Potential divider

[prasannan82]

Hi all,
A potential divider can be made by using the ratios of two resistor values.but with the same ratio many other different resistors can be chosen.like 1k,2.2k and 10k,22k and 100k,220k.But which resistor values to be used?What does that depend on and how?

 

[Nigel Goodwin]

Like everything else, it depends on ohms law - which you can use to calculate them, instead of a simple ratio. The actual values are dependent on the other values they connect to (input and ouput impedances) and what you are using them for.

 

[gramo]

Say you had a 10V power supply and two 10K resistors in series to earth, the total resistance is 20K, therefore the total current going through the circuit is I = V / R

10V / 20000ohm = 0.0005 (500uA)

If two 1K resistors are used, then the total resistance = 2K, so the total current would be

10V / 2000ohm = 0.005 (5mA)

 

[prasannan82]

My application is like this.
I have to build an voltage attenuator ckt to attenuate input to 1.2v(using potential divider technique) whose input is an audio from milli volts to max 4 volts.But the output of my amplifier should clip only at 1.2v input to it.How much value resistor should i use?

 

[Nigel Goodwin]

You don't mention the input impedance the potential divider has to feed, nor the source impedance that feeds the divider - both of these are crucial.

 

[prasannan82]

the input impedanc for the divider is 300 ohm and the output of divider should go to the amplifier whose input impedance is from 60k to 150k.

 

[Nigel Goodwin]

OK then, and you want to go from 4V down to 1.2V - so the ratio of resistors needs to be 2.8 to 1.2 - so a 28K and a 12K should be fine, 28K may need to be made by paralleling resistors, or just use a 27K instead.

 

[prasannan82]

But why not 2.8k and 1.2k resistors.

 

[prasannan82]

What is the proper rule for that?

 

[Nigel Goodwin]

It depends on the input and output impedances like I said, in this case the source impedance was very low (300 ohms) so can be ignored - otherwise it goes in series with the top resistor of the potential divider. The other consideration is the load impedance, which was 60K minimum - this is in parallel with the bottom resistor.

As a 'rule of thumb' the bottom resistor needs to be no higher than 1/5 of the load - which is why I choose 12K (the 12 actually just came from the 1.2 value for the divider output).

So the values I gave were probably as high as you should go - you can safely reduce them, but shouldn't really increase them.

 

[prasannan82]

OK.Thank you very much.

 

[huttojb]

Wouldn't you use a T type Attenuator or a Pi type Attenuator for this sort of application??

Jason

 

[Nigel Goodwin]

Wouldn't you use a T type Attenuator or a Pi type Attenuator for this sort of application??

Well he doesn't mention the application?, but you would only do that where you're wanting impedance matching, which is much less common (such as RF circuits).