Why does two PCBs with same components and same functionality draw different amount of current?

by Jayakumar.k
August 13, 2021

Two same transistors can be different to the point that if the conception of the circuit doesn't take that into account, the result can differ. As example, you rarely see two BJT in parallel since one will lightly differ from the other and that would be enough for one transistor taking almost all the current while the other, in parallel, could barely conduct.

by vanderghast
August 13, 2021

Thanks for your replay. I think your replay holds good when both the PCBs are connected parallel. What if i observe diffrent currents when i test one after the another.

by Jayakumar.k
August 14, 2021

If you take a look at the datasheet of a transistor, like the P2N2222A, you should be able to see that its beta, or $h_{fe}$ can vary, for a given $I_C$ , $V_{CE}$ and temperature, from 100 to 300, or a factor of 3. There are ways to minimize that variation in the circuit, but if the designer didn't, you may have a variation by a factor of 3, just there.

by vanderghast
August 14, 2021

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