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Schottky diode vs silicon diode: which should you use?

A silicon diode and a Schottky diode both let current through one way and block it the other. The difference is the price of passing: a silicon diode drops about 0.7 V, more at high current, while a Schottky drops about 0.3 V. That halves the voltage and heat lost. In return a Schottky leaks more current backwards and is made for lower reverse voltages. Pick the Schottky when every tenth of a volt counts and the voltages are low.

Forward drop, measured

Put each diode in series with a 12 V supply as a reverse-polarity guard:

Load currentSilicon (1N4001)Schottky (1N5817)
110 mAdrops 0.83 V, load gets 11.17 Vdrops 0.31 V, load gets 11.69 V
about 1 Adrops 0.96 Vdrops 0.42 V

Both drops grow with current, but the Schottky's stays well under half the silicon's. Plug the supply in backwards and both block completely: the load sees 0 V and nothing is damaged.

Live experiment

Plug it in, then backwards

A 12 V supply runs the right way round for a second, then reversed. Guard the load with a plain wire, a silicon diode, a Schottky, or a diode fitted backwards. At 110 mA silicon leaves 11.17 V and the Schottky 11.69 V. Switch to a load near 1 A and watch both drops, and the heat, grow.

Try it yourself: free lesson

“The wrong way round” is lesson 6 of 6 in Solder Strait, PCB Design World 2. Free with an account. Lessons unlock in order, so you’ll do the 12 before it first.

The heat

P = VF × I

At nearly an amp, the silicon diode turns about 0.87 W into heat, all the time, just for protection. The Schottky makes about 0.40 W: half a watt less to get rid of, and half a volt more for the load. On a battery-powered board, that's energy the battery no longer spends warming a diode.

A worked choice

A load needs at least 11.6 V from a 12 V supply. Silicon gives about 11.2 V, so it fails. A Schottky gives about 11.7 V, and passes. Read the drop at your current before you pick the part: a datasheet's forward-voltage curve gives it.

Where each belongs

Schottky: reverse-polarity guards on low-voltage supplies, the freewheel diode in a buck converter, blocking diodes on solar chargers, and OR-ing two supplies together. Their speed helps too: Schottky diodes switch faster than ordinary silicon ones, because they don't have to clear stored charge when they turn off.

Silicon: mains rectifiers and anything with high reverse voltage, and circuits where leakage would spoil a measurement, like a peak detector holding a voltage on a capacitor.

At high currents neither diode is ideal. Laptops and power tools use a MOSFET wired as an "ideal diode", which drops only millivolts; reverse polarity protection covers how that works.

Free lesson

Now build it yourself

Each lesson comes in three short parts: see the idea, try it on a live board, then build it yourself, with quick checks as you go.

Try it yourself: free lesson

“The wrong way round” is lesson 6 of 6 in Solder Strait, PCB Design World 2. Free with an account. Lessons unlock in order, so you’ll do the 12 before it first.

Or start from the very first lesson, no account needed

Work it out

Practice questions

Three quick questions from the lessons. Pick an answer to check it.

  1. Question 1 of 3The load wants 11.6 V from 12. Silicon gives 11.2, so…
    Show the answer

    Answer: A Schottky, dropping about 0.3 V, gives 11.7 and makes it. The Schottky's smaller forward drop is the whole reason to use one here.

    • Why not “No diode”? With no diode the load gets 12 V, but nothing protects it if the supply is plugged in backwards.
    • Why not “Two silicon diodes”? Two diodes in series drop twice as much, about 1.6 V, leaving roughly 10.4 V.
    • Why not “A zener”? A zener conducts forwards like a silicon diode, at about 0.7 V or more, so it can't reach 11.6 V either.
  2. Question 2 of 3The Schottky's price is…
    Show the answer

    Answer: A little reverse leakage and a lower reverse voltage rating. Schottkys leak a little backwards and don't stand as much reverse voltage. For a 12 V supply that's a fine trade.

    • Why not “A bigger drop”? The Schottky's drop is smaller, about 0.3 V against silicon's 0.7 V. That's why it's chosen.
    • Why not “Slower”? Schottky diodes switch faster than ordinary silicon ones, which is why switching supplies use them.
    • Why not “Nothing”? Every part trades something. A Schottky leaks more in reverse and is rated for lower reverse voltages.
  3. Question 3 of 3Why does a solar charger need a blocking diode?
    Show the answer

    Answer: After dark, the panel's own diodes would drain the cell. With no light the panel is just a string of diodes, and a charged cell would push current back through them all night.

    • Why not “To limit the panel's current”? The panel limits its own current. The diode is there to stop current flowing backwards.
    • Why not “To raise the panel's voltage”? A diode can only take voltage away: it always costs its forward drop.
    • Why not “The panel makes AC”? A solar panel makes DC. The diode's job is to block reverse current after dark.
Keep practising: free lesson

“The wrong way round” is lesson 6 of 6 in Solder Strait, PCB Design World 2. Free with an account. Lessons unlock in order, so you’ll do the 12 before it first.

Questions people ask

Why does a Schottky diode have a lower forward voltage?

It's made from a junction between a metal and a semiconductor instead of between p-type and n-type silicon. That junction has a lower barrier for current to climb over, so it conducts at about 0.2 to 0.45 V instead of silicon's 0.6 to 1 V.

What are the disadvantages of Schottky diodes?

They leak more current in reverse, and the leakage climbs steeply as they warm up. They're also made for lower reverse voltages: the 1N5817 is rated for 20 V, where the silicon 1N4001 handles 50 V and the 1N4007 1000 V.

Can I use a Schottky diode for reverse polarity protection on 12 V?

Yes, as long as its reverse voltage rating is comfortably above the supply, including any spikes. A 1N5817 is rated for 20 V; for more margin, or for 24 V systems, the 40 V 1N5819 or a higher-rated part is the safer pick.

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