Bypass Diodes Explained: Hot Spots, Partial Shade & String Layouts

What Are Bypass Diodes and Why They Matter

Bypass diodes are small, built-in electrical components inside most solar panels. Their job is to protect the panel and maintain power output when part of it is shaded or damaged.

Without bypass diodes, even a small shaded section could cause a massive voltage drop — reducing output from the entire panel or even the whole solar array.

How They Work

A solar panel is made up of multiple “strings” of cells (typically 18–24 cells per section). Each string has a bypass diode wired in parallel.

• When all cells receive sunlight, the diode is inactive (open circuit).

• If one or more cells in a string become shaded, cracked, or dirty, those cells can block current flow.

• The bypass diode activates automatically, allowing current to “bypass” the shaded section and keep the rest of the panel generating power.

This prevents both:

• Hot spots — overheating of shaded cells due to reverse current, and

• Output collapse — the entire panel dropping to near zero volts.

Typical Bypass Diode Layouts

Most modern A-grade mono shingled panels (like the ones on your Star Power system) use multiple bypass sections for improved partial-shade tolerance.

What Happens During Partial Shading

Here’s a simplified example using a 12 V 36-cell panel (2 diodes):

Key takeaway:
When a single section is shaded (like from a roof vent or antenna), only that segment’s power is lost — not the whole panel.

Hot Spots Explained

If a shaded cell has no bypass protection, the bright cells still push current through it in reverse. That shaded cell turns into a resistor, heating up rapidly — sometimes over 120 °C.

This can cause:

• Burn marks or “snail trails”

• Localized delamination of the encapsulant

• Permanent damage or fire risk

Bypass diodes automatically prevent this by redirecting current around the shaded section.

Bypass Diodes and String Layouts

When wiring multiple panels, bypass diodes protect each panel individually — but poor string layout can still create large mismatches.

Best Practices for System Wiring

• Avoid mixed orientations or tilt angles in the same string.

• Keep panels in the same sunlight conditions (don’t mix shaded and unshaded panels in one string).

• If unavoidable (e.g. different roof faces or portable setups), use:

  • Parallel wiring, or

  • Separate MPPT inputs for each section.

Example:

If you have two roof panels and one gets shaded by a roof vent in the afternoon:

• Series wiring = output limited to shaded panel’s current.

• Parallel wiring = other panel keeps full output.

How to Test Bypass Diodes

If you suspect a diode fault:

1. Use a multimeter in diode mode.

2. Test across each diode (accessible inside the junction box).

   • Healthy diode: one-way voltage drop ~0.4–0.7 V.

   • Faulty diode: open circuit or shorted both ways.

3. Replace if faulty — usually inexpensive (~$5–$10) and soldered to the terminal block.

Summary: Why Bypass Diodes Are Crucial

Pro Tip for Caravan & RV Installers

When designing roof layouts:

• Place vents, antennas, and awnings away from the middle section of the panel (most have central diode groups).

• If shade is unavoidable, prefer shingled or half-cut panels — they have more diodes and finer segmenting, giving better real-world performance under partial shade.