4-wire milliohm tester for DMMs

Based on a Maxim IC application note, this simple battery-powered adaptor converts an ordinary digital multimeter (DMM) into a 4-wire milliohmeter that accurately measures the resistance of wiring, motor coils, solenoids, high current inductors and meter shunts. It could also be used to locate shorts on a PC board.

The circuit is essentially a constant current source which applies 1A, 100mA or 10mA to the unknown resistance via two test leads, according to the range switch (S2) setting. In use, you set your DMM to the 2V range and connect it across the resistance being measured. This forms a 4-wire connection to the resistance being measured. This method of connection avoids the problems of resistance of the test leads.

Pressing switch S1 passes the selected current through the resistance being measured. With 1A selected, the DMM reading will be in Ohms. A 1# resistance, for example, will read 1.000#.

With 100mA selected by range switch S2, you have to multiply the multimeter readings by 10 to get a value in Ohms or by switching the DMM to 200mV, the reading will once again be in Ohms. Finally, if the 10mA range is selected, the DMM should be set at 200mV and the reading will need to be multiplied by 10 to give the correct value in Ohms.

The overall accuracy will depend on that of the DMM, the op amp’s input offset voltage ( [pm] 70uV maximum) and the tolerances of resistors R1, R2, R3, R4, R5 & R6. These resistors could be trimmed to improve the accuracy if you have suitable standards.

The constant current source works as follows: The LM-336 provides a 2.5V reference to the non-inverting input (pin 3) of op amp IC2 and this is compared with the voltage developed by the Mosfet (Q1) current through resistors R4, R5 and R6.

Note that S1 disconnects the 9V battery when it is not pressed and this means that there is no current drain from the 1.5V battery either. If you use a D cell here it should produce thousands of measurements. The current drain from the 9V battery when S1 is pressed is only about 30μA so it should last for years.

SILICON CHIP.

Constant voltage charger uses LM317 The ability to adjust the output voltage of the LM317 3-terminal regulator makes it ideal as the basis for a simple constant voltage charger. These two circuits, taken from the National Semiconductor Linear Brief 35, have the advantage of no filter capacitors and the final voltage from the charger is adjusted by potentiometer R3. In the top circuit, R1 defines the charger output impedance and enables a "taper" charge characteristic to be obtained. In the lower circuit, transistor Q1 provides control of the peak charging current, an important point with smaller batteries. Note that the transformer secondary voltage will need to cater for the voltage drop across the regulator and depending on the charging current, the LM317 will need to be mounted on a heatsink.

Fine & coarse power supply control

Often a circuit calls for a multi-turn potentiometer in order to be able to give sufficiently fine control. However, 10-turn potentiometers are not cheap.

This circuit idea is an economical alternative. It uses a dual gang 50kΩ linear potentiometer as the coarse control and a 500Ω linear potentiometer as the fine control.

A portion of both the 50kΩ pot elements are effectively connected in series with the 500Ω potentiometer between them. However, because of the method of connection, the total resistance in circuit is always constant and is equal to 50kΩ + 500Ω = 50,500Ω.

As you can see, as the coarse 50kΩ control is wound up, less of the top element is in circuit while more of the bottom element comes in, thus keeping the overall resistance constant. The result is a very consistent fine and coarse control operation.

The circuit could be used with the constant current load featured in Circuit Notebook in the February 2000 issue. It is also equally applicable to the Li’l PowerHouse 40V 1A Power Supply described in June 2000 and in this month’s issue.

Karl Sundberg,

Ashfield, NSW. ($25)