BT139-600G Triac Equivalent

allison allisonportable at
Sun Dec 17 08:23:27 CST 2017

On 12/17/2017 08:24 AM, Peter Coghlan via cctalk wrote:
>> I have a suspicion that this component may be faulty on the input side of my
>> H7826 PSU. A little tester I have does not recognise it, it is possible that
>> the currents it uses are too low for this particular triac, but I am not
>> sure.
> What is the function of this triac in your power supply?
> I have come across a triac which is used to switch between 115V and 230V input
> in the PSU of a DEC 3000/300.  My triac went short circuit which caused some
> release of magic smoke when the PSU was used on 230V.  I also had difficulty
> finding a replacement for it so I ended up leaving it out.  The PSU worked
> fine on 230V only without it once I replaced the other damaged components.  
> If your triac has a different function, ignore the above.
> Regards,
> Peter Coghlan.

The common use is to create a hard short circuit across the output if
the output voltage
exceeds a safe limit.  The logic is that a power supply is cheap
compared to the rest
of the computer.  The typical output where this is done is the 5V rail.

The tester likely cannot test a triac its not a simple transistor or
diode.  They are easy to test
a light bulb (12V) and a DC source.


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