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Forum Index : Microcontroller and PC projects : MicroMite vs CMOS logic chips...
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Grogster Admin Group  Joined: 31/12/2012 Location: New ZealandPosts: 9078 |
Hi all. 
When connecting to the likes of 4000 series CMOS logic gates etc, do you need to worry about series resistors, or are the inputs to most 4000 series chips of suitably high impedance, that current limiting series resistors are not really required? Smoke makes things work. When the smoke gets out, it stops! |
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TassyJim Guru  Joined: 07/08/2011 Location: AustraliaPosts: 5923 |
CMOS has a very high input impedance. Around 10^12 ohms and 5pF capacitance. The only time a series resistor would do anything is if the input is driven outside the voltage limits. Then the input protection diode would come into play. If the CMOS is fed from a different supply, the loss of one supply is the biggest risk. Jim VK7JH MMedit MMBasic Help |
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| Grogster Admin Group Joined: 31/12/2012 Location: New ZealandPosts: 9078 |
Understood - thanks. Smoke makes things work. When the smoke gets out, it stops! |
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| TassyJim Guru Joined: 07/08/2011 Location: AustraliaPosts: 5923 |
With CMOS the high impedance can be a problem with stray signals getting in. Often, it is worthwhile putting an external resistor to ground on vulnerable inputs. Jim VK7JH MMedit MMBasic Help |
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| Grogster Admin Group Joined: 31/12/2012 Location: New ZealandPosts: 9078 |
Acknowledged - thanks again. Smoke makes things work. When the smoke gets out, it stops! |
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