Universal Test Probe

  
The compact test probe provides rapid `measurement` of voltage levels at digital gates, fuses, diodes, batteries, and
Universal Test Probe - schematic

others. It does not provide absolute values, but rather it provides a good indication of correct operation or otherwise. Measurements are carried out with pins A and B. If the potential difference between A (the reference pin) and is 1.9 to 2.0 V, D2 will light. If the voltage at is < 1.4 V higher than that at A, D3 will light. Finally, if the potential at is > 11 V, with respect to that at A, Dl will light. Transistor T5 is used as a zener diode. The probe allows the measurement of alternating voltage. The maximum input voltage is highly dependent on the dissipation allowed in Rl. For example, when this resistor is a 0.5-W type, the input voltage can be as high as 200 Vrms. The current drawn by the circuit depends on the number of lighting LEDs: it is not more than 10 mA at a supply voltage of 3 V. In quiescent operation, the current is so low (about 5 ) that an on/off switch is not necessary.




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