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Although this question sounds basic but, I have been trying to implement this precharge circuit to safely turn on the inverter without overloading the system, however especially when I powered it on, the relay turned on in no time(didn't seem the delaying(timing) works) and I have no clue why, although using a MOSFET do solves it but it gets hot due to turning it on in the linear region.

Here the 3904 here is just to charge the relay slowly and turn it on, after C4 gets charged. The TIP122 here is just to charge the inverter slowly from the capacitor till the relay turn the whole system on(by connecting the inverter to the ground).

enter image description here Note:

The 600 milliohm (R5) here symbolize an inverter(I know it should be kind of inductance and capacitance but just for simplicity) which will draw maximum 24V 40A 1000W.

I need just to charge the capacitor first (C4) then using with the TIP122 to charge the inverter before the relay turns on so the system doesn't get overloaded.

Please if you need more information write in the comments below.

Edit: sorry for the mistake that happened in Q2 I had corrected the schematic.

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    \$\begingroup\$ are you certain that the schematic diagram is correct? ... R2, R3, C3, C4 and Q2 are separate from the rest of the circuit ... they only share the power supply \$\endgroup\$ Commented Jul 24, 2023 at 3:36
  • \$\begingroup\$ can you please clarify it for me, by meaning if they were separate from the rest of the circuit? \$\endgroup\$ Commented Jul 24, 2023 at 16:03
  • \$\begingroup\$ slide the Q2 collector connection along the V+ node all the way to the battery ... slide R1/C1 along the V+ node and along the GND node until they are next to Q1 ... look at the schematic diagram again \$\endgroup\$ Commented Jul 24, 2023 at 16:23
  • \$\begingroup\$ HELL!! I didn't ever notice the Q2 thing it should be connected to the inverter - side(GND or pin 4 of the relay) although I checked it before asking the question. \$\endgroup\$ Commented Jul 24, 2023 at 16:33

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schematic

simulate this circuit – Schematic created using CircuitLab

This shows that there is reasonable delay (about 300 ms) from application of voltage to closing the relay. My previous post had erroneously used 100 mF for C3.

Voltages

Load current

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    \$\begingroup\$ (10000 μF isn't really 100 mF. Renumbering components may make discussion cumbersome.) \$\endgroup\$ Commented Jul 24, 2023 at 5:15
  • \$\begingroup\$ I corrected the circuit component values and labels. Seems to work well enough. Perhaps C1 is staying charged and not enough time between tests. \$\endgroup\$ Commented Jul 24, 2023 at 6:14
  • \$\begingroup\$ (For an entirely irritating display of simulation results, switch voltage scale to logarithmic.) I guess I don't get what current gain circuitlab is using for the TIP222 - shouldn't \$V_{R3} (V_{cap})\$ be anywhere from 7 to 22 V? \$\endgroup\$ Commented Jul 24, 2023 at 9:36
  • \$\begingroup\$ @PStechPaul thanks for this great simulation. Should there be a parallel resistor with C1 to make it later discharge? \$\endgroup\$ Commented Jul 24, 2023 at 16:18
  • \$\begingroup\$ @greybeard I had simulated it on Falstad too(I used a MOSFET because the transistor didn't wants to work as intended) hope this helps to clarify it: tinyurl.com/28djef7u \$\endgroup\$ Commented Jul 24, 2023 at 16:29

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