the driver FETs/transistors, and first, particularly in higher-milage supplies, the friggin' transformer. They just wear out, believe it or not, due to temp cycling and 1000V spikes on the switching edges from parasitics, etc.

Answer 3

If it's in a glass envelope, then it is probably a transient voltage supressor (TVS) or metallic oxide varistor (MOV). TVSs are back-to-back zener diodes so there is no 'orientation' a ssociated with the part. Likewise MOVs are bi-directional devices.

Both are used as ESD protection, and for conducted EMI supression.

If DA1 is intermittently 'short', then it would definitely interfere with the operation of the power supply. If DA1 is indeed a transient suppression device, then removing it (temporarily) will not affect the operation of the power supply.

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Answer 4

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