There are applications where the ramp generated by a relaxation oscillator can’t be used because it is exponential. In fact, the charge of a capacitor through a resistor follows an exponential curve.
However, it is possible to modify a load circuit of a capacitor by causing the resistor to be replaced by a constant current source and with that the ramp generated is linear. Such a circuit can serve as a time base for viewing waveforms and samplings much more accurately than the ramp generated by a common RC.
It is precisely this solution that we present based on the known circuit 555 and which is shown in figure 1.
The transistor may be the BC558 or equivalent and the maximum operating frequency is in the range of 100 kHz for the 555. Capacitor C2 and resistor R1 should eventually be changed to obtain better linearity. The waveform of the signal generated by this circuit is shown in figure 2.
Circuit tripping is done with the negative transition of the input signal. Eventually a resistor of 10k to 47k should be connected between the trigger input and the positive of the power supply.
In figure 3 we have a printed circuit board suggestion for the implementation of this linear ramp generator.
MATERIAL LIST
CI-1 - 555 - integrated circuit, timer
Q1 - BC558 or equivalent - general purpose PNP transistor
C1, C2-10 nF - ceramic capacitors or polyester
R1 - 10 k ohm x 1/8 W - resistor
R2 - 39 k ohm x 1/8 W - resistor
R3 - 100 k ohm x 1/8 W - resistor
Several:
Printed circuit board, power supply, wires, solder, etc.
