Hello, as an electronics beginner, I am trying to understand how the pulldown resistors in this circuit work.
I have written a description as to how I understand it.
Can someone please take a look at the attached schematic and my text, to confirm if my understanding is correct.
Thanks kindly for any help.
"MY UNDERSTANDING"
The source of the logic circuit is a pair of high or lows from two Picaxe outputs on pins 6 and 7. These two signals feed respectively into the bases of two NPN transistors (Q3 and Q4), therefore enabling them to act as switches controlling the VCC source. The emitters of Q3 and Q4 are directly connected to ground in order to 'channel to ground' any floating signals (created by susceptibility to electrical noise) when Q3 and Q4 are supposed to be turned off, i.e. a floating signal could potentially cause Q5, Q6, Q7 or Q8 to activate unintendedly.
The 2 pull-down resistors R8 and R9 become necessary when Q3 and Q4 are open in order to prevent a short circuit. Care was taken to ensure they were not too high in value otherwise when either of the two PNP transistors, Q7 or Q8 were activated, their base current would pass through R8 or R9 and potentially generate a voltage of higher than 0.6V. If that point came up more than about 0.6V there is a danger of the other NPN transistors, Q5 or Q6, being turned on when they shouldn't, e.g. if the pull-down resistor was too high and the current from a PNP transistor was elevating that to 1V then you would also have an NPN transitor inadvertently turn on.
By having the pulldown resistor smaller than the base resistors, it stops the voltage from 'lifting it's feet'.
I have written a description as to how I understand it.
Can someone please take a look at the attached schematic and my text, to confirm if my understanding is correct.
Thanks kindly for any help.
"MY UNDERSTANDING"
The source of the logic circuit is a pair of high or lows from two Picaxe outputs on pins 6 and 7. These two signals feed respectively into the bases of two NPN transistors (Q3 and Q4), therefore enabling them to act as switches controlling the VCC source. The emitters of Q3 and Q4 are directly connected to ground in order to 'channel to ground' any floating signals (created by susceptibility to electrical noise) when Q3 and Q4 are supposed to be turned off, i.e. a floating signal could potentially cause Q5, Q6, Q7 or Q8 to activate unintendedly.
The 2 pull-down resistors R8 and R9 become necessary when Q3 and Q4 are open in order to prevent a short circuit. Care was taken to ensure they were not too high in value otherwise when either of the two PNP transistors, Q7 or Q8 were activated, their base current would pass through R8 or R9 and potentially generate a voltage of higher than 0.6V. If that point came up more than about 0.6V there is a danger of the other NPN transistors, Q5 or Q6, being turned on when they shouldn't, e.g. if the pull-down resistor was too high and the current from a PNP transistor was elevating that to 1V then you would also have an NPN transitor inadvertently turn on.
By having the pulldown resistor smaller than the base resistors, it stops the voltage from 'lifting it's feet'.
Attachments
-
159.2 KB Views: 62