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Protection for overvoltage and negative voltage
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NuttyMonk



Joined: Jun 30, 2020
Posts: 60
Location: UK

PostPosted: Sun Jun 13, 2021 8:26 pm    Post subject: Protection for overvoltage and negative voltage Reply with quote  Mark this post and the followings unread

Hi all,

I've been wondering recently about protecting IC's and other components such as PIC's and microcontrollers from overvoltage and negative voltage which is out of the range that the inputs on those components can take.

I was recently looking at the Envelope Generator from Electric Druid and it had the schematic shown below as a method of doing this.

As you can see it uses a common emitter setup with a 2N3904 transistor with a 1N4148 diode providing the negative voltage protection. The problem is that this setup means there is a drop in voltage of about 0.6V

I reckon it can only be used in the situation which they suggested which was for protection for a clock signal input. In other words the voltage only needed to go HIGH and LOW and didn't need to be an accurate or exact replication of the input voltage

Does anyone know of any other ways to provide overvoltage and negative voltage protection which gives an exact copy of the input voltage at the output?

Cheers

NM


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PHOBoS



Joined: Jan 14, 2010
Posts: 5215
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PostPosted: Mon Jun 14, 2021 4:29 am    Post subject: Reply with quote  Mark this post and the followings unread

my preferred method is using 2 (schottky) diodes and a resistor.
Posted Image, might have been reduced in size. Click Image to view fullscreen.
note: I usually don't use R2.

Grumble did some tests with an ATMEL processor (ie an arduino) and a single 1K resistor in series with an input seemed to do the trick.
you can read about it here: https://electro-music.com/forum/topic-68022.html

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NuttyMonk



Joined: Jun 30, 2020
Posts: 60
Location: UK

PostPosted: Mon Jun 14, 2021 9:17 am    Post subject: Reply with quote  Mark this post and the followings unread

Thanks Phobos,

That looks interesting. I've simulated it in Falstad and i can see that it still allows about 0.5-0.6V beyond the supply rail and ground but for most applications that would be fine i think, judging from some technical specification sheets i have just looked at.

What's the advantage of using a Schottky over a 1N4148? Is it just faster at responding?

Cheers

NM
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PHOBoS



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PostPosted: Mon Jun 14, 2021 9:30 am    Post subject: Reply with quote  Mark this post and the followings unread

Silicon diodes (like the 1N4148) have a forward voltage of around 0.6V which is what's causing the overshoot.
For schottky diodes this voltage is lower, around 0.3V, so it should give less overshoot.

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NuttyMonk



Joined: Jun 30, 2020
Posts: 60
Location: UK

PostPosted: Mon Jun 14, 2021 9:49 am    Post subject: Reply with quote  Mark this post and the followings unread

Ah, nice.

Well, i think that's one more problem solved.

Thanks for your help Phobos.

NM
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Steveg



Joined: Apr 23, 2015
Posts: 169
Location: Perth, Australia

PostPosted: Mon Jun 14, 2021 4:56 pm    Post subject: Reply with quote  Mark this post and the followings unread

This article may also be of interest: https://sound-au.com/appnotes/an015.htm
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NuttyMonk



Joined: Jun 30, 2020
Posts: 60
Location: UK

PostPosted: Mon Jun 14, 2021 6:31 pm    Post subject: Reply with quote  Mark this post and the followings unread

That article looks good Steveg. I generally like the explanations on https://sound-au.com/

Cheers

NM
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