what are pulldown resistors used for?

[lebevti]

when are are pulldown resistors used? what situations?

can someone give me an example circuit that uses them and why it uses them?

thanks.

 

[KeepItSimpleStupid]

One way to answer this question is: CMOS inputs need to be pulled up or down when unused. When the inputs are used, the inputs need to be placed in the lowest power state. You want that state to consume the least amount of power.

Another example is that you have a dis connectable module and when the controller is removed you don't want random things to happen, so you either pull up or pull down the input to a safe state so when the connector is removed nothing bad happens. e.g motor runs astray and/or input oscillates

Some processors and IC chips have selectable pull ups and./or pull downs. Take a look at the ADP5589

 

[Vizier87]

pulldown resistors.... like when you want to detect a "HIGH" on a pin which is an input, you pull it down so that it'll definitely ground noise or any voltage resulting from drift current and so on.

 

[colin55]

There are dozens of answers
Here's another one:
When you have a circuit that has a high impedance line, (also called a high impedance input), the line can detect static and create an unwanted HIGH. By putting a “pull-down” resistor on the line, it will remain LOW until a signal is delivered to the line to make it HIGH.

Most gates, such as TTL and CMOS inverters, buffers etc have a low or high input impedance but the input line does not sit as a HIGH or LOW. It generally sits midway between a HIGH and LOW state.
If you leave the input line “floating” it will generate a high-frequency signal on the output.
This action heats up the chip and wastes energy.
To prevent this, the input line is held at a HIGH state by connecting a resistor between positive rail and the input line. This is called a PULL-UP resistor.
Or held at a LOW state by connecting a resistor between input and 0v rail. This is called a PULL-DOWN resistor.

 

[sheldonstv]

Heres an example for you ......

 

[lebevti]

ok thanks, that helps...

now, what is the difference between TTL and CMOS...is it simply the voltage level? TTL being +5V and CMOS being 3.3V? is it safe to say that both TTL and CMOS are in relation to digital logic and digital circuits?

furthermore, is it safe to say that pull-down resistors wouldn't be used in analog circuits?

 

[colin55]

TTL works on 5v rail.
CMOS works on 3v to about 15v
The input resistance (impedance) of TTL is classified as very low. It may be 5k to 30k.
The input resistance (impedance) of CMOS is classified as very HIGH. It may be 1M to 50M.
The pull-up or pull-down resistor(s) for TTL can be 1k to 10k
The pull-up or pull-down resistor(s) for CMOS can be 47k to 10M

You may have a transistor driving an input to an analogue stage and the transistor is configured as an emitter-follower.
The emitter will be connected to the 0v rail via a resistor. You can call this resistor the LOAD resistor or a PULL-DOWN resistor.
When the transistor "turns-on" it amplifies the positive portions of the signal and the emitter rises. This voltage is delivered across the load resistor or "pull-down" resistor. At the same time the signal is delivered to the next stage in the circuit.
You can also have a "pull-up" resistor on the base of the emitter-follower so the stage driving the emitter-follower only has to pull the base towards the 0v rail. The pull-up resistor returns the base to the positive rail.

 

[KeepItSimpleStupid]

TTL is really a family. CMOS is Complementary Metal Oxide Semiconductor. You can have a CMOS OP amp. CMOS is more of a generic term.

See: https://en.wikipedia.org/wiki/Logic_family

For a list of some logic families. The "FAMILY" has a set of characteristics. The one thing that's usually defined is what is a high and what is a low and Typical FAN OUT (How many gates will an output drive)

CMOS only consumes appreciable power when switching. When an output is at logic 1 or zero nearly no power consumed.

Some of the early computers I worked with used DTL and RTL logic.

The HCT logic family has the Hi/Low levels of TTL logic and the low power and output characteristics of CMOS.

"pull downs" would not be called a pull down in an analog circuit, but they would exist. As an example to set the input Z of an amplifier too 600 ohms, a 600 ohm resistor is placed from the input to ground.

On a non-inverting amplifier, the (-) lead is attached to ground through a resistor. It' doesn't have to be, BUT it improves bias current compensation.
Capacitors, since they are not idea, may require resistors to drain them of excess charge.

Since OP amps can be used as comparators, you could pull-up the output or with the addition of a diode and a pull-up convert it to an Open Collector output.

So, in a sense, yes.

 

[crutschow]

TTL works on 5v rail.
CMOS works on 3v to about 15v
................

The CD4xxx series of CMOS logic will work from 3V to 15V. The high-speed CMOS 74HCxxx logic series and its variants generally only work at 5V.

 

[Dean Huster]

I never saw switches mentioned. If you want a manual RESET on a counter with an active-LOW RESET, you use a pull-up resistor to keep the RESET input HIGH and connect a switch to a LOW so that when it's pressed, the input will go LOW and RESET the counter. The pull-up resistor prevents shorting the supply (HIGH) to ground (LOW).

In TTL, pull-down resistors should be avoided on active HIGH inputs because the input draws more current through the pull-down resistor to get it LOW than it would draw through a pull-up resistor to pull it HIGH. The problem surfaces with the fact that you have a large window of acceptable voltages that are considered HIGH to the input and a very narrow window of acceptable LOW voltages. A HIGH might be anywhere from 3v to 5v while a LOW must be between 0v and 0.7v and many chips will not operated reliably unless that LOW is at 0.1v or less. To get that 0.1v, it takes a low value of pull-down resistor. If you want to add a switch for manual operation of the function, pressing that switch places 5v across that resistor and you may find that you need a power rating of 1w to make sure that the resistor doesn't not overheat with extended button-pushing. Often times, it's easier (and smarter) to use a "left-over" inverter to pull down on an active-HIGH input, using the push-button on the inverter input with a pull-up resistor. The inverter will pull down harder on the active-HIGH input without the problem of having to have a high-wattage pull-down resistor hanging off the pushbutton.

It's really disgusting that I've written a tutorial on designing pull-up/pull-down circuits but I can't find where I've put it. Danged old age!