Photoresistor vs. Phototransistor: Key Differences Explained

This article compares photoresistors and phototransistors, highlighting the key differences between them. It also covers the advantages and disadvantages of each component.

What is a Photoresistor?

Introduction:

• A photoresistor is a passive component whose resistance changes in response to light.
• It’s typically disc-shaped with two leads, as shown in Figure 1.
• When light shines on the disc’s surface, the resistance between the leads decreases.

In darkness, a photoresistor can have a resistance as high as 10 MOhms. In bright light, the resistance can drop to as low as 500 Ohms.

• It’s a passive, non-polarized component.
• It offers equal resistance in both directions and can be used with either AC or DC circuits.

One common application for a photoresistor is as a variable resistor controlled by light intensity. As the incident light intensity increases, the resistance decreases, exhibiting photoconductivity.

Photoresistors come in various sizes, ranging from small (5 mm in diameter) to large (25 mm in diameter). They can be used to generate a variable voltage.

What is a Phototransistor?

• A phototransistor is a transistor controlled by light exposure, similar to a photodiode controlling a regular bipolar transistor.
• It can be either a bipolar transistor or a field-effect transistor (FET).
• The phototransistor is encased in resin, plastic, or a metal shell with a window.

• A phototransistor typically has two leads connecting internally to its collector and emitter (or source and drain in a FET).
• The base of the transistor (or gate in the FET) responds to light and controls the current flow between the leads.
• In the absence of light, a bipolar phototransistor permits a leakage current between the collector and emitter of 100 nA or less.
• When exposed to light, it can conduct up to 50mA. This is a key difference from photodiodes, which cannot pass as much current.
• Figure 1 (above) depicts the most common variant: the Bipolar NPN phototransistor.
• Examples of phototransistors include: Optek/TT Electronics OP506A, Vishay TEKT5400S, and Vishay BPW17N.

Difference Between Photoresistor and Phototransistor

The following table highlights the key differences between photoresistors and phototransistors: