Fixing Your Failed Solar Panel

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Fixing Your Failed Solar Panel

This article assumed you have already determined what's wrong with your solar panel. If you haven't determined how your solar panel failed, see our "Determine What's Wrong with Your Solar Panel" article to make that determination.

It's most likely that your solar panel failed because its internal Schottky bypass Diode has failed. The solar panel is easy to repair if that is the case. This article will walk you through determine the Schottky bypass diode that is in your solar panel, potential upgrade, and the actual repair.

How to Determine the Diode Spec

To know exactly what diode is in the solar panel, you'll need to cut it out of the circuit. Before you do so, make sure you have tested the diode to determine that it has failed. Cut the lead on both side of the diode as close to the diode as possible. Once the diode is out of the circuit, its label confirmed the part.

In our failed solar panel example, the diode was embedded under insulation and adhesive, but once exposed, it was clearly marked, "15SQ045". It is a 15‑amp, 45‑volt Schottky bypass diode. This is a common diode used in 100‑watt panels.

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Find a Replacement Schottky Bypass Diode

To find a replacement Schottky bypass diode, you need to match:

Type: Schottky diode
Current rating: at least equal to or higher than panel Isc
Voltage rating: comfortably above panel Voc

We can simply replace the diode with the exact same one. And we can get them quite inexpensively on Amazon and other places (see "Related Links" section below). But our solar panel failed after 2 years. Does that mean we need to replace the diode every 2 years? We can. However, wouldn't it be better if we didn't have to? You can replace the diode with the same model, but if you want a diode that won't fail, you should upgrade to one that is over-rated.

For example, best long‑life replacements for our 15SQ45 are as follows. These Schottky diodes are massively over‑rated for a 100‑watt panel and will run cool for decades:

STPS20L45 (20A, 45V) - top choice, extremely reliable
20SQ045 (20A, 45V) - stronger version of your original
15SQ060 (15A, 60V) - higher voltage tolerance
MBR2045CT (20A dual diode - use one side)

Any of these will outlive the panel itself. But will a Stronger Diode Harm the Panel? The short answer is "No". A stronger diode runs cooler, handles shading better, reduces stress on the junction box, prevents catastrophic short failures, extends panel lifespan. There is zero downside to upgrading.

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Why Does STPS20L45 Have 3-Pins?

Great question, and it's one that confuses a lot of people the first time they upgrade from a simple axial diode (like the 15SQ045) to a higher‑grade part like the STPS20L45.

STPS20L45 has 3 pins because it's a TO‑220 package. But electrically, it's still just a single diode. The STPS20L45 is packaged in a TO‑220 case -- a very common high‑power semiconductor package. TO‑220 devices always have three leads, even if the internal component only needs two. For the STPS20L45:

Pin 1 = Anode
Pin 2 = Cathode
Pin 3 = Cathode (duplicate)

Yes - pins 2 and 3 are the same node. They are physically tied together inside the diode. This is done for:

Lower resistance
Higher current handling
Better thermal performance
Easier PCB or wiring layout

How you wire it in a solar panel

You only need two connections:

Anode → panel's negative side of the cell string
Cathode → panel's positive side of the cell string

Since pins 2 and 3 are both cathode, you can use either one, or twist/solder both together for extra mechanical strength. Electrically, it makes no difference. The STPS20L45 is so reliable because it is 1) 20A rated (100 watt panel only makes ~6A); 2) 45V rated (100 watt panel Voc is ~19V); 3) Schottky (low forward drop → runs cool); 4) TO‑220 package (excellent heat dissipation). This is why it's considered a "won't fail" diode for solar bypass use.

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How to Replace Schottky Bypass Diode in Solar Panel

When you replace it, match orientation exactly. The diode will have a band marking the cathode. Install the new one with the band in the same direction as the old one. Solder or crimp it in place where the old one was. If you reverse it, the string won't behave correctly.

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