Electronics > Beginners
Not measuring 0v/gnd through ground resistor
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bitman:
On the attached schematic (part of a larger clock control circuit of pure 7400 chips) I have a strange issue. On the bread board this works - the selector switch changes the output selector but on the bread board, I measure 1.2v on pin 10 and 11 which means I'm in the bad TTL zone and the selector isn't working. This is a very basic setup - I'm pulling down the input lines to gnd through a 100K resistor - but still I get 1.2V when measuring on the two pins.

I would really like to understand why that is. I'm thinking a smaller resistor is needed but I am not sure I understand the reason for it.  Note the LEDs that shows selected settings work as expected. When the switch is pulled high I do get 5v on the two pins. It's just not going to gnd as I expected.
nigelwright7557:
100k is far too big.
bitman:

--- Quote from: nigelwright7557 on October 13, 2019, 09:50:47 pm ---100k is far too big.


--- End quote ---

Ok - but why? I'm trying to understand the principle (it's simply picked to make the number of different resister sizes as low as possible).
rstofer:
Ohm's Law and the datasheet

Grabbing a 74xx datasheet:
https://www.jameco.com/jameco/products/prodds/910901.pdf

IIH is 40 uA max so there is a current flowing into the pin when high.
VIH is as low as 2.0V - this is the minimum voltage on the pin that represents a logic 1
So, the 40 uA flowing through the pull-up resistor can't drop more than 5.0V - 2.0 or 3.0V

3.0V = 40 uA * R or R = 75K Ohms.  This is NOT a good value to use even though it meets the specs

I use 4.7K or even as low as 1K because I need enough current flow through the mechanical contacts of a switch to keep the circuit controlled.  Contacts tend to have some amount of oxidation and it takes voltage to poke through it.  It takes current flow to keep the connection 'made'.  That's why they invented switches with bifurcated gold plated contacts for low level switching.

floobydust:
Always remember TTL gate inputs float high, you have to sink current (~0.4mA) to take them low.
So your DIP switches are best going to ground, or the 100k should be much lower value.
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