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I'm trying to diagnose a faulty current limiting circuit in a piece of lab equipment from back in the 80s. The fault is too much current.

The tail end of the circuit is below. Q3 represents a complex control/feedback circuit, and R3 is the test load. For the purposes of testing I've tied Q3 base low. Thus the emitter-base voltage of Q1 is 0V.

My understanding is this should shut down the darlington, but I'm seeing a base-emitter voltage on the darlington of approximately 2v.

Does this suggest Q1 or Q2 is faulty? Is there any way of testing the darlington without pulling it out of circuit?

schematic

simulate this circuit – Schematic created using CircuitLab

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    \$\begingroup\$ If you pull the base of Q1 to ground, then Q1 will conduct. 2Volts then sounds pretty reasonable for the base of Q2. That's 2* 0.7 Volts for the two base/emitter junctions in Q2 plus the voltage drop across R3. \$\endgroup\$ – JRE Jan 23 '17 at 6:27
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    \$\begingroup\$ I've pulled the base of Q3 to ground, so this should push the base of Q1 high? \$\endgroup\$ – scrooge Jan 23 '17 at 6:28
  • \$\begingroup\$ @JRE: not the base voltage of Q1 is 0V but the emitter-base voltage. \$\endgroup\$ – Curd Jan 24 '17 at 10:06

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