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 Forum index » DIY Hardware and Software
CMOS input protection
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fonik



Joined: Jun 07, 2006
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PostPosted: Mon Dec 11, 2006 3:50 pm    Post subject: CMOS input protection
Subject description: what value should the zener diodes have?
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hi all,

i built jürgen haibles wasp filter clone last week. and it sounds great. i think i will upload some sound samples next week.

i have one concern: the input signal goes straight into a 4069 which is powered from GND to 5V. i also have some oscillators with a hot output and i want to protect the CMOS input. i understand that i can achieve this using one zener diode from input to 5V and one from GND to input.
what value should they have? 5V or 2.5V?

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matthias
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blue hell
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PostPosted: Mon Dec 11, 2006 4:43 pm    Post subject: Reply with quote  Mark this post and the followings unread

You indeed should protect cmos inputs, they are easily damaged.

From http://www.fairchildsemi.com/an/AN/AN-140.pdf

Circuit a) is the "standard" circuit, b) is used with a special cmos buffer device, also read text on page 6 in document linked above.

For circuit a) use scottky diodes, those start to conduct at around 0.2 .. 0.3 V of forward biasing (normal si diodes would do so at 0.6 .. 0.7 V).

To limit current flowing from the input into (or out of) the power supply (through the protection diodes) the input resistor could be enlarged to some k-ohms (might be useful when running from batteries).

This would also help against static discharge on touching the input (or connecting something external to it). They do have ESD protection, but they might still go.


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cmos input protection
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prot.jpg



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Jan
also .. could someone please turn down the thermostat a bit.
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Last edited by blue hell on Thu Aug 23, 2007 2:24 pm; edited 1 time in total
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fonik



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PostPosted: Mon Dec 11, 2006 11:37 pm    Post subject: Reply with quote  Mark this post and the followings unread

and once again you where a light in the dark...
as you might recognize i am learning a lot these days.

i should use schottky BAT 85 diodes, i guess?

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blue hell
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PostPosted: Tue Dec 12, 2006 4:40 am    Post subject: Reply with quote  Mark this post and the followings unread

fonik wrote:
i should use schottky BAT 85 diodes, i guess?


Yes it should be schottky, and BAT 85 should be ok.

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Jan
also .. could someone please turn down the thermostat a bit.
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ello



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PostPosted: Mon Aug 20, 2018 10:29 am    Post subject: Reply with quote  Mark this post and the followings unread

hello, do i understand it right, that the circuit shown in pic A protects for example a cd4040 powered on 5V against a 12V gate signal comming from another module?

edit: i created it on a breadboard using a 100k resistor and bat85 diodes and it seems it works (it is 5.2V on the out, does a cmos powered on 5V work with that or will this be enough to kill the chip?)

and should i use a diode before the resistor to keep negative voltage (like from an lfo) outside, too, or isn't that neccessary
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fonik



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PostPosted: Mon Aug 20, 2018 1:27 pm    Post subject: Reply with quote  Mark this post and the followings unread

snother way of protection was to use a sturdy opamp that accepts a large differential voltage on its inputs that is way above the actual Vcc (i.e. 5V) it is powered from.
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ello



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PostPosted: Mon Aug 20, 2018 11:50 pm    Post subject: Reply with quote  Mark this post and the followings unread

any hint on what opamp this would be? as i already fried some opamps in the past, i am a bit concerned Smile

but if there is a less part counting alternative i am indeed interested
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fonik



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PostPosted: Tue Aug 21, 2018 2:13 am    Post subject: Reply with quote  Mark this post and the followings unread

i just played a little bit in LTSpice and maybe the best and most rock solid method was to use a 'programmable' comparator.
here you power the comparator from the rails, while you program the output to be 0V to 5V. see attached picutre.
and this has one big advantage: the input can now work with everything, even a sine wave could be used to drive the CMOS.


