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This is a part of a well known product created by Markus Frejek and now maintained by Karl-Heinz Kübbeler (more info). It is the famous "transistor tester" or "component tester" (theses guys here).

Part of the schematic

So, how does the transistor T3 (inside the red box) turn on?

I guess T3 base is not connected to ground because T1 (inside the blue box) isn't active, or is it?

In order for to T1 activate, the MCU needs to pull up T1 base via R8 but it needs power to do so and power is only delivered to the MCU via T3.

I know there's something wrong with my beginner's analysis. Could some pro show me the way?

By the way, the full schematic can be found here (page 10).

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    \$\begingroup\$ Notice that the Test button will turns-on both T3 and T2 at the same time. Next, MCU turns on T1 to keep T3 permanently turned on. \$\endgroup\$
    – G36
    Aug 13 at 15:07

1 Answer 1

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Pressing the test button will allow current to flow from T3's base through T2's base-emitter junction to ground. The T3 base current will turn on T3 to power up the MCU.

The button needs to be held on long enough for the MCU to wake up and turn on T1 which maintains the power-on.

When the MCU decides to go to sleep it will turn off T1 and, provided the test button has been released, the circuit will power down.


From the comments:

Isn't T2 off at that point? I thought BJTs base-emitter current is related with emitter-collector current (beta?). There is no VCC feeding the collector.

That's only true when the transistor is operating in its linear region. If there is no VCC then it can't and the base-emitter junction behaves like a diode.

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  • \$\begingroup\$ "Pressing the test button will allow current to flow from T3's base through T2's base-emitter junction to ground". Isn't T2 off at that point? I thought BJTs base-emitter current is related with emitter-collector current (beta?). There is no VCC feeding the collector. \$\endgroup\$
    – Rodo
    Aug 13 at 17:11
  • \$\begingroup\$ @Rodo, see the update. \$\endgroup\$
    – Transistor
    Aug 13 at 17:50
  • \$\begingroup\$ this is a clever design! thanks for explanation! \$\endgroup\$
    – raffs
    Aug 13 at 22:06

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