Installing a Schottky Diode in a Solar Panel Circuit. Which Way Round Should it be?

March 13, 2010

Trying to figure out which way to install a Schottky diode in solar panel wiring can be very confusing. Does it really matter which side of the panel the diode is installed. Should the silver band (or arrow) point to the panel, or away from it? Hopefully, this article will clarify things.

In the absence of another barrier (a charge controller for instance) a Schottky diode is installed in the solar panel wiring to prevent the batteries from back-feeding into the panel in low light conditions, particularly at night. Without the diode the batteries will lose their charge in no time and the panel may suffer damage.

The Schottky diode is an electrical check valve, to use a plumbing term. It allows maximum current to flow in the forward direction with minimal voltage drop, and zero current (or close to zero) in the reverse direction. Typical Schottky voltage drops are 0.15-0.45 volts in the forward direction.

It’s important to use a Schottky diode that has an overall voltage rating that exceeds the max voltage output of the panel. For instance, a 30V Schottky will work fine for an 18V panel, but would be inadequate for a 48V panel. Likewise, the amperage rating of the diode should exceed the max current output of the panel.

Schottky DiodeHere is a drawing of a Schottky diode along with it’s electrical symbol. The silver band identifies the cathode, or negative side, of the diode.

If we think of current flow in the conventional sense as flowing from the positive side of the solar panel to the negative side then the corresponding “full flow” current direction of the Schottky diode is from the anode to the cathode. Think of it as current entering the end opposite the silver band and exiting the end with the silver band.

Now, which side of the panel do we install the diode? Normal convention suggests that we should place it in the positive lead from the panel as shown in the following diagram. It’s certainly easier to visualize the current flow. However, it would work equally well in the negative lead but we would need to turn the diode around so the negative silver-banded end (cathode) is pointing to the panel.Solar Panel and Schottky Diode

Once you’ve connected your panel, diode and battery, check that the system operates correctly by testing the voltage drop across the panel in full light and no light conditions. If the voltage across the panel remains at battery voltage in no light conditions then you have the Schottky diode installed backwards and your panel will start to heat up as the energy in the battery is dissipated.

While this article is primarily about which way round to install a Schottky diode, it is necessary to point out that the diode does not provide any voltage regulation. A panel that puts out excessive voltage  for extended periods will shorten battery life which quite possibly will result in a major rupture of the battery as it overheats. Please go here for further discussion on this topic.

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