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I have done DC motor control with an H-bridge circuit before. I used a 3.6 V, 9000 mAh (ERC26500) battery and had no problems.

Now I want to use a latching solenoid and I need 7.2 V for that, so I will use 2 batteries in series. At the same time, I will regulate the 7.2 V voltage and reduce it to 3.3 V. This 3.3 V will power the STM32.

I had a problem when I made this circuit. The instantaneous current of the battery was not enough for the solenoid. That's why I added an ultra capacitor to the circuit. After adding the capacitor, the solenoid started working without any problems. However, while the solenoid is operating, it also draws some current from the battery and the STM32 does not receive enough power.

That's why I want to disconnect the battery and ultracapacitor before the H-bridge circuit works. So, the solenoid will only be fed from the ultracapacitor and the battery will continue to supply only the STM32.

I decided to use a depletion type MOSFET (Q1 in the schematic) so that the battery does not feed the solenoid. Is there any other method to break the connection between battery and capacitor for 1-2 seconds?

enter image description here

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  • \$\begingroup\$ Some observations and questions. The maximum recommended current for your battery is 150mA with 300mA pulse capability for 100ms. This is quite low for a solenoid; do you have a datasheet? How long do you intend to pulse the solenoid off of your ultracapacitor? \$\endgroup\$
    – vir
    Commented Aug 8, 2022 at 20:24
  • \$\begingroup\$ You won't be able to drive the high side PMOS devices on a 7.2V rail using 3.3V GPIO from your ST32, although perhaps you are regulating it to 3.3V by some means not shown. And I'm not sure how Q1 will isolate the pulse cap from the battery. Perhaps you can use an opto-isolator. \$\endgroup\$
    – PStechPaul
    Commented Aug 8, 2022 at 20:24
  • \$\begingroup\$ @vir, Yes, the instantaneous power of the battery is not enough for the solenoid. So I want to charge the supercapacitor first and feed the solenoid from the supercapacitor. The solenoid draws 1.8A current and 500ms is enough. But I will feed it for 1 second to avoid a problem. I am planning to use a supercapacitor with 2.6A pulse current. \$\endgroup\$
    – harmonica
    Commented Aug 9, 2022 at 4:37
  • \$\begingroup\$ @PStechPaul I will regulate the voltage to 3.3V with a regulator. I don't know how to isolate the battery and capacitor from each other when the H-bridge starts working. I think opto-isolator will not do exactly what I want. \$\endgