Relaxation Oscillator with Variable Duty Cycle

A “Relaxation Oscillator” is just a fancy name for a circuit which generates a repetitive output signal (Wikipedia says this must be nonsinusoidal). Most examples are for 50% duty cycle designs. Here, build one using the NXP NXC2200.

Components

• Comparator (op-amp will work too)
• 3x equal value resistors (R1-R3)
• 1x small signal diode
• 1x capacitor in the 1-20nF range (C1)
• 1x resistor in the 100 to 500k range (R4)
• 1x resistor approximately 20% the size of R4 (R5)

Wire up the circuit like this:

Relaxation Oscillator - EveryCircuit
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Note that in this example, R1-R3 are 510k, C1 is 8.2nF, R4 is 330k, and R5 is the combination of 50k and 30k – simulation is a bit wonky when it comes to making this converge.

Testing

Wire up the comparator/op-amp, and take a look on your scope. The pulse width seen at the output should have a variable duty cycle. This is a direct result of the time constant for charging C1 being smaller than the time constant for C1 to discharge (due to the diode allowing for a parallel resistance only when C1 is charging).

Uses

Timing circuits, periodic enable lines (or wakeups), triggers for sampling, etc. The shorter on-time can allow for lower quiescent current when attempting to enable a power-hungry device that doesn’t need to be on for the full 50% duty cycle.

Additional Notes

• The quiescent current of this circuit can get fairly low; on the order of microamps, depending on component selection.

• If repeatable thresholds/oscillation frequencies are not a concern, this can be made pretty cheaply with a Schmitt Trigger instead of comparator/op-amp.