#805 3DU5C Phototransistor

A quick review and test of the 3DU5C metal-encapsulated silicon phototransistor, used for light detection in the visible and infrared spectrum.

Notes

The 3DU5C metal-encapsulated silicon phototransistor has a peak sensitivity at 880nm in the infrared spectrum, but it is relatively good sensitivity through the visible spectrum. This makes it very useful in a wide range of applications.

Specifications:

• NPN Silicon Phototransistor
• Model : 3DU5C
• Working Voltage (max) : 10V
• Reverse Breakdown Voltage : 15V
• Dark Current : 0.3uA
• Photocurrent : 0.5-1mA
• Power Consumption : 30mW
• Peak Wavelength : 880nm
• Body Size : 7 x 5mm/ 0.28” x 0.2”(L*D)
• Total Length : 28mm/ 1.1”
• External Material : Metal
• Weight : 3g

The package has equal length leads. The emitter is identified by the metal tab on the can.

Comparing Photodiodes and Phototransistors

See also:

• The Art of Electronics 12.6.1 Photodiodes and phototransistors (3rd Edition).
• Infrared and Photodetectors WEbinar by DigiKey with Würth Elektronik

Photodiodes and phototransistors are both light-sensitive semiconductor devices, but they behave quite differently.

Photodiodes:

• A diode optimized to generate current when light hits its junction
• Light creates electron–hole pairs → produces a small photocurrent.
• Used in photoconductive mode (reverse-biased) or photovoltaic mode (like a tiny solar cell).
• Characteristics:
  • Fast response (nanoseconds to microseconds).
  • Low gain — the photocurrent is small because there’s no internal amplification.
  • Very linear and precise.
  • Wide wavelength range (UV → visible → IR depending on material).
  • Can be used for high-speed or analog light-measurement.
• Common uses:
  • Remote controls (as a raw sensor or inside modules).
  • Optical communication receivers.
  • Light meters / lux sensors.
  • Fiber optics.

Phototransistors:

• A transistor (BJT or sometimes photo-darlington) where light replaces the base current.
• Light generates base current → transistor amplifies it → much larger collector current.
• Characteristics:
  • High gain — much more sensitive than a photodiode.
  • Slower (microseconds to milliseconds) because transistor junctions store charge.
  • More nonlinear behavior.
  • Often packaged in IR receiver modules.
  • Requires some biasing and has more electrical complexity.
• Common uses:
  • IR object detection.
  • Slotted opto-switches / rotary encoders.
  • Simple ambient light detection.
  • Counter/frequency detection at modest speeds.

In summary:

Test Circuit Design

This is a quick test of the sensitivity of the 3DU5C:

• positioned on the high-side of a 3.3kΩ resistor, so voltage measured at the emitter will increase with brightness
• a 3-wire voltmeter attached to monitor the voltage

Note: this is similar to the tests described in Make: More Electronics Experiment 4: Measuring Light

Designed with Fritzing: see 3DU5C.fzz.

Test Results

Responds well to a wide range of light sources:

• LED lamp
• sunlight
• red, green, blue LEDs
• IR LED

The following is a scope trace of the voltage being excited with a blue LED:

Credits and References

• “2PCS 3DU5C Phototransistor Transistor Silicon Phototransistor Transistor Metal Package 2-pin NPN” - aliexpress seller
• https://www.alibaba.com/product-detail/3DU5C-Silicon-Phototransistor-Two-Pin-Metal_1601271212767.html - example alibaba source
• The Art of Electronics 12.6.1 Photodiodes and phototransistors (3rd Edition).
• Infrared and Photodetectors WEbinar by DigiKey with Würth Elektronik
• Make: More Electronics Experiment 4: Measuring Light