r/electronics • u/1Davide • Dec 23 '17
Tip [Tip] Selecting Schottky diodes
When selecting Schottky diodes, be aware that typically you have 2 choices:
- Low forward voltage (more efficient, runs cooler), but high reverse current
- Low reverse current (critical when normally reverse biased), but high forward voltage
For example, both of these are Schottky diodes with 100 mA 30V specs, SC-79 / SOD523 package:
- DB2S30800L, low forward voltage: 420mV fwd at 100 mA, 120µA rev at -30V
- DB2S30900L, low reverse current: 580mV fwd at 100 mA, 2µA rev at -30V
In this plot of reverse current vs. forward voltage for 40 V, 1 A Schottky rectifiers you can see there's not a strict correlation between the two, that there is a lot of variation.
You should also consider standard (non-Schottky) diodes, that do not have the disadvantages of Schottky diodes (low voltage breakdown, high reverse current):
- For fast turn off, high power: ultrafast rectifier diodes
- For fast speed, low power: PIN diodes, Tunnel diodes
- For low voltage drop, high power: "super-barrier" rectifier diodes
2nd tip: to get a low voltage drop, you can select a diode that is rated for a much higher current than needed
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u/arewehavinfunyet Dec 23 '17
This is pretty cool. Are there any links that help with component selection like this?
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u/1Davide Dec 23 '17 edited Dec 23 '17
Ideally, right here in /r/Electronics: I hope that some of the electronic gurus who frequent this sub would post more tips like this one.
If we get more tips, I'll start compiling them in the Wiki.
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u/tonyp7 Dec 24 '17
Thanks for the tip. I’m always picking the lowest fwd voltage without thinking much about the reverse current.
That being said I feel like you can have the best of worlds if you are willing to pay a few cents more (Diodes Inc powerdi schottky for instance)!
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u/fatangaboo Dec 23 '17 edited Dec 23 '17
Tip: Hobby projects that run off 9V batteries, including guitar pedals, benefit greatly when you install a 1Amp Schottky Rectifier in series with the +9V battery lead. The voltage dropped at ~5 mA operating current is negligibly small, but the diode protects you and your circuit board against SOME IDIOT trying to plug in the battery backwards. Invest 25 cents and remain insured against idiocy for the entire lifetime of the project. Recommended diodes: 1N5817 (good), SB120 (better), SB320 (best).
I learned this in the 1970s when disassembling secret miniature radio transmitters ("bugs"), sold for several hundreds of dollars by clandestine Spy Shops, in the classified ads of Popular Science et al. Jealous husbands or amateur gumshoes who buy a bugging transmitter, are quite likely to destroy it by plugging in the battery backwards. Solution: make it idiot proof using a Schottky diode. Yes really.
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