Simple Tester Probe Circuit

You might think you can just use a multimeter to test all electronics, but that is not always safe for special parts called semiconductors.

This article shows you how to build a simple tester specifically for semiconductors that wont damage them.

This tester is easy to build and uses a safe amount of current to test different types of semiconductors.

What is a Tester Probe Circuit:

The circuit tester probe serves as a versatile measuring and testing tool for swiftly assessing the state of a electronic circuits.

Additionally, it is employed to verify the accuracy of certain electrical components and devices.

Circuit Working:

Parts List:

T1 and T2 form a voltage controlled LF oscillator.

The loudspeaker acts as a load in the oscillator circuit.

The oscillator frequency is determined by C1, R1, R4 and the external resistance between the measuring leads.

Resistor R3 serves as the collector resistance of T2, and C2 acts as low frequency decoupling for this resistor.

Oscillator Operation:

The oscillator frequency can be adjusted for testing purposes.

The loudspeaker provides an audible indication during testing eliminating the need to constantly refer to the testing device.

Safety Features:

The circuit ensures that no harm is caused to the circuit under test.

• Diodes D1 and D2 are incorporated to prevent the circuit under test from damaging the tester components.

Power Efficiency:

The circuit draws minimal current when there is no electrical connection between the testing prods.

This design ensures a longer battery life approximately equivalent to the shelf life of the battery.

Formulas:

The time constants connected to these cycles of charging and discharging are approximately correlated with the oscillation frequency (f):

f = 1 / (T_charge + T_discharge)

where,

• The charging time constant, or T_charge is dependent on R and C.
• The discharging time constant, T_discharge is dependent on R and C during the discharging process.

Methodological Analysis:

Nonetheless, we may perform a qualitative analysis of the circuit to determine the parameters influencing the frequency and to approximate a range:

Charging Phase: One BJT activates when the voltage across the capacitor C drops, enabling the capacitor to charge via a particular path that is dictated by the resistors.

The resistance value and the charging current determine the charging rate.

Discharging Phase: The BJT changes characteristics and shuts off when the capacitor hits a specific voltage threshold.

Through a new set of resistors, a second conduit for the capacitor to discharge is formed.

The resistance that is present affects the discharging rate as well.

How to Build:

• Connect T1 and T2 as per the specified configuration.
• Integrate resistors R1, R3 and R4 into the circuit.
• Add capacitors C1 and C2 for frequency control and decoupling.
• Incorporate diodes D1 and D2 for protection against potential damage.
• Connect the loudspeaker as the load in the oscillator circuit.
• Ensure proper wiring of measuring leads for accurate testing.

How to test:

Testing the DIY semiconductor tester probe involves a few simple steps to ensure its functionality and accuracy.

Power On:

• Ensure that the battery is properly installed in the circuit to power the semiconductor tester.

Connect Measuring Leads:

• Connect the measuring leads to the corresponding points on the circuit.
• Ensure that there is no electrical connection between the testing prods before connecting to the semiconductor or IC.

Identify Test Points:

• Identify the test points on the semiconductor or IC that you want to test.
• Refer to the datasheet of the component for guidance on the correct test points.

Touch Test Points:

• Gently touch the identified test points on the semiconductor or IC with the measuring leads.
• The loudspeaker should produce an audible indication, confirming that the circuit is operational.

Listen for Indication:

• Listen for changes in the tone or intensity of the audible indication from the loudspeaker.
• This variation can help identify different states or characteristics of the semiconductor under test.

Disconnect and Repeat:

• After testing one set of test points disconnect the measuring leads, and repeat the process for other test points on the semiconductor or IC.

Verify Results:

• Verify the results against the specifications or characteristics of the semiconductor or IC being tested.
• Confirm that the observed indications align with the expected behavior.

Turn Off:

• Turn off the semiconductor tester when testing is complete to conserve battery life.

Conclusion:

This DIY solution provides a reliable method for passive testing without the risk of causing harm to the circuit under examination.

References

Test probe

Datasheet 2N2907