I have recently found out that there are FAKE transistors in the market. I have a built a circuit design and when I bought the transistor from the Lajpat rai market in Delhi; I assembled the circuit on a test bench for measurements. But I was surprised that the BD139 did not work with 10mA at 30V and they began overheating. They heat up so much that bubbles appear on the front epoxy and it cracked. I thought that maybe it was damaged so I replaced the whole circuit with a new transistor set. But they also didn’t hold and this time they burnt a resistor along with them. I saw flames engulfing the 1/4 Watt resistors.
So I again made the circuit using the minuscule BC546B. Which neither get warm nor went into thermal runaway.
on the BD139 it was marked that it was manufactured by NXP. And I then checked the NXP website and found out that the NXP closed its manufacturing of this transistor in 2001.
There are a lot of BD139 transistors circulating in the market. It is essential that you must buy from a good source.
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