A bypass diode is wired across each section of 12 to 18 cells with its cathode to the positive side, so that when a cell is shaded the string current flows around the section instead of being forced through the shaded cell as a reverse-biased resistor. The diode limits how much power the shaded cell can be made to dissipate, but the section it bypasses stops contributing voltage, so a 36-cell panel with one shaded section drops from about 18 V to about 9 V and may stop charging a 12 V battery.
Part of Solar Panel Construction
Why partial shading damages a series string of solar cells, where bypass diodes go, which diode to use, and the cost of using them.
In A Series String Every Cell Carries The Same Current
Cells in series share one current (Cell Interconnection and Panel Wiring). A cell that is shaded can generate only a small current, but the rest of the string insists on pushing its current through it, so the shaded cell is driven into reverse bias and turns into a resistor. The carrier says it “becomes a resistor and heats up”. The power it dissipates is its reverse voltage times that current, and its reverse voltage can be as large as the combined voltage of the cells that are still lit.
The remedy in the carrier is a diode across each sub-string of 12–18 cells, with the cathode to the positive end and the anode to the negative end. With all cells lit, the section voltage reverse-biases the diode and it does nothing. When a cell in the section is shaded, the section’s voltage falls, the diode turns on, and the current goes through the diode instead.
Which Diode: The Germanium One Or The Tube One
There are two articles named “diode construction”. The one to use is Diode Construction in the electronics folder, a germanium point-contact and alloy junction diode with about 0.2 V forward drop (0.6 V for silicon), 5–50 Ω forward resistance for the point-contact type and over 100,000 Ω reverse. A bypass diode carries the whole string current, so it must be rated for the panel’s short-circuit current: a small point-contact diode is not. The practical source is a silicon rectifier diode salvaged from an alternator, a power supply or a controller (Power Electronics). The other article of that name, Diode Construction in the radio-telephone folder, is a vacuum-tube diode built from a bulb filament; it needs a heated filament and drops volts, and has no place on a panel.
What You Need
- Diodes rated above the panel’s short-circuit current — salvaged silicon rectifiers; the charge-controller article names 1N4001–1N4007 types for small currents (rating is general knowledge, about 1 A, not in the corpus)
- A multimeter — diode test and voltage, see Metering
- Solder, flux and wire — Soldering Basics
- A place to mount the diodes where they stay dry — Junction Boxes
Method: Fit Bypass Diodes
Step 1: Mark the sections. Divide the string into sections of 12–18 cells and bring a tap wire out of the ribbon at each section boundary.
Step 2: Check the diode. With the meter’s diode setting, confirm that it conducts one way only.
Step 3: Wire it across the section, cathode (banded end) to the positive end, anode to the negative end.
Step 4: Measure the panel in sun. The voltage should be unchanged from before. A diode wired backwards short-circuits the section and the panel’s voltage will fall by that section’s share, so check at once.
Step 5: Shade one cell with a card and check that the voltage falls by about one section and not to zero.
Step 6: Seal the diodes inside a junction box or the panel.
Worked Example: What One Shaded Cell Costs
Take the carrier’s panel: 36 cells at 0.5 V, 0.5 A, 18 V. A shaded cell can be forced to pass the string current while the rest of its section pushes against it. As an upper bound, the shaded cell dissipates the power the lit cells in its protected section could produce, (cells − 1) × 0.5 V × 0.5 A. The corpus does not give the shaded cell’s reverse characteristic, so the real figure is lower, and depends on the cell.
- No bypass diode, 36 cells: 35 × 0.5 × 0.5 = 8.75 W in one small cell.
- Diode per 18 cells: 17 × 0.5 × 0.5 = 4.25 W.
- Diode per 12 cells: 11 × 0.5 × 0.5 = 2.75 W.
Smaller sections give a smaller worst case. The price is the output lost when a diode conducts. With two 18-cell sections and one shaded, the panel gives 18 cells × 0.5 V = 9 V, less the diode’s drop of about 0.6 V, because the diode replaces the bypassed section. A 12 V battery needs about 15.8 V at the panel to charge, so a single shaded cell stops charging altogether until the shade passes; the cells are saved, not the harvest.
Blocking diode, a different job. A diode in the line from panel to battery stops the battery discharging through the panel at night. It costs its drop times the current: at the 4.1 A of a 59 W panel at 14.4 V, a 0.6 V silicon diode loses 0.6 × 4.1 = 2.5 W and a 0.2 V germanium diode 0.2 × 4.1 = 0.8 W. Do not confuse the two: a bypass diode sits across a section, a blocking diode sits in series.
Common Mistakes
- Diode backwards — it short-circuits the section; check the polarity and the voltage
- A diode rated below the panel current — it burns out in the first shade
- Using the wrong “diode construction” article — the vacuum-tube diode is not a bypass diode
- No diode at all on a long string — the shaded cell overheats
- Leaving the diode exposed to rain — it corrodes
Safety
A panel under a diode is still a live string, and shaded cells get hot
Panel wiring work is done on a string that makes voltage whenever light falls on it, and a series string reaches dangerous voltage even though a cell does not: three 18 V panels make 54 V, six make 108 V. Work with the array covered or disconnected at the array and treat it as live. A cell forced to dissipate several watts in a small area can crack the glass or scorch the encapsulant, so inspect for brown spots after any shading event (Lethal Current).
What’s Next
- Learn next: Panel Encapsulation and Framing — sealing the diodes and cells inside the panel
- Combine with: Shunt Regulator Design — another place a diode protects the system