Any idea on how to make a simple ohmmeter using solid state devices and not magnets and coils?

Answer 1

Use a 7106 or 7107 IC. They provide analog to digital conversion and drivers for a liquid crystal display. The source in "references" provides kits to make a panel mounted ohm or volt meter using these ICs.

"Google" these ICs and you will find more.

Answer 2

There are two basic approaches to ohmmeter circuits - constant current and constant voltage.

Most digital ohmmeters use a circuit that forces a constant known amount of current through the unknown resistance (up to the voltage limit of the internal power source) and then measure the voltage drop across the unknown. If an appropriate current is chosen, then the voltage can be directly interpreted as the resistance.

For example, if 1 mA of current is forced through an unknown resistance and the voltage measured across it is 3.56 volts, then the value of the unknown resistance is 3.56 K ohms. (From Ohm's Law). Lower values of unknown resistance would use higher values of constant current, such as 10 mA or 100 mA.

So a meter such as you describe would require a power source that produces a selectable value of constant current and a voltage measuring device such as the DVM chip mentioned in a previous answer.

Ohmmeters using a constant voltage source generally employed a moving-coil meter movement to measure the current flow when the unknown resistance was connnected as part of the circuit. The simplest was the series ohmmeter, where the ohmmeter source voltage and full-scale meter current were used with Ohm's law to calculate a total full-scale series resistance. The resistance of the meter movement was then subtracted from this to obtain the value for a series calibration resistor. When the probes were touched together, (zero ohms measured resistance) the meter would indicate full scale, which was marked as zero ohms. As greater values of unknown resistance was measured, the current would be less, indicating a greater resistance measurement. Open test leads would allow zero current, so infinite resistance was at the left end of the scale.

Using a digital voltmeter IC, this type of circuit could be made if the voltmeter IC measured the drop across a sense resistor in the series chain comprised of a voltage source, calibration resistor, sense resistor, and the probes to the unknown resistance. The laws of the series circuit could be used to calculate the resistance for each voltage reading. Unfortunately, the voltage readings will not be directly proportional to the measured resistance, and a lookup table would be required to interpret the value of the measured resistance.

Another variation of the constant voltage ohmmeter circuit would use the same basic circuit (Voltage source, calibration resistor and sense resistor), but the resistance to be measured would be connected in parallel with the sense resistor. The voltage across the sense resistor would be maximum when the test leads were open (infinite resistance), and the voltage reading would decrease as lower values of resistance were measured. Shorting the measuring probes together would develop zero volts measured, corresponding to zero ohms. The advantage of this circuit is that the scale of the old meters read with zero at the left and infinite at the right. This circuit also is non-proportional, requiring a lookup table or calculation algorithm to convert the measured voltage into a measured resistance.

Generally, with digital meters, the simplest method is the constant current meter circuit.

Answer 3

Generally speaking, an ohmmeter is used to measure the resistance of a deenergized electrical component, and therefore must have its own power supply (battery?). It is used to measure a range of resistors from nil resistance to high resistance and must not short out its own battery when measured resistance is very low. Existing digital ohmmeters do not use magnets and coils but do use solid state.

Answer 4

Simple Ohmmeter