# How do I figure out which Schottky diode to use?

I'm not well-versed in the domain of electrical engineering, so please bear with me if this question is too basic.

I'm putting together an automatic watering system for my greenhouse. Basically, it's a solar panel > voltage regulator > schottky diode > battery > timer > water pump.

The Schottky diodes I've used so far came with the solar panels. However, I need extra ones. This is where the problem arise. What type of Schottky diode do I need?

I'm using two 9V/2W/~200mAh solar panels connected in series (18V) and I've been looking at a 40V 3A (1N5822) Schottky diode. Would this be a reasonable Schottky diode to use? The panels are connected in series in order to trickle charge a 12V lead acid battery at 13.8V.

1. How do I know which Schottky diode is the right one for my use?
2. What considerations do I need to take?
3. How do I see what the voltage drop for this type of diode is?
• What is the role of Schottky diode presently? – User323693 Jan 11 '17 at 7:11
• The Schottky diode prevents the lead acid battery from discharing through the solar panels during night / low sun. – sbrattla Jan 11 '17 at 7:26

A 'feature' of Schottky diodes is high reverse leakage (when compared to ordinary diodes).

The part you have chosen has 20mA of reverse leakage (max) at 100C ambient

In reverse, if you had that amount of leakage at that temperature, there would be up to 276mW of power dissipation in the device nd this would lead to a further 11 degrees of temperature rise. This would cause increased leakage.

In this particular case, the part does indeed look suitable provided you can live with a few mA of leakage.

You can see the exponential nature of leakage current vs. temperature for these devices in the graph from the datasheet:

Always check for the reverse performance for a Schottky device; under some reverse bias conditions, they can go into thermal runaway.

• Instead of Schottky one can also go with a FET for least voltage drop. Correct me if I am wrong – User323693 Jan 11 '17 at 8:02
• When you talk about leakage, does that - in practical terms - translate into a battery discharge from the battery to the diode when the diode is "blocking" current? In other terms, at ~25C, will the diode "consume" ~2mA from the battery? Is this different from voltage drop, which I understand as what gets lost when current travels across the diode? – sbrattla Jan 11 '17 at 8:14
• @sbrattla Yes, that leakage is discharging the battery. Usually th eratio of leakage to normal use is sufficient to make this negligible, but if you leave it for months in the dark, it will flatten the battery. – Neil_UK Jan 11 '17 at 8:22
• In the OP's application the diode will run cool (about at ambient) when blocking- the high reverse leakage is typically more of a problem in switching power supply applications where the diode goes from conducting high current to blocking and back again many times per second. There is a bit of a tradeoff between in voltage and reverse leakage. Higher current lower rated voltage Schottky diodes will tend to have more reverse leakage and less forward voltage (and thus losses). – Spehro Pefhany Jan 11 '17 at 8:23
• @sbrattla If the reverse leakage of a schottky is a problem, then you could switch to a silicon diode. You lose 0.4v in the charge phase, but the reverse leakage is now truly negligible. Alternatively a FET will give the best of both worlds, but with extra control complexity. – Neil_UK Jan 11 '17 at 9:02