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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?

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.

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

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.

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.

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.

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

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

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...

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.

Do they have different rules in New Hampshire?

Yes, they do. I'm originally from Vermont, and have to point out that New Hampshire is the upside-down state (look at any map).

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