是什么使上拉/下拉电阻强或弱?


30

“强”上拉(上/下)电阻将是一个相对较低的值,而“弱”一个电阻将是一个相对较高的值。

例如,将使用下拉电阻将I / O引脚保持为低电平,但是按下该引脚与V CC的按钮会使该引脚处于高电平,因为从V CC流向该引脚的电流要大于从该引脚流向该引脚的电流。引脚接地。

在这种情况下,似乎可以使用任何电阻值来将引脚保持为低电平,而按下按钮始终会“覆盖”该引脚。那么,什么将决定下拉电阻的强弱呢?

“强”与“弱”仅在将一个这样的电阻与电路中的其他电阻(例如内部下拉电阻)进行比较时才适用吗?

Answers:


30

强势意味着低电阻意味着高抵抗力。当然,是相对术语,也是如此。必须从上下文中推断出此关系的参考。

低电阻上拉/下拉电阻是好事,因为所形成的负载电容(通常,输入栅极电容,并且PCB迹线电容)时间常数是小的,所以上升/下降时间将是短的。

上拉/下拉电阻是好的,因为从非预期的耦合和EMI噪声电流将导致更小的噪声电压。(想想欧姆定律)

一个高电阻上拉/下拉电阻是好的,因为它不会从驱动电路对电阻器的工作需要多大的电流。因此,电池使用寿命更长,零件可以更小,并且不会变热。

当然,您通常需要所有这些东西,但电阻器不能兼而有之。有关的讨论通常是澄清,其中这些问题(或者其他)是为特定的应用更为重要。


You say, "strong means low resistance. Weak means high resistance." But a high resistance pull-up becomes strong when the drive strengths of the other components are relatively low.
travisbartley

1
@trav1s of course it's relative. If I didn't want relative terms, I'd say simply a 4.7kΩ pull-up resistor.
Phil Frost

That wasn't clear from your answer.
travisbartley

1
@trav1s better?
Phil Frost

8

A "weak" pull resistor is usually a high value resistor that only allows a small amount of current through, and can quickly be overwritten, but takes longer to reassert itself.

A "strong" pull resistor is usually a low value resistor, allows more current through, takes longer to be overwritten, but can quickly reassert a line.

They are completely relative to your needs, not just other pull resistors like internal ones.

In your button scenario, the time it takes to switch from one state to the other isn't important, so weak vs strong doesn't apply there. But weak vs strong does apply in the practical matter of Current Consumption. A strong pull resistor would, when the button is pressed, cause a large drain of current from vcc through the resistor to ground. A weak pull resistor would cause a small drain of current. Theoretically any resistor would work, but for practical purposes, a weak resistor is used because unnecessary high current drains can cause issues and can easily be avoided by sizing the resistor correctly.


I understand the difference of times to reassert the line by weak vs strong pull resistors. But I don't understand why does the strong takes longer to be overwritten. Could you explain?
Rafael

1
@Rafael think of the line as a boat (a capacitor). Think of the pull-up as a hole. The bigger the hole, the harder/longer it is to empty the bowl. You sink faster with a bigger hole, so you need to work harder. The amount of time it takes to empty the boat is longer.
Passerby

@passerby The bigger the hole the easier/shorter it is to empty the bowl.
HörmannHH

3

Does "strong" vs "weak" only apply when one such resistor is being compared to other resistances in the circuit, such as an internal pull-down resistor?

Yes, this is exactly it. Strong and weak simply refer to the relative drive strength of the component. A pull up/down resistor's value has no association to whether it is strong or weak. Only in knowing the context of the other connections to the net can you determine if a pull-up is strong or weak.


Consider a pull-up resistor connected to an open switch connected to ground. Now, how does having a LOW-value resistor make it "stronger"? The resistance of the switch and wire to ground will have a far high impact on Vin levels when the resistance of the pull up is getting closer to that of the resistance of the ground switch. I'm missing something. Help!
HörmannHH

0

There are other things to consider when selecting the value of a pull-up or pull-down. For example, depending on the capacitance of the circuit, too week of a pull-up/down will limit how quickly the voltage change occurs. On the other hand, too strong of a pull-up/down will draw excessive current through whatever is trying to pull the other way. These are often considerations in selecting the pull-ups for an I2C (open drain) bus, for example.

However, the place I see "weak pull-ups" typically used is inside microcontroller chips, typically on I/O pins. These are mainly used to guarantee that an input won't float if not connected. The pull-ups are weak both to limit their effect on the external circuitry and to limit the power dissipated inside the chip.


0

When you put a large resistance for ground coupling, the voltage developed across it would prevent the node from getting to ground potential. On the other hand, if you put small resistance to the ground, the node potential would be more close to V(gnd). If R(gnd) is high, it would not be able to pull down your node to zero potential. So, you can consider this as "weak" pull down, and vice-versa. Of course, this is just for comparison purpose only (with other components in your circuit)


What has this to do with the concept of pull up and strength which is to counteract resistance and hence voltage drop in the ground lead connection? When talking about strength of the pull up its in the context of not necessarily knowing what the ground resistance and currents from noise can be,
HörmannHH
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