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Pull-up Resistors?

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Gandledorf

New Member
I understand what a pull-up resistor is for, when it is needed, and why they are important, but I am a little shakey on the theory behind how they work (my Analog skills are a little on the low side). Can anyone share a good explanation?
 

Russlk

New Member
The load, whatever it is, requires a positive current (current into the load) in order for the voltage to rise. The pullup resistor provides that current. I=V/R as always. The open collector is able to sink current and pull the voltage down.
 

defcon31

New Member
and is there a way to know how large that pullup resistor has to be, or does it has to be determined experimentally?
 

Nigel Goodwin

Super Moderator
Most Helpful Member
defcon31 said:
and is there a way to know how large that pullup resistor has to be, or does it has to be determined experimentally?

It depends entirely on the load - think of the open-collector output as a simple switch, it can be on or off - when it's off it's as if it's not there at all. The pull up resistor has to provide enough current for whatever it's feeding - one example would be an LED, the open-collector gate is connected across the LED (which goes to ground), the top of the LED goes to +ve via a resistor as normal. When the gate is turned off the LED lights, as usual, fed from it's current limiting resistor - when the gate is turned on, it bypasses the current round the LED, extinguishing it.
 

Sebi

Active Member
Also need pull-up resistor for logic inputs to definiate the logic level (High). The datasheet give the max value: for TTL 1k, LSTTL 15k, CMOS 1Mohm.
 

Russlk

New Member
I would like to point out that M means milli as in .001. There is probably no confusion in this case, but another could be confusing. Megohm has to be spelled out as in 1Meg.
 

Sebi

Active Member
Thanks, Russlk for correction.(in my country upper case M= mega, lower case m= milli) sorry...
 

Roff

Well-Known Member
Russlk said:
I would like to point out that M means milli as in .001. There is probably no confusion in this case, but another could be confusing. Megohm has to be spelled out as in 1Meg.
Do they have different rules in New Hampshire? I usually use M for mega, e.g., MHz, and m for milli, e.g., mH. Here are a couple of references I found (I know, just because they exist, the aren't necessarily right).
https://whatis.techtarget.com/definition/0,,sid9_gci499008,00.html
https://www.americantech.org/pdf/PREFIX.pdf
 

Noggin

Member
whats really annoying is when I found a capacitor that had an M for micro. Imagine my disappointment when I discovered that my newly found 100 megafarad cap was a measly .1 milli :( I coulda been rich...
 

Roff

Well-Known Member
Noggin said:
whats really annoying is when I found a capacitor that had an M for micro. Imagine my disappointment when I discovered that my newly found 100 megafarad cap was a measly .1 milli :( I coulda been rich...
With a mug like that, you'll soon starve if you're not rich. :D
 
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