Question

What will be the resistance of a photoconductive cell in darkness?

(a) 1000-3000Ω

(b) 100MΩ

(c) 250-500Ω

(d) None of the mentioned

The question was posed to me by my college professor while I was bunking the class.

Asked question is from Instrumentation Amplifier in portion Operational Amplifier Applications of Linear Integrated Circuits

Answer 1

The correct answer is:

(b) 100MΩ

Explanation:

A photoconductive cell (or LDR - Light Dependent Resistor) exhibits a very high resistance in darkness and this resistance decreases when exposed to light. In complete darkness, the resistance of a typical photoconductive cell is very high, often in the range of 10 MΩ to 100 MΩ or more, depending on the specific type of LDR.

• In Darkness: The absence of light means the photoconductive material has minimal conduction, leading to a very high resistance.
• In Light: The resistance decreases significantly as the photoconductive cell absorbs photons, allowing more current to flow.

The other options:

• 1000-3000Ω and 250-500Ω are much lower resistance values that would apply when the photoconductive cell is exposed to light, not in darkness.
• None of the mentioned: This is incorrect since the correct range is covered by 100MΩ.