Voltage Dividers
What it does
Voltage dividers chop voltages into smaller parts, like salami!
Resistors
Most voltage dividers are just two resistors connected in series, where an input voltage is applied across these resistors and where a new output voltage emerges from the connection between them. See simulation below.
Formula
Vout = Vin * R2 / (R1+R2)
Formula (R1=R2)
Vout = Vin / 2
- 12 volt is applied over R1 and R2 as input voltage.
- Because both resistor values are the same (1k Ohm) the output voltage is exactly 12V / 2 = 6V.
- Please try different values in the simulator.
Potentiometers
Potentiometers are often used as variable voltage dividers. Internally you change the resistor values and with this you change the resulting output voltage. Whenever you turn a knob on a synthesizer you are most likely turning a potentiometer.
- In the left version we have a potentiometer similar to the two resistors.
- In the right version I changed the ground reference to -12V instead. When the wiper of the potentiometer is centered the output voltage will be exactly between +12V and -12V = 0V.
- Please play with the wiper position in the simulator.
Passive crossfader
When you use dynamic voltages instead of fixed voltages you get crossfader.
- Please play with the potentiometer, and notice the output.
Min-Max
If you look at the first potentiometer example you will see that the output ranges between 0V to 12V. However you often want to tweak the output range without changing the input voltage (because that's simply a easy voltage you already have in your circuit). For this I use often extra fixed resistors on one or both sides of the potentiometer.
Formula Minimum (Potentiometer Fully Closed)
Vout = Vin * R3 / (R1+R2+R3)
Formula Maximum (Potentiometer Fully Open)
Vout = Vin * (R2+R3) / (R1+R2+R3)
- Notice R1 and R3.
- Please try different values in the simulator, and play with the wiper position.
- Do you want to have more knob control over a smaller voltage range? Increase the fixed resistor values relatively to the potentiometer value.
- Do you want to have more range? Decrease the fixed resistor values relatively to the potentiometer value.
Logarithmic
Standard potentiometers are lineair. When you turn lineair potentiometers at 50% (centered) the voltage output is 50% of Vin, while with logarithmic potentiometers you would get a lot smaller voltage output (10% or so). This can be favourable to control certain signals. You can create your own Logarithmic potentiometers with the circuit below.
| Wiper | 0% | 25% | 50% | 75% | 100% |
|---|---|---|---|---|---|
| Lineair | 0V | 3.0V | 6.0V | 9V | 12V |
| Log | 0V | 1.0V | 1.7V | 3.1V | 12V |
Formula
Vout = Vin * R2 * RL / (R1 * RL + R2 * RL + R1 * R2)
- The table gives only the calculations for this circuit.
- Notice at 0% and 100% both potentiometer types are equal.
- Notice until 75% the output voltage of the logarithmic is fairly low.
- Notice at the final stage 75-100% the potentiometer jumps up to full voltage.
- Please play with the fixed resistor to change the logarithmic behaviour.
TIP
A logarithmic scale is useful when a large range is required together with a fine control over the lower voltages.
Anti-logarithmic
The output voltage of the anti-logarithmic potentiometers jumps up at the start and then slowly goes to its maximum.
| Wiper | 0% | 25% | 50% | 75% | 100% |
|---|---|---|---|---|---|
| Lineair | 0V | 3.0V | 6.0V | 9V | 12V |
| Anti-Log | 0V | 8.9V | 10.3V | 11.V | 12V |
- The table gives only the calculations for this circuit.
TIP
The anti-logarithmic scale is useful when a large range is required together with a fine control over the higher voltages.
LOAD
We learned from the log and anti-log section that if a simple voltage divider is connected to any parallel resistor that resistor affects the voltage output. This effect is unfortunatly often something we don't want. If we would add directly to our simple voltage divider a LED, a VCO, a speaker or actually any other circuit these circuits could have the same effect as the load resistor due to their impedance. We might get very unwanted effects.
Potentiometer values
You might think a voltage divider is a voltage divider, so the value of a potentiometer doesn't matter so much. What really happens if we change the potentiometer value with a parallel "load" ? Well see below.
- Notice I have only changed the potentiometer values.
- Notice the output voltages. The resistor in the LOAD area represents any circuit with a certain impedance.
- Notice the smaller the potentiometer, the closer the voltage output goes to our ideal 6V.
TIP
- Smaller potentiometer values:
- Consume more current.
- Can handle small load impedances well.
- Are less noisy.
- Larger potentiometer values:
- Consume less current.
- Can handle small load impedances not very well.
- Are more prone to noise.
Load impedance
What if you change a "load" parallel to the potentiometer?
- Notice I only changed the "LOAD values".
- Notice the output voltages. The resistor in the LOAD area represents any circuit with a certain impedance.
- Notice the smaller the impedance of the load (RL) is the more effect the load has on the output voltage.
- Notice the bigger the impedance of the load (RL) is the closer the voltage output comes to our ideal 6V.
TIP
To get rid of these "load effect" use a buffer! See Buffers.