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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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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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Last edited by Blue Hell on Thu Aug 23, 2007 2:24 pm; edited 1 time in total
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fonik



Joined: Jun 07, 2006
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Location: Germany
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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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