Long time delays – why they are difficult to achieve with just RC networks
It is difficult to achieve timing delays of more than a few minutes with the standard 555 circuit because of the way electrolytic capacitors behave. When the capacitor plates are charged a small current passes through the dielectric – this is called the leakage current. A real capacitor behaves like a perfect capacitor in parallel with a leakage resistance. To achieve long time delays, a large capacitance in required. Unfortunately, these are only available as electrolytic or tantalum types which have high leakage currents. The circuit shown in fig 1 shows how the leakage resistance modifies the time delay.
The time constant of the 220µF capacitor charging though a 10M resistor is:
But the leakage current of an electrolytic capacitor (at max working voltage) is typically 10 nA/µF
For the 220uF capacitor the leakage current is:
If this leakage current is driven by the 6V supply, the leakage resistance R2 is:
The leakage resistance R2 forms a potential divider with the timing resistance R1 (fig 1). When charge stops flowing into C1, the output voltage is set by R1 and R2, in this case settling at 1.5V. This is nowhere near the voltage required at the threshold pin of the 555 to end the timing period. In other words, the capacitor leakage current not only affects the accuracy of the timing equation for large time delays but may cause the capacitor to stop charging at a voltage significantly lower than the supply voltage, which prevents the timing period from ending.