# Voltage divider Q

Discussion in 'General Electronics Chat' started by protomor, Dec 30, 2008.

1. ### protomor Thread Starter Member

Dec 23, 2008
22
0
If I use an R1 resistor of 100 ohms and an R2 resistor of 200 ohms, I get 6v from a 9v source (for example). The equation becomes (200/300)*9. or 2/3 of the voltage in (12v in would give 8v out; in this scenario).

Since the ratio is what gives the over all output. What (other than heat) would be the down sides of using 2k ohms and 1k ohms? Or 20k ohms and 10k ohms? or even 1 ohm and 2 ohms?

2. ### hgmjr Retired Moderator

Jan 28, 2005
9,030
218
Typically, the values of resistors in a voltage divider circuit are chosen to keep the current consumption of the circuit to a minimum without compromising the reliable function of the overall circuit in which the voltage divder is being used.

hgmjr

3. ### protomor Thread Starter Member

Dec 23, 2008
22
0
so lower the better.

Can you give an example of what compromising the reliable function of the overall circuit would be?

4. ### hgmjr Retired Moderator

Jan 28, 2005
9,030
218
Lower current consumption from the power source is generally desireable. Lower current consumption corresponds to higher values of resistance.

One example is the bias network on the input of an common-emitter transistor amplifier. If the values of the resistors are too low then the effective input impedance to the amplifier is lower which is generally undesireable. If the values of the resistors are chosen too high then changes in the base current of the transistor will have a greater influence the bias voltage setpoint.

hgmjr

Apr 5, 2008
18,899
3,717
Hello,

It will work if the "load" has a very high resistance.
With low resistance the voltage wil change.

Here is an example without and with load.

Greetings,
Bertus

File size:
14.4 KB
Views:
38
6. ### protomor Thread Starter Member

Dec 23, 2008
22
0
so bertus, in your example how would you get it back to 8v? use 1k and 2k instead of 100 and 200?

@hgmjr. that example kinda went over my head. I don't know what a bias network is or an emitter transistor amplifier. I'm really new to this.

Apr 5, 2008
18,899
3,717
Hello,

When you change the voltage divider to 1 and 2 Ohm the voltage will go back to 8 Volts.
This will give a lot of current in the divider that is converted to heat and will shorten battery life.

Greetings,
Bertus

8. ### protomor Thread Starter Member

Dec 23, 2008
22
0
I think I get it.

Lowering the resistance values in a voltage divider raises consumption while higher resistance lowers consumption.

So, generally, you want to use higher valued resistors unless (like in Bertus' example) it disrupts the circuits normal function.

9. ### hgmjr Retired Moderator

Jan 28, 2005
9,030
218
It sounds like you have grasped the concept.

Study and practice DC circuit analysis to explore the topic in greater depth.

hgmjr

Apr 5, 2008
18,899
3,717
Hello,

Another possebility to keep the voltage at higher resistances is using a buffer.
This can consist of an opamp.

Greetings,
Bertus

File size:
18.6 KB
Views:
34
11. ### protomor Thread Starter Member

Dec 23, 2008
22
0
wow that sounds like a great idea if I ever run into that problem. I just looked up opamps and it sounds really useful.