Simple RC Delay Circuit

In the process of circuit design, some circuits that require timing or delay are often encountered. For precise timing, many chips or MCU internal timers on the market can be implemented, but for some timing that does not require too precise. Based on the principle of cost saving and energy investment, the simplest is a simple RC delay circuit, one capacitor and one resistor can achieve a rough delay function.

① The principle of RC delay

As shown in the figure, after the 3.3V power supply passes through R1, it will charge C1, and the voltage at point A will increase from 0. for the subsequent circuit, point A will increase to the effective voltage required by the subsequent circuit. The time T can be regarded as the delay time.

② Calculate the time constant of the RC delay circuit

In circuit design, when it is not necessary to be too precise, the time delay generated by the RC delay circuit can be calculated directly by RC, for example, R1=10K, C1=0.1uf in the figure. R*C=1ms. Note that R in RC is converted to ohms (Ω) and C is converted to farads (F). After that the product is in seconds (s)

③ Calculate the delay time of the RC delay circuit

The process of RC delay is the process of charging the capacitor C, which uses the capacitor charging and discharging formula:
Vt=V0+(V1-V0)*[1-exp(-t/RC)]
V0 is the initial voltage value on the capacitor, V1 is the voltage value that the capacitor can finally be charged to or put into, and Vt is the voltage value on the capacitor at time t.
For example, a capacitor with a voltage of E charges a capacitor C with an initial value of 0 through a resistor with a resistance value of R. At this time, V0=0, V1=E, and the voltage on the capacitor at time t is: Vt=E*[1- exp(-t/RC)].
If it is a discharge process, the capacitor whose initial voltage is E is discharged through the resistor whose resistance is R. According to the formula V0=E, V1=0, the voltage discharged on the capacitor at time t is: Vt=E*exp(-t/ RC).
For the above picture, V0=0, V1=3.3V, RC=1ms. So Vt=3.3*[1-exp(-t/1ms)]. If for the subsequent circuit we need point A to go up to 1.5V. Then the delay time t can be calculated. Or we want to delay the time by 3ms. After determining the delay time, we can calculate the other parameters of the RC circuit by configuring any two of the three variables of power supply, resistance and capacitance.

④ advantages and disadvantages

The RC delay circuit is simple and practical, and the cost is low. But it is only suitable for short delays in imprecise scenarios. According to the principle, limited by the storage capacity of the capacitor, a long time delay cannot be realized. In actual use, if a longer delay time is required and certain accuracy is required, a time relay can be selected. In automatic control, RCs and relays are commonly used, and switches are combined into circuits with different functions such as slow suction or slow release, and rapid suction. In short, according to the actual usage scenarios and functions, or the needs of the actual design, an appropriate delay circuit can be selected.

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