I allowed myself to slightly change the circuit - the value of the resistor in the emitter circuit of the second transistor was reduced from 100 ohms to 51 ohms. Thus, the circuit is able to check the integrity of circuits and radio elements whose DC resistance is >5 ohms.
Those. if the resistance of the circuit under study is 5 ohms or less, the probe shows a short circuit (the LED is on), otherwise, the circuit is considered unbroken (the radio element is not broken).
Principle of operation. On the first two transistors, a comparator (comparison device) is assembled, included in the diagonal of the measuring bridge on resistors connected to the emitters of the transistors. The ratio of the resistances of the upper resistors to the resistance of the lower ones is the same, this is the bridge equilibrium condition. It is easy to see that at the indicated ratings, the equilibrium condition is met with a tested resistance of 5 mo.
The input diode is switched on to limit the voltage supplied to the circuit under test (up to Upr=0.2-0.3 V for germanium diodes). This allows you to test semiconductor pn junctions in a small signal mode, i.e. serviceable transitions of diodes and transistors (silicon) at this voltage - do not shift (do not open). And with broken transitions, a short circuit is indicated. It is clear that most inductors cannot be tested with this probe (low winding resistance).
With such a probe, you can check the elements WITHOUT SOLDERING THEM FROM THE SCHEME.
This scheme is taken from the magazine (some old, I don't remember the number) "Young Technician".
Author: A. Belousov, engineer; Publication: cxem.net
See other articles Section Measuring technology.
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