Pulling down resistors

Question

Please i want to know more about the pull down resistor. For a circuit of about 12v, what value of resistor do i need? Simply put, should a pull down resistor be selected according to the voltage of the circuit? Or just any type of resistor would work out? what do u think Guys? Cause i tried make a voltage sensor with an LM324 IC, when the NEG(-) pin is higher than the POS(+) pin, it normally should not give any output, but in my case, it does. Not until i use my meter prob to touch the the positive terminal before the output will turn off. So i use a higher resistor as a pull down resistor to the positive pin, and it worked, but the thing is, i have to adjust my voltage down to the value of my pull down resistor voltage before i could achieve it. So my question is, do i need a specific value of resistor to perform a pull down job for me?

Pull-up and pull-down resistors are used so that you a) don't short your power supply through a pin or transistor when you turn on said pin and b) bring your pin (quickly) to a known voltage state when turned off. At bare minimum, you'd need to know how much current your pin or transistor can handle to determine what resistor would adequately limit current. — Bort, Mar 11 '19 at 14:38
related, if not duplicate: [Chosing a pulldown resistor](https://electronics.stackexchange.com/q/45291/7036) — Nick Alexeev, Mar 11 '19 at 16:29

score 1 · Answer 1 · answered Mar 11 '19 at 14:57

A pull down resistor is used with output drivers that have high side switches, not to force something to 0V by exceeding the recommended output current. The LM324 is a opamp with a push pull output driver, the only thing your doing by adding a large pull down is wasting current. Although the LM324 can support short circuits (some op amps can't), it's not a good idea to use pull downs on the output. There are better ways to change op amp circuits to get the design results, like adding another stage or using a second stage in comparator mode.

If a pull down is used, the value of the resistor can't exceed it's power rating. If it is a 1/4W resistor, the smallest value of resistor that can be used is P=V^2/R or R=V^2/P

for a 1/4 at 12V the smallest resistor would be 576Ω but down't use a pull down in the application your describing.

Pls tell me, why is my LM324 Not working the way it out to, untill i use the pull down resistor? — Iresi Jw Flux, Mar 11 '19 at 15:31
@IresiJwFlux Give us a schematic and we can give you an answer. — Hearth, Mar 11 '19 at 15:35

score 0 · Answer 2 · answered Mar 11 '19 at 15:44

There is a bias current that flows in or out of any op-amp input pin. In the case of CMOS op-amps the polarity is uncertain and it tends to be very low.

The LM324 input stage (and the rest for that matter) uses bipolar transistors so the input bias current is relatively well defined. At room temperature it is typically about -20nA and the worst case specification is about -500nA. The minus sign means that it flows out of the input (and without anything connected, the input will rise towards the positive supply voltage).

If you want to hold the input to within, say, 5mV of ground then the resistor you need would be typically 250K but to allow for the temperature range and especially bad units, it would require as low as 10K. Use Ohm's law, as I did, to figure out what you really need in your application. If you only need to hold it within 100mV then multiply those values by 20.

Pulling down resistors

2 Answers2