Fast and slow blo still blowing found a diode testing bad

hi i’m try to i’d this diode

fuses blow immediately upon power and this diode is showing low resistance in both directions (in circuit )
but it has strange cathode stripe
it’s about 6mm long
i’m hoping someone has seen this and can id.
thank you!

Possibly SMBJ45A. It lists MV as a part marking in the datasheet.

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Please note that as a general rule, diodes cannot be meaningfully tested in-circuit.

While the ID suggestion given seems reasonable, I’d estimate less than a 5% chance that replacing it would solve the malfunction.

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thanks for the training tip
this machine was hit with a major voltage spike

ive never heard of a tvs diode before
when i take it out of circuit, will it test just like a regular diode with multimeter, with current flowing in one direction ? or do i just have to switch it out
and hope for a 5% miracle :sweat_smile:

TVS (Transient Voltage Suppression) diodes are basically zener diodes that are built and tested in regard to their ability to absorb brief, but substantial pulses of energy in the reverse direction. They’re available in unidirectional or bi-directional flavors, the latter basically being two unidirectional devices back-to-back so as to provide a symmetric behavior.

Unidirectional devices appear to a typical multimeter as a standard diode, whereas bidirectional devices appear open in both directions.

Many devices will “work” when over-voltage protection devices are removed, but are vulnerable to damage in such a state. If such protective components are found to be damaged, there’s a good chance that whatever they were protecting are damaged also.

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thank you! i got a hold of a good board with that same part and just like you explained
the good tvs unidirectional tested just like a diode
so i seem to have a burned tvs because it’s testing a dead short in both directions

part should be arriving soon from digikey in a few days, hopefully i’ll get lucky and nothing else was damaged as it failed

thanks again for tvs explaination

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tvs diode fixed it!!
thanks for all the help and training me on tvs diodes

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