Method tests continuity in AC and DC

-October 08, 2012

A specialized test gadget that tests continuity quantitatively in both AC and DC modes could prove to be an indispensible bench test instrument. This article describes such a continuity test set up, which provides two different display options.

The meter (see Figure 1) can be used in AC mode to test/estimate capacitors, inductors, transformers etc. that are sensitive to AC current, while the LEDs can be used in AC/DC mode to test diodes, transistors, transformers, etc.

Figure 1: Photo of an actual prototype with simple switching arrangement using jumper pins and a couple of switches.

The LEDs serve to indicate the direction of current flow, which is preferred when testing components like diodes, transistors or sometimes simply continuity.  The meter, for instance, can also be used to detect the primary and secondary sides of a transformer. It can also estimate capacitance or inductance provided that a reference capacitor is chosen. By choosing an adequate frequency and noting the deflection of the meter for the reference capacitor/inductor and then comparing this with the deflection produced for the unknown capacitor/ inductor, it is possible to have good idea of the value of the unknown capacitor/ inductor. If carefully built, this test set up can indicate the value of the capacitor/inductor directly on the meter scale, which would be linear with the value of the capacitor/ inductor (from the FSD side).

Because the LEDs indicate the direction of current flow in AC mode, they are useful in testing diodes (pn junctions) while simultaneously knowing their sides. In the case of transistors, care must be taken not to reverse breakdown the BE junction which may degrade the transistor. If a breakdown occurs, you can try putting a pair of signal diodes antiparallel with each other in series with the LEDs. Each color of the LED corresponds to a particular orientation of the pn-junction. The LED color can correspond to the junction side type for a particular probe tip (A/B, here, for example, the crocodile clip red-N_side, green-P_side).

Performing two tests for a transistor can unambiguously tell about its type and base terminal. The frequency of the AC can be chosen as desired by means of a three-throw (or more) switch that can cover a wide frequency range. This enables the testing of wide values of capacitors and inductors (including transformers as well). The circuit in this example is powered by a pair of rechargeable NiCd cells (3.6V, 60mAH). This makes the tester highly portable, compact, and complete (see Figure 2).  It should be noted that this instrument should have supply voltage less than ±4.5 V for the transistor rectifier to work properly. Useful formulas corresponding to time period of the waveforms are

where VT is the threshold voltage of the NAND gate, V is the supply voltage, and R1/R2 are the resistances as indicated in Figure 2. These formulas hold better if a high resistance is included at the input of G2.  The frequency of the astable is simply the inverse of their sum, i.e f = 1/(t G1high + t G2high).

Figure 2: Schematic circuit diagram of general continuity tester.  The potentiometer of 50k serves to adjust the deflection of meter to full scale when the test probes (A,B) are shorted. A series variable resistance of 5k along with the 50k helps in fine tuning this adjustment. The low base resistor for the four transistors favors estimation of low inductance. It is the main contributor to the R/L time constant.

Functioning of the Circuit

The tester comprises an AC generator built around the common NAND chip CD4011, a low voltage transistorized AC rectifier to drive the meter, and a few switches that help to select the appropriate display and mode. The AC is of the square wave type produced by the astable oscillator comprising G1 and G2. G3 and G4 act as buffers.

The desired frequency is chosen by throwing switch S1 to the appropriate position. The effective resistance of a capacitor goes as 1/(ωC), while that of an inductor goes like ωL, for a sine wave. Since the square wave has the primary Fourier component as a sinusoidal wave at frequency of the square wave, one can roughly assume that the effective resistance is given similarly by the test component at the applied square wave frequency. However, the deflection of the meter is better understood by transient analysis (similar to [2]).

To run the meter, the AC has to be rectified to DC before application to its terminals. This is acheived by using four transistors (instead of diodes) in standard configuration. The voltage drop across the collector-emitter of a transistor is much smaller (~ 1/20th of a diode voltage, i.e ~30mV ; 1/5th of a Schottky diode) compared to the forward voltage drop across a diode (0.6V or 600 mV; 0.15V – 0.5V). So, the transistors form a better AC bridge rectifier than the ordinary diode bridge rectifier. The necessary forward biasing of the transistor base-emitter junction is achieved using the voltage drop across a pair of signal diodes. Each diode-pair serves to forward-bias a type of transistor.  Since the instrument is used to compare values in order to estimate the component value, the transistorized rectifier serves its purpose. It can rectify a signal as low as 0.5V very well, with a mere difference of ~30mV between  peak-to-peak [1]. In comparison, a conventional rectifier using silicon diodes would conduct weakly at this low amplitude.  

If a 1 volt peak-to-peak AC is applied to the transistorized AC rectifier, one would see that the rectified output will almost overlap with the applied AC with any offset hardly distinguishable [1]. So the rectifier alone is another useful byproduct that can be used in some other application. Another switch is required to switch the power on/off to the circuit. All together, a set of five switches control the circuit. The test probes are designed so that connections can be made to the component under test by one hand alone. One of the probes has a spring for flexibility, whereas the other is rigid. The spring is made a bit longer so that connections with the component can be easily established. This circuit has been built and and it works satisfactorily; see the illustrations on the following pages.

>>Illustrations of test setup
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