Schottky for surge & polarity protection?

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- 12th July 2008, 23:33
kevj

Schottky for surge & polarity protection?

This is a basic electronics question, but everything I find on google goes into tons of detail of how exactly a schottky is constructed and it's characteristics, but I can't find straight up practical application examples for it.

I've got 2 things I'm trying to solve, and I think a schotty may be a solution for them both; but I'm not sure how exactly it would be implemented...

1) Surge / spike protection. I've got a radio receiver that will operate in an area that experiences a high current EMP spike as the attached hardware activates. The spike induces between 2 and 15 volts in an open wire on a scope. I'm a bit concerned that the EMP could be coupled into the antenna and result in a voltage spike into the radio receiver and burn it out. We've tested it to the extreme (lots and lots of test spikes) and it keeps working fine, but I'd feel much better knowing I've done the "right" thing in preventing this. I think a schottky could be used to direct a high voltage spike (maybe anything more than 1 to 2 volts) directly into the ground plane. I also think a zener could be used? Whatever it is, it would have to be really fast acting as the spike is really short - maybe 100nS.

2) Polarity protection. Assume my circuit is all complete with a voltage regulator, pic, and varous components. It's powered from a single battery cell, and I want to add reverse polartiy proection (if the user puts the batt in backward). I put a batt in backward and it seems fine but again, I'd like to add some intentional protection for this.

Specific help or links to example schematics much appreciated. If I could just see how these things are used I think I could figure out how to use them for my application.

Thanks!
- 13th July 2008, 00:26
skimask

Quote:

Originally Posted by kevj

2) Polarity protection. Assume my circuit is all complete with a voltage regulator, pic, and varous components. It's powered from a single battery cell, and I want to add reverse polartiy proection (if the user puts the batt in backward). I put a batt in backward and it seems fine but again, I'd like to add some intentional protection for this.

Assuming your circuit, regulators, etc. can handle it (i.e. drop out voltages, etc), a couple of diodes could make it so that it wouldn't really matter if a person did plug the battery in backwards...but at the same time, ground would act a bit goofy since it would be offset just a bit.
- 13th July 2008, 01:07
BrianT

Anti-parralel diodes shottky diodes should do it.

During blizzards in the Antarctic we used to get 25-35 mm sparks off unterminated antennae caused by static buildup from the passing ice crystals. That would rip the front ends right out of the receivers until we tried a pair of anti-parralel shottkys across the input. The earth braid to ground had to be beefed up to reduce noise.

HTH
BrianT
- 13th July 2008, 18:39
kevj

Quote:

Originally Posted by BrianT

until we tried a pair of anti-parralel shottkys across the input. The earth braid to ground had to be beefed up to reduce noise.

Thanks Brian. I hear what you're saying, but just can't picture how exactly such a thing would be implemented.

I've got an antenna - a piece of wire... with a signal that goes to an input of a "radio stage", it sits inside a grounded SMA connector with no electrical connection to the signal line obviously.

If I were to tie a single shottky from the signal line (antenna) to ground, as it has a low forward voltage drop, but it still does have "some" voltage drop - the very low voltage signal itself shouldn't be effected, but if a spike were to happen, it would cause the schottky to "open" and that voltage would be quickly dumped to ground. Am I picturing this right? Sounds too simple.

Also, as EMP is a wave, it's actually kind of an A/C signal induced in the antenna - so should I put a pair of schottky's - one pointed either direction?

Thanks everyone. Don't know why I'm having such a hard time picturing this.
- 13th July 2008, 23:16
BrianT

Nearly there

A single diode won't do it unless you know the pulse is always the same polarity which is highly unlikely in RF systems. The shottky diode does not 'open' as you mention. It will be reverse biased or forward biased depending on the polarity of the incoming signal. The diode will withstand some reverse bias and breakdown at some voltage from 20 to 400 depending on the diode specs. That 400 volt signal is being poked straight into your radio front end so you must eliminate this reverse breakdown by using two diodes where one protects the other and the voltage across the pair is never more than the forward voltage drop of the diode - about 250 mV

Your 'single wire' antenna has both an earth part (the outer of the SMA) and the active line (the centre pin on the SMA). Two shottky diodes in "anti-parallel" means one has the anode to the antenna with cathode to earth and the other is the reverse of that. The forward voltage drop of the shottky is about 250 mVolts so no matter what the polarity of the offending spike is, the diodes clamp the voltage to no more than 250 mV. RF, which is typically below 10 mV rides through unaffected.

HTH
BrianT
- 17th July 2008, 10:31
kevj

Outstanding. Thank you very much Brian - exactly what I was looking for.
- 18th July 2008, 11:14
Ioannis

1 Attachment(s)

Have a look at the file I attached. I think it is exactly what you want.

Ioannis
- 19th July 2008, 04:38
languer

Understand the uses of TVS and Schottky Devices

Transorbs (like SMBJ) and other TVS (transient volt suppressors) are great for transients. In fact, for lightning protection and such TVS devices are used widely (these are what you find in your surge protection power strips). Schottky diodes would never whidstand lightning transients (and live to see another day).

But schottly diodes have very little capacitance, transorbs generally have a great deal of capacitance. So if you hang a transorb directly across the antenna input, you may as well not worry about reception / transmission (since you will have none).

In your case, a schottky diode in an anti-parallel configuration (as was suggested) maybe all you need. Should you require to add the TVS (there are some very good bidrectional ones - look up AVX's TVS) to the antenna line, you will want to have an RF choke (RF inductor) in series with the TVS. The choke connects between the antenna line and TVS, and the TVS betweeb the choke and power rail (GND most likely). Lookup the following appnotes, they have very good info:
What is a Silicon TVS and How Does it Work?
Selecting the Optimum Transient Voltage Suppressor