Resistor Voltage Dividers - Ratio vs. Values?

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Two fiddly voltage divider questions...

In the standard 2x R Voltage Divider you can change the values of the resistors around and still get the same Vout as long as the ratio between them is the same: e.g.

To get +2.5V from +10V I can use a 3K:1K combo, a 300R:100R combo or a 30R:10R combo...

1) In general use - is there a reason to choose larger R values over smaller ones? Is there a point where noise or current draw becomes an issue with bigger or smaller resistors?

2) Is there a best practice for putting the larger value or the smaller value as the Resistor to Ground? I'n asking this one because I have a circuit breadboarded at the moment with about a half dozen Rx voltage dividers - all with 100R resistors to Ground and I'm starting to worry if that's potentially a lot of signal leaking into Ground... or maybe I just worry too much

:???: Thanks.

 

[chuckey]

It all depends on the impedance of the voltage source and the impedance loading the output terminal. For instance a diveide by 10 scope probe just relies on a serial 10M ohms resitor, the other half of the attenuator is the 1 M ohm input resistance of the scope. This so the probe will not load the circuit. If you are putting an artificial centre tap on a power supply so instead of 0 + 12V, you want + 6V, 0V, -6V, then the resistors should take about 10 times the current difference between the two supplies.
Frank

 

[7@rB]

1- of course, since the value of the resistance of the resistor controls the value of the caurrent will pass through this circuit ( V= R x I )

2- the same case, in these resistor even they are connectod to ground there will be a current passing through them.

 

[BradtheRad]

Often a resistive divider has to compete, yet cooperate, with something else.

* One common situation that comes to mind is biasing a transistor. Making a divider using low resistances will drown out your input signal. It will also use up (waste) several milliamps between the supply rails.

Very high resistances will be unable to compete with your input signal, unless your input is also very high resistance.

* Another situation is a gain (volume) control. If the potentiometer is very high ohms, it will cause extreme change within a narrow region at one end.

On the other hand, if you are biasing a mosfet, the pot can be very high ohms. This is because a mosfet input is also very high ohms.