Transistor Relaxation Oscillator Circuit

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The relaxation oscillator circuit configuration provides a very easy way of creating a simple oscillator circuit, in this case using a single transistor and very few other components.

The relaxation oscillator is a well known principle and can be used with a variety of components including an operational amplifier, field effect transistor, vacuum tube of valve, but in this case we will use a single transistor.

Although this simple one transistor circuit is very convenient for many situations, it would not be recommended for a circuit design as it relies on the uncontrolled reverse breakdown of a junction. Additionally the thresholds used in the circuit are not accurately defined and the frequencies and performance will vary from one transistor to the next - soem transistor may jot even work, whereas another of the same type may.

With this not of caution, this one transistor oscillator is still an interesting circuit to explore and show some circuit operation.

What is a relaxation oscillator

The simple one transistor oscillator circuit is a relaxation oscillator. This is a particular type of oscillator and it is worth describing what it is to give an understanding of the term so that a better insight into its operation can be gained.

Essentially a relaxation oscillator is one where the circuit has a switching device, in this case a transistor, although other devices including relays tunnel diodes, comparators, operational amplifiers FETs and other electronic components can be used.

In the circuit, a capacitor (or inductor) repetitively charges until it reaches a certain level, where upon the circuit switches and the capacitor discharges until it falls below another threshold at which point it charges again.

This process continues indefinitely until power is removed from the overall circuit.

In summary, for a relaxation oscillator, the active device switches between charging and discharging modes, whereas other oscillator circuits use feedback to excite oscillation in the resonant circuit.

Often relaxation oscillators are used to generate low frequency signals, and they are often chosen because of their simplicity rather than their performance.

There are various forms of relaxation oscillator - even a two transistor astable multivibrator falls into the category of a relaxation oscillator.

One transistor relaxation oscillator circuit

There are many formats for the relaxation oscillator, some have an audio output, others demonstrate the circuit with an LED in the emitter side of the transistor which flashes in line with a low frequency oscillation from the circuit.

It is a matter of selecting the various components within the circuit to provide the required results.

This circuit design relies on the reverse breakdown of the transistor and as such it is not a very elegant circuit. Also the reverse breakdown characteristics of individual transistors vary and therefore it may be necessary to try a few transistors to see which works best.

The basic circuit is given below, and from this, it can be seen that it is particularly simple, requiring just a few electronic components - this is its attraction.

From the circuit it can be seen that the base of the transistor is not connected and the transistor itself appears to be the wrong way round.

To understand about these unusual aspects of the circuit, it is necessary to look at why this circuit works, and then at its operation.

The essence of the circuit is that the transistor operates in a reverse biassed mode. The emitter of the NPN transistor is connected to the positive junction of the charging resistor and capacitor and the negative line is connected to the collector, often via an LED, etc.

The emitter junction within the transistor has a relatively low breakdown voltage - often between about five and severn volts. If this breakdown voltage is exceeded then the junction will breakdown and conduct electricity, even though it is nominally reverse biassed.

It is worth noting that the voltage used to drive the circuit must be sufficiently high to enable this breakdown to happen, but not so high that the transistor is damaged. Also the current must be limited to prevent too much current flowing in the reverse breakdown condition. 12 volts or thereabouts is normally sufficient.

It is worth noting that in normal operation of a transistor, the collector emitter junction is reverse biassed, but as the breakdown voltage of this junction is much higher, the circuit would not work.

It is also worth noting that for this circuit to work, the base should not be connected and left pen circuit, otherwise the voltage required for breakdown might be increased.

To explain the operation, once the power is applied to the circuit, the capacitor C1 starts to charge up through the resistor R1. The voltage across C1 rises and as the transistor TR1 is in parallel with it, the voltage across the transistor rises as well.

The voltage rises more quickly at first and then the rate slows - it rises exponentially towards the supply voltage.

However, when the voltage across the base emitter junction rises to the breakdown voltage, the transistor passes current. At this point the waveform on the capacitor has reached its peak voltage.

AT the switch on point of the transistor, the capacitor then discharges. The rate is governed by the effective resistance down to ground, i.e. that of the transistor plus, often an LED. Remember also that there is still current coming in from resistor R1, so this will also have some effect on the discharge rate.

With current passing through the transistor the LED turns on and glows.

However, a point is reached where the transistor breakdown ceases, and at this point the LED turns off, but also the capacitor C1 starts to charge again, and the cycle is repeated for as long as power with a sufficient voltage is applied to the circuit.

Even though the discharge also occurs exponentially, the aiming point is below the turn off point and as a result it often appears to be more linear.

It is worth noting that the voltage across the capacitor can be calculated from the following formula.

Where::
    V_C is the instantaneous voltage across the capacitor at time t
    V_S is the input or supply voltage
    e is constant with the value 2.7182
    t  is the elapsed time since the application of the input or supply voltage
    RC is the time constant of the RC charging circuit

The RC time constant is obtained by multiplying the value of the capacitor in Farads by the value of the resistor in Ohms.

The time constant, often denoted by the Greek letter τ represents the time required for the voltage across the capacitor to reach approximately 63.2% of its final value after a change in voltage is applied to such a circuit.

The one transistor relaxation oscillator circuit is one of the simplest oscillator circuits out there. However it is not one that runs the transistor in a reliable mode - in fact the variation in breakdown voltage means that some transistors, even of the same type will work better than others and some not at all. It is a circuit that should not be used for any widely used project, but one that can be useful for the odd breadboard or hook-up where a simple oscillator may be required.

  Written by Ian Poole .
  Experienced electronics engineer and author.

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