What diode to use?

Newbie here...

What kind of surface mount diode should I use? My project will be powered with a 3.7v coin cell battery or 5v USB. I want to protect again the power being connected backwards, but waste as little power as possible with the diode.

Any suggestions?

Use a shockley diode as it has less voltage drop across it in forward bias mode.

Schottky.

Close, but Shockley was a different guy,

The higher the continuous current rating of the schottky diode is the lower its forward voltage
will be for a particular current level, so you might be able to get down to 0.3V or so loss with
suitable device. Shame germanium diodes aren't still generlly available - they might be even better.

Trying to read some data sheets (for instance a SOD-123 20V Vr Schottky diode) the lowest Vf I've found is about 400mV. Does this mean with a 3.7v battery I'd be getting 3.3V through the diode (losing about 10%)?

Obviously I'm no electronics engineer, just trying to learn.

Thanks

Yes that is what it means and that is why you might not want to use one.
With a normal diode this would be 700mV.
A geranium diode would be about 250mV but they are hard to get nowerdays.

You can use a mosfet to provide reverse-polarity protection without significant voltage loss. See attached schematic. You need to choose a P-channel mosfet that will turn on fully @ about 3V Vgs, and that has a low enough Rds(on) for the current draw of your circuit.

Note that the mosfet conducts current between the source and drain opposite to the normal direction. This is intentional.

Grumpy_Mike:
A geranium diode would be about 250mV but they are hard to get nowerdays.

I don't advise using geraniums in electronic circuitry, they have high failure rates at low temperatures due to frost damage :slight_smile:

I don't advise using geraniums in electronic circuitry, they have high failure rates at low temperatures due to frost damage

But they do smell good.

Grumpy_Mike:

I don't advise using geraniums in electronic circuitry, they have high failure rates at low temperatures due to frost damage

But they do smell good.

Geez, who would have expected this out of GRUMPY_Mike?

I noticed on the data sheets of a couple voltage regulators I was considering for my project that they have reverse polarity protection. The sheets don't explain, maybe it's too obvious. Can this take the place of the diode?

You have a link to the data sheet? Kinda hard to tell what they are used for without seeing that. Does the data sheet specifically say they are for reverse protection?

Linear technology has a bunch of them.

sample;-

Undervoltage, Overvoltage and Reverse-Supply Protection

A couple voltage regulators I looked at are:

http://www.micrel.com/_PDF/mic5205.pdf
http://www.micrel.com/_PDF/mic5203.pdf

All the sheets have is a bullet point about a reverse polarity protection feature.

I misunderstood the post when I asked for the data sheet. Bottom line, yes you can drop the diode if the regulator has reverse protection.

I noticed some coin cell battery holders (with a metal clip on one side) that won't contact the + and - ends of the battery if it's installed backwards. Low-tech reverse polarity protection - clever design.