# How to self start a MOSFET multivibrator circuit

I have built a simple multivibrator circuit using IRF530s for the switchers. It does not start unless I introduce noise by touching one of the timing caps. I tried to include a circuit image but as a new user could not. It is a textbook multivibrator only using power MOSFET instead of Bipolar Transistors. How can I make this circuit start at power up?

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If you provide a link to the image, we'll edit it into the question. – Dave Tweed Jan 10 '13 at 17:00
You can see the circuit at: circuitlab.com/cs56fxj3 I have tried different timing values and voltage supplies. Both in the simulation and in the lab the circuit will not self start, but always starts with just a small touch of my finger. – user17689 Jan 10 '13 at 17:52
– user17689 Jan 10 '13 at 18:02
@user17689 How does one execute a small touch of the finger on the circuit sim? :-) – Anindo Ghosh Jan 10 '13 at 18:14
no time to write an answer, but see virtual.cvut.cz/dyn/examples/examples/electronic/mosmulti/… – Phil Frost Jan 10 '13 at 19:25

## 2 Answers

Instead of applying a static 20 V power supply or a slow start PSU, pick a voltage source that starts instantaneously. When simulating, pick a step voltage source that is 0V for the first second, then after 1 second make it step to 20 volt. This step introduces enough noise in the circuit to make it start.

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Some observations:

1. Why are R1 and R2 connected form gate to GND and not from gate to V+? IRF530 is an N-Channel-MOSFET, and it need a continuous positive voltage on the gate to remain "on". Your circut can only provide positive spikes at the gates through the capactors, and maybe through Crss (Miller Capacitance), but no sustained positive gate voltage.

2. In your simulation, the circuit will have trouble starting up because it is exactly symmetrical. Disturb the symmetry (e.g. by making R3 = 1 k$\Omega$ and R4 = 1.001 k$\Omega$), and it may start. A real circut will have enough imperfections to start, no need to buy a 1.001 k$\Omega$ resistor in Schillicon Valley.

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