Homemade simple magnetic material tester

This circuit is shown in the figure below. IC1 constitutes a square wave oscillator, and the square wave signal it outputs is transformed by the impedance of the Darlington emitter output device composed of Ql and Q2, and then transmitted to the test coil L1. L1 and L2 are a two-wire parallel-wound mutual inductor with only three turns, and the magnetic core to be tested is set inside. The voltage induced by the secondary L2 is rectified by the germanium diode and sent to the microammeter for display. Generally speaking, as the frequency increases, the current value displayed on the microammeter also increases. This change is obvious for a good quality magnetic core; this change is not obvious for a poor quality (i.e., low magnetic permeability and poor frequency characteristics).

The tester can find some magnetic cores with known performance in advance, draw the curve of test frequency and meter current, and use it as a comparative reference for unknown magnetic cores to judge the performance of the magnetic cores to be tested (E-shaped, mouth-shaped, pot-shaped, etc.).

The potentiometer P1 in the circuit is used to adjust the frequency, so it should be installed on the panel. First calibrate it with an oscilloscope and draw the frequency scale.

After the potentiometer P1 is connected in series with the resistor R1, the frequency adjustment range should be between 20 and 100kHz. If it is not accurate, the resistance value of R1 should be adjusted. The microammeter head I used is 500μA, so there is no need to connect R2 in series. If you use one below 300μA, you need to connect an appropriate adjustable resistor R2 in series. Because of the function of Q3, as long as the oscillator works, the light-emitting diode D1 will ignite, so D1 can be installed on the panel as an indicator light. L1 and L2 are hollow skeleton-free coils made by winding three turns of an ordinary electric lamp wire (double wire) about 70cm long, as shown in the right figure below.

For the power supply part, since there is no 220V to 18V converter on hand, it is better to use a voltage doubling rectifier. Although the voltage is large, it will be fine for the whole circuit.