The post explains how an efficient sawtooth generator circuit can be built using just a handful of ordinary parts such as BJTs and resistors.
Sawtooth generators are often recommended in many divisions of electronics. We for that reason include a new design for a circuit that makes use of components that you can get in about any container of add-ons (or ‘rubbish’ to unbelievers). The fundamental model of the circuit as demonstrated in figure 1 works with a 9 V battery as the supply.
The circuit by itself could be conveniently comprehended: capacitor C1 is linearly charged by constant current source T1, R1, P1.
Transistors T2 and T3 are employed as replacement for a silicon controlled rectifier (SCR) and in case you bear this in your mind, the circuit is relatively simpler to understand.
The ‘SCR’ is not really, as usual, fired by a pulse. Rather, the ’gate' is biased by voltage divider R3/D2 and when the ‘anode to cathode’ voltage surpasses this bias, the 'SCR' fires. C1 then discharges quickly via the ‘SCR’ and current limiting resistor R2.
Once the voltage across the capacitor has dropped to approximately 1.4 V, the current from the 'SCR' is becoming low enough for it to cut off. C1 once again charges and the routine repeats.
The ensuing sawtooth output voltage is demonstrated in figure 1. The frequency of the output voltage may be tweaked within a range of about 102 ; with the values proven the frequency range is 5...500 Hz.
The smaller C1, the more quickly it is going to charge, and the higher the frequency. The circuit was tested in our laboratories with frequencies up to 100 kHz, however higher frequencies are usually achievable.
The amplitude of the sawtooth voltage will depend on the ‘gate’ bias across zener diode D2 and it can consequently be changed by switching this diode. It will, however, be borne in your mind that tne zener voltage should not be over fifty percent the supply voltage to make sure appropriate functioning of the generator.
If an exponential ramp is necessary rather than linear one, T1 can basically be removed and R1 hooked up straight to the supply voltage. C1 will likely then charge straight from the supply and automatically offer an exponential waveform in this simple sawtooth generator circuit.
Hildy says
Hello. That is not an SCR in your schematic. It is a PUT, a type of Unijunction transistor. (The SCR diagram has the gate terminal on the cathode side.) It still works pretty much as you described. Once the Anode voltage exceeds the gate voltage (as set up by the voltage divider) it conducts and releases the stored energy of C1.
admin says
You are right, thanks for the useful information.