# Why doesn't my opamp relaxation oscillator oscillate?

I have designed a relaxation oscillator with an opamp. It is supposed to oscillated at 50Hz, but it doesn't. I haven't built a physical circuit, I'm trying to simulate it in CircuitLab.

I calculated the oscillation frequency with the circuit element values in the schematic as

$$f = \left( T_c + T_d \right)^{-1} = 50.17Hz.$$

Where, $T_c$ and $T_d$ are charging and discharging times of the capacitor respectively;

$$T_c = RC \ln \left( \dfrac{(+12V) - \dfrac{R_2}{R_1 + R_2} (-12V)}{(+12V) - \dfrac{R_2}{R_1 + R_2} (+12V)} \right) = 9.97ms, \\ T_d = RC \ln \left( \dfrac{\dfrac{R_2}{R_1 + R_2} (+12V) - (-12V)}{\dfrac{R_2}{R_1 + R_2} (-12V) - (-12V)} \right) = 9.97ms.$$

What am I doing wrong here?

simulate this circuit – Schematic created using CircuitLab

Without the $R_i$ resistor:

With the $R_i$ resistor:

• Of course! The oscillators start with the ambient noise. I forgot that. Adding the $R_i$ resistor initiated the oscillations. Thanks. – hkBattousai Aug 6 '14 at 13:19