“and any threats you have made to me, obviously come from someone who is not prepared to learn” I am experienced enough to know this is also vastly exaggerated, which doesn’t make it a more reliable statement. “This circuit is one of the worst I have seen” “Before putting a circuit on the web as being a “Quality-Design”, you should ask on a forum for advice and improvements.” Totally lacks, powerful words, you get off on that don’t you? “It totally lacks ELECTRONIC ENGINEERING” “The circuit is not an accurate timing circuit.”Īccurate enough, more accurate than a circuit that discharges at start up. It is so much easier to criticise than to have the imagination to design eh? We have a saying here in my coutry which is literally translated:”the best pilots are on the ground”… Or you could say:”the best swimmers are on the dry”… “if the connections to the gates of T1 and T2 are reversed, the discharging components will turn ON for a brief period”Ī brief period eh? I see another time delay i that then, because if they would be connected to discharge at start up as you describe by swapping the gates of the two fets it will keep discharging C3 and the circuit won’t charge at all. “Altering the circuit to the first two FET’s WILL discharge the cap on start-up” You’d be surprised, just one OB truck in a broadcasting company is laced with these. “There is little or no external interest in this circuit” Diode D1 prevents back EMF when T1 switches off. Capacitor C2 keeps the base bias of T1 steady so that relay clicking can be avoided. Relay remains latched as long as the voltage level in the mains is normal. The relay connected to the collector of T1 energizes and power will be available through the common and Normally Open contacts of the relay. When the voltage in C1 rises above 3.1 volts, zener conducts to trigger T1. The circuit is a zener controlled switch.Ĭapacitor C1 charges through R1 and VR. It gives power to the device only after one to two minutes of delay after the power is switched on. The time delay relay circuit described here is intended for this purpose. If a short delay is provided, such damages can be avoided. The spurious spike in the power supply when power resumes is due to heavy magnetic flux in the distribution transformer in the mains network. Inrush current at power on or power resumes after a power failure can cause unexpected damage in SMPS based power supply of electronic devices. This prevents deleterious effects due to inrush current and spurious spikes at power on. The SMPS based power supply of these modern electronic devices is vulnerable to spikes in the mains line so it gives a time delay of one minute before applying power to the device. I would have accepted Dowwe66 first solution, except that it left GPIO 18 high all of the time (which would have caused the Pi to shutdown immediately after the shutdown monitor daemon became active) and also that the UART TX cannot be used as a path to ground for the EN pulldown (internal resistance while powered off is about 27M).Īll in all, I learned a lot, and now have a working solution.Protect your equipments with this tiny 12V time delay relay circuit. Wesley's solution may have worked, but the component count was too high. And using a voltage divider instead of two diodes in series to step down the voltage for GPIO 18 actually reduced the component count. Thanks to Douwe66 for giving me the idea to use a higher value (100uf) capacitor, which let me use the existing 100K pulldown to complete the RC network. I had a similar solution early on, but the RC values were such that the resistor (2M) was too high a value to function as a pull down, and so EN was always high, which kept the Pi powered on. I found a simple solution as shown below. After shutdown, GPIO goes low, the capacitor discharges, and the. ![]() Raspberry Pi reads GPIO 18 and a script instructs the Pi to power
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