The standard schematic symbol for a photodiode is a diode (a triangle pointing to a vertical bar) with two small arrows pointing inward toward the PN junction, representing incoming photons. Physically, the cathode is typically marked by a flat edge on the epoxy lens or a shorter lead on through-hole packages, while surface-mount variants like the Vishay BPW34 use a notch or printed band to denote the cathode.
Photodiode Symbol, Pinout, and Marking Reference
The table below maps the schematic symbols you will see on diagrams to the physical markings you will encounter on the bench. Use this to verify your components before soldering.
| Symbol / Marking Type | Visual Description | Standard / Variant | Practical Meaning & Bench Application |
|---|---|---|---|
| Standard Photodiode Symbol | Triangle and bar with two arrows pointing IN | IEEE 315 / IEC 60617 | Generates current when illuminated. Used in photoconductive (reverse bias) or photovoltaic (zero bias) circuits. |
| Enclosed Symbol (Circle) | The standard symbol enclosed in a circle | IEC 60617 (Historical/Discrete) | Indicates a discrete physical package rather than an integrated silicon block. Common in older European schematics. |
| THT Physical Marking (5mm) | Flat edge on the plastic lens base; shorter lead | Industry Standard (EIA) | Flat edge / short lead = Cathode. Long lead = Anode. Always verify with a DMM if leads are trimmed. |
| SMD Physical Marking (e.g., BPW34) | Notch on the package corner or a printed line | Manufacturer Specific (Vishay, OSRAM) | Notch/Line = Cathode. The BPW34 is a PIN photodiode; the intrinsic layer requires 5V-12V reverse bias to minimize junction capacitance for high-speed data links. |
| Phototransistor Symbol | Transistor symbol (NPN) with inward arrows, no base lead | IEEE 315 / IEC 60617 | Not a pure photodiode. Has internal gain (amplification) but much slower response time (microseconds vs nanoseconds). Example: TEFT4300. |
Standard Variants and Rows People Get Wrong
While the inward-pointing arrows are nearly universal today, historical and regional standard variations still cause confusion on the bench, especially when repairing legacy equipment or reading imported schematics.
IEEE 315 (US/ANSI) vs. IEC 60617 (International)
In the US, IEEE 315 standards dictate the triangle-and-bar diode base. Older IEC 60617 standards sometimes represented diodes using a rectangle and a line, though modern IEC has largely harmonized with the triangle to reduce global confusion. If you see a rectangle with a line and inward arrows on a legacy European schematic, it is a photodiode, not a specialized resistor.
The 'Rows People Get Wrong' Notes
- Arrows Pointing OUT: This is the most common schematic error made by junior designers. If the arrows point away from the junction, it is a Light Emitting Diode (LED), not a photodiode. Photodiodes absorb light; LEDs emit it.
- Confusing Zener and Photodiode Symbols: A Zener diode symbol features a triangle pointing to a bar with bent ends (resembling a 'Z'), but it has no arrows. If you see arrows, it is an optoelectronic device, regardless of the bar shape.
- Assuming the Flat Edge is Always Cathode: While true for 99% of standard 5mm through-hole IR and visible light photodiodes, some specialized UV or high-speed avalanche photodiodes (APDs) in metal TO-18 cans use the isolated tab as the anode. Always check the specific manufacturer datasheet (like the Vishay BPW34 datasheet) before applying reverse bias.
Safe Interpretation When Markings Are Faded or Missing
On the bench, you will frequently encounter salvaged photodiodes with clipped leads, sanded-off markings, or yellowed epoxy that obscures the internal die structure. Here is the exact decision path to identify the component and its polarity without guessing.
Step 1: The Multimeter Diode Test
Set your digital multimeter (DMM) to the Diode Test mode (usually indicated by a diode symbol). Place the red probe on one lead and the black probe on the other.
- Forward Bias Reading: If you read between 0.3V and 0.5V (typical for IR photodiodes) or 0.5V to 0.7V (visible spectrum), the red probe is on the Anode and the black probe is on the Cathode.
- Reverse Bias Reading: If the meter reads 'OL' (Over Limit) or '1', the probes are reversed. Swap them to confirm the forward voltage drop.
Step 2: The Photovoltaic Verification Test
To definitively prove the component is a photodiode and not a standard switching diode (like a 1N4148), use the photovoltaic effect.
- Set your DMM to the lowest DC millivolt (mV) range.
- Connect the red probe to the suspected Anode and the black probe to the Cathode.
- Shine a bright white LED flashlight or direct sunlight onto the component's lens.
- Expected Result: The voltage will immediately jump, often reading between 100mV and 350mV depending on light intensity. A standard diode will remain at 0.0mV. This confirms it is a photodiode and verifies your polarity identification.
Step 3: Dark Current and Shunt Resistance Check (Advanced)
If you are sorting high-precision photodiodes for a low-noise transimpedance amplifier (TIA) circuit, you need to measure dark current. Apply a 5V reverse bias through a 1MΩ resistor and measure the voltage drop across the resistor in a completely dark room (cover the sensor with black electrical tape). A high-quality PIN photodiode like the BPW34 will show a dark current of roughly 1nA to 2nA at room temperature. If you measure microamps of dark current, the junction is likely degraded or it is a phototransistor, not a photodiode.
Frequently Asked Questions
How do I tell a photodiode symbol from an LED symbol on a schematic?
Look strictly at the direction of the arrows relative to the diode triangle. On a photodiode symbol, the arrows point inward, toward the PN junction, indicating that external light energy is entering the device to excite electrons. On an LED symbol, the arrows point outward, away from the junction, indicating that electron recombination is releasing energy as photons (light) into the environment.
Which way does conventional current flow in a photodiode circuit?
This is a frequent source of confusion because it operates differently than a standard forward-biased diode. In a photodiode operating in photoconductive mode (reverse bias), conventional current flows from the Cathode to the Anode (reverse leakage current). When photons strike the depletion region, they generate electron-hole pairs, which are swept across the junction by the reverse electric field, increasing this reverse current proportionally to the light intensity. In short: light increases reverse current.
Why does my photodiode symbol have a circle around it?
The circle enclosing the diode and inward arrows is an older IEC 60617 convention used to denote that the component is a discrete, packaged device rather than an internal junction within an integrated circuit. In modern schematic capture software (like KiCad or Altium), the circle is often omitted to save space, but you will still see it on legacy military, aerospace, and European industrial schematics. It does not change the electrical behavior or pinout of the component.
Can I use a photodiode in forward bias like a normal diode?
Technically, the PN junction will conduct if forward-biased above ~0.4V, but it will be completely blind to light. The strong internal electric field required to separate photon-generated electron-hole pairs only exists when the diode is reverse-biased (or at zero bias in photovoltaic mode). If you wire it in forward bias, it acts as a standard, useless silicon diode and will not function as a light sensor.
