zener Flyback diodes issues and pull-in & hold current issues in this Relay circuit

Description: Although this may be a basic question, there is still some struggle with it. In this schematic, two zener diodes D1 and D2 are connected back-to-back across relay coil L1. The breakdown voltage (BVds) is -30V for Q1. The question arises whether 15V zeners (Vz = 15V) can be used for D1 and D2 instead of 5.1V zeners. Concerns are raised about potential damage to the relay coil or contacts during the turn-off phase of the relay. The relay being used is a 5V DC standard coil. Additionally, to reduce the steady-state current consumption of the relay coil, an RC circuit is proposed. When Q1 turns on, the uncharged capacitor temporarily acts as a short circuit, allowing maximum current to flow through the relay coil and closing the relay contacts without chatter. As the capacitor charges, both the voltage across and the current through the relay coil decline. The circuit reaches a steady state when the capacitor has charged sufficiently such that all current through the relay coil is flowing through R1. The contacts will remain closed until the drive voltage is removed. The best placement for the RC circuit is questioned, whether it should be in section A or B of the schematic. Section B appears to be the preferred choice since when Q1 turns off, capacitor C1 can discharge via R1 to ground. The inquiry continues about how C1 would discharge if the RC circuit is placed in section A. There is a concern about potential side effects of placing the RC circuit and whether there is a better solution. The schematic also includes a 6V DC standard coil with a resistance of 48.5 ohms, and C1 is set to 10uF. Assuming the R1C1 circuit is placed in section A, with a power supply of +5V and a hold-on voltage drop of 3V across the relay coil, the required current through the coil is approximately 62mA. Consequently, the voltage drop across R1 at steady state is calculated to be 2V, resulting in R1 needing to be 32.33 ohms for the steady-state current of 62mA. The datasheet indicates an operate time of 15ms at worst case. The RC time constant is calculated as 48.5 ohms x 10uF = 0.485ms. Therefore, C1 will be nearly fully charged within approximately 2.425ms after Q1 is turned on. Upon turning off Q1, back EMF is generated and clamped to 3.3V by zener D2 (Vz = 3.3V) plus the diode D1 drop of 0.7V. The voltage across C1 then becomes -2V + (-3.3V - 0.7V) = -2V. The charge on C1 remains at 20uC, and since capacitance is constant, the charge must decrease as the voltage across C1 drops from +2V to -2V immediately after turning off Q1. The question arises whether the 62mA current will charge or discharge C1, given that the voltage across C1 is 6V as soon as Q1 is turned off. There is confusion regarding the current flow between R1, C1, D1, D2, and the relay coil after Q1 is turned off. Additionally, it is noted that "current in an inductor will not change instantaneously." If there is a flyback diode D1 (which could be a small-signal or Schottky diode, with zener D2 removed), the inquiry is whether there would be even a brief current spike after Q1 is turned off. If a current spike occurs, it may exceed the maximum peak forward current rating of a diode selected for 200mA, potentially damaging it. The steady-state current through the relay coil is 62mA, and the question is whether this current will ever exceed 62mA, even momentarily, after Q1 is turned off.

In this circuit, the configuration of zener diodes D1 and D2 serves to protect the relay coil L1 from overvoltage conditions during the switching events. The choice of zener voltage ratings is critical, as using higher voltage zeners could potentially lead to insufficient clamping of the back EMF generated when the relay is de-energized, which could damage the relay contacts or coil. The RC circuit is designed to limit the steady-state current through the relay, ensuring that it operates efficiently without chatter during activation.

The placement of the RC circuit within the schematic is vital for its performance. By positioning it in section B, the capacitor C1 has a clear discharge path through R1 to ground when Q1 turns off, allowing for a controlled decay of current, which is essential for protecting the relay components. Conversely, if the circuit is placed in section A, the discharge path may not be as effective, potentially leading to unintended voltage spikes across the relay coil.

The calculations regarding the current and voltage across R1 and C1 illustrate the importance of correctly sizing these components to ensure the relay operates within its specified parameters. The relationship between the resistance, capacitance, and the required current through the relay is governed by the RC time constant, which directly influences the response time of the relay activation and deactivation.

Finally, the concern regarding the flyback diode and the potential for current spikes is valid, as inductive loads can generate significant back EMF that may exceed the rated specifications of the diode. The flyback diode must be selected with adequate current handling capabilities to prevent damage during transient conditions. Understanding the dynamic behavior of the circuit during switching events is crucial for ensuring reliable operation and longevity of the components involved.Although this may be a basic question but I`m still struggling with it. In this schematic, two zener diodes D1 and D2 are connected back-to back across relay coil L1. The BVds = -30V for Q1. Can I use 15V(Vz = 15V) zeners for D1 and D2 instead of 5. 1 V zeners Will the relay coil or contacts can get damaged during turn-off of relay If required, I `m using this relay (5V DC Standard Coil). Also, to reduce steady state current consumption of relay coil, I wanna use the RC ckt shown aside in schematic. As soon as Q1 turns-on, uncharged capacitor temporarily appears as a dead short, causing maximum current to flow through the relay coil and closing the relay contacts without chatter.

As the capacitor charges, however, both the voltage across and the current through the relay coil decline. The circuit reaches steady state when the capacitor has charged to the point that all the current through the relay coil is moving through R1.

The contacts will still remain closed until the drive voltage is removed. Which is the best place to put this RC ckt - section marked `A` or `B` in schematic. Will it make any difference Section-B seems to me the best choice, as when Q1 turns-off, capacitor C1 can discharge via R1 through ground. How will C1 discharge when instead I place RC ckt at section-A Am I missing something here Does putting this RC ckt has any side-effects Any better solution Say in above schematic I have 6V DC Standard coil(see datasheet above), 48.

5 ohm relay. And take C1 = 10uF say. Assume that R1C1 ckt is placed at section-A in schematic above. The power supply is at +5V. For a Drop of 3V(Hold-on voltage) across relay coil, the current must be 62mA approx. through coil. So drop across R1 at steady state is 2V. For a current of 62mA through relay coil at steady state, R1 must be 32. 33 ohm. Now in this data sheet, the operate time is given to be 15ms worst case. From above data we have RC = 48. 5ohm x 10uF = 0. 485 ms. So, as soon as Q1 is turned on, the C1 will be almost fully charged in 2. 425 ms. Similarly, as soon as Q1 is turned-off, due to back emf generated and clamped to 3. 3V by zener D2(Vz = 3. 3V) plus diode D1 drop of 0. 7V, the voltage across C1 will be -2V + (-3. 3V - 0. 7V) = -2V. But charge on C1 is still 20uC. Since capacitance is constant, so charge must decrease as voltage across C1 decreased from +2V to -2V instantly after turning off Q1. Will this 62mA current will charge or discharge the C1 The voltage across C1 is 6V as soon as Q1 is turned off right I didn`t get how currents will flow b/w R1, C1, D1, D2 and relay coil as soon as Q1 is turned-off.

"Current in an inductor will not change instantaneously" - While there is a flyback diode D1(Say, D1 is not zener but a small-signal or a schottky diode, and zener D2 is removed in the schematic above), as soon as Q1 is turned-off, will there not even be a current spike(not even for few usecs) I`m asking this becoz, if there is a current spike then the amount of current that will flow during this spike(say > 500mA in this case) might damage the flyback diode if I had selected a diode with max peak forward current rating of around 200mA or so only. 62mA is the amount of current that is flowing through the relay coil when Q1 is on. So, will the current through relay coil never exceed 62mA - not even for a moment(say for some usecs) after Q1 is turned off