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5V CMOS input protection using comparator
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Last edited by fonik on Tue Aug 21, 2018 2:20 am; edited 1 time in total
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fonik



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PostPosted: Tue Aug 21, 2018 2:14 am    Post subject: Reply with quote  Mark this post and the followings unread

looks more complicated than it is. one 8pin IC and 4 resistors.
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ello



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PostPosted: Tue Aug 21, 2018 2:21 am    Post subject: Reply with quote  Mark this post and the followings unread

thank you. from what i see there, it is much more place on the board and more parts. is there a remarkable benefit from this approach? since my test with the third diode in front kept negative voltages away, too
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fonik



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PostPosted: Tue Aug 21, 2018 2:46 am    Post subject: Reply with quote  Mark this post and the followings unread

yes, it is one DIP8 and a resistor more. but as i said, you now can process anything, not just pulses.
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ello



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PostPosted: Tue Aug 21, 2018 2:59 am    Post subject: Reply with quote  Mark this post and the followings unread

ok. thank you very much
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blue hell
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PostPosted: Tue Aug 21, 2018 6:22 am    Post subject: Reply with quote  Mark this post and the followings unread

fonik wrote:
[...]you now can process anything, not just pulses.


Could you explain that a bit please? as in .. why could the diode protection not process "anything" but just "pulses"? Not getting it ...

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Jan
also .. could someone please turn down the thermostat a bit.
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ello



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PostPosted: Tue Aug 21, 2018 9:21 am    Post subject: Reply with quote  Mark this post and the followings unread

from what i understand is that some (or all) circuits don't understand a rising voltage as a gate signal. at least my test i just did brings me to that conclusion. when i create a triangle wave form between 0 and 8 volt and feed this into the protection circuit it doesn't trigger the dinky taiko. however if i shorten the rise time there is some point at which it counts as a trigger signal again.

so, if the comparator circuit creates a nice trigger signal at one point, it would be the solution for a wider scaled input scenery. if i get all this correct Wink
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blue hell
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PostPosted: Tue Aug 21, 2018 9:26 am    Post subject: Reply with quote  Mark this post and the followings unread

Ah .. ok ... hysteresis added ... had assumed the CMOS input port would have done that. And was reasoning the other way around .. that the protection diodes would be better in passing "any signal" (as in that it will work for analog inputs too).
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also .. could someone please turn down the thermostat a bit.
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ello



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PostPosted: Tue Aug 21, 2018 10:04 am    Post subject: Reply with quote  Mark this post and the followings unread

matthias, could you tell me where you found the lm311 symbol for LTspice (which i installed just today)?

all sources i found seem to be broken and I don't know the program well enough to create it by myself.

or, is there a general ressource site, for example to find bat85 and other parts?
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ello



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PostPosted: Sat Aug 25, 2018 10:42 am    Post subject: Reply with quote  Mark this post and the followings unread

hallo nochmal, ich hab folgendes im falstad simulator getestet und das scheint recht zuverlässig 12v gates zu erzeugen. wäre das ein geeigneter weg um mit 12v gespeisten cmos zu arbeiten? oder ist da noch etwas zu beachten??

als opamp würde ich jetzt einen tl074 nehmen.


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diophantine



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PostPosted: Mon Jul 08, 2019 7:46 pm    Post subject: Reply with quote  Mark this post and the followings unread

Sorry to bump, but it seems this thread has been bumped a few times!

What's the best way to handle such a circuit when you also need a pull-down (or pull-up) resistor on one of the CMOS inputs?

My first inclination was to do this:
Posted Image, might have been reduced in size. Click Image to view fullscreen.

But then I realized that the two resistors form a voltage divider, and when using 100k for both, 5V in will only give 2.5V to the CMOS pin (ignoring the effect of the diodes). Unfortunately this won't work with every chip I'm using...

Assuming I want to keep the input resistor at 100k (modular "standard"), should I change R11 to 1M or something to minimize the effect of the voltage divider?

Or should I add another resistor, like this?
Posted Image, might have been reduced in size. Click Image to view fullscreen.

Ideally I'd like to keep the part count as low as possible. For context: I'm basically gutting an old Ed-Lab 700 and replacing all the circuits with new equivalent circuits that won't be fried by modular signal levels.
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blue hell
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PostPosted: Tue Jul 09, 2019 11:11 am    Post subject: Reply with quote  Mark this post and the followings unread

You can add the pull up/down resistor on the other end .. teh K / P4 point
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diophantine



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PostPosted: Tue Jul 09, 2019 3:31 pm    Post subject: Reply with quote  Mark this post and the followings unread

Blue Hell wrote:
You can add the pull up/down resistor on the other end .. teh K / P4 point

Thanks, I gave that consideration at some point & then forgot about it... I think too much reading of datasheets & application notes that always put the pull-down closest to the pin.

But after further thought it should work fine as long as the module feeding it has an output resistor (usually always the case), and that value is usually pretty small so the voltage divider effect should be minimal.

Back to KiCad! Thanks!
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