Understanding Ohm Meter Readings for Components

An ohm meter is a device used to measure electrical resistance, which is the opposition to current flow in a circuit or component. The unit of resistance is the ohm ($\Omega$). When measuring a component like a resistor, the ohm meter applies a small voltage across it and measures the resulting current to determine its resistance value based on Ohm's Law ($R = V/I$).

What is an Open Resistor?

An "open resistor" refers to a resistor that has malfunctioned and broken internally, creating an open circuit. In an open circuit, there is a complete break in the conductive path, preventing any current from flowing through it, regardless of the voltage applied across it.

Resistance of an Open Circuit

According to Ohm's Law, resistance is Voltage divided by Current ($R = V/I$). In an open circuit, even if a voltage ($V$) is applied, the current ($I$) that flows through the break is zero. When you divide any voltage by zero current ($R = V/0$), the result is theoretically infinite resistance. This signifies that the circuit path is completely blocked.

Ohm Meter Measurement of an Open Resistor

When an ohm meter is connected across an open resistor, it attempts to push a small current through it. Because the resistor is open, it behaves like a break in the circuit, meaning no current can flow through it. An ohm meter is designed to interpret the inability of current to flow (or extremely low current) when a voltage is applied as a very high or "infinite" resistance reading. Different ohm meters may display this in various ways, such as "OL" (Over Limit), "INF", or a symbol resembling infinity ($\infty$).

Analyzing the Options

Let's consider the given options in the context of measuring an open resistor with an ohm meter:

• Low but not zero: This reading indicates a small amount of resistance, much lower than what an open circuit represents. A normal, low-value resistor would show a reading like this.
• Infinite: This reading indicates a complete break in the circuit, where no current can flow. This is precisely the characteristic of an open resistor or an open circuit.
• Zero: This reading indicates no resistance at all, like a perfect conductor or a short circuit where current flows freely. A resistor that has failed by becoming a short might read close to zero.
• High but within tolerance: This indicates the resistor is functioning correctly, but its measured resistance is on the higher end of the value range specified by its tolerance percentage. It's not indicative of an open circuit.

Based on the behavior of an open resistor acting as an open circuit, the ohm meter will measure infinite resistance.

Conclusion

An open resistor creates an open circuit, which exhibits theoretically infinite resistance. Therefore, when checked with an ohm meter, an open resistor will read infinite.

Revision Table: Ohm Meter Readings Summary

Additional Information on Component Testing

Using an ohm meter is a basic way to test the integrity of components like resistors, wires, and switches. Here are some key points:

• Always test components when they are *out* of the circuit or when the circuit is *powered off* to avoid damaging the meter or getting incorrect readings due to parallel paths.
• An ohm meter requires a complete circuit path to measure resistance. Connecting it across an open component provides this path through the meter itself and the component leads.
• "OL" or infinity on a digital multimeter typically means the resistance is higher than the meter's maximum range, effectively indicating an open circuit.
• Zero or near-zero resistance indicates continuity, which is expected for a good wire or a closed switch, but usually indicates a fault (short) in a component that should have higher resistance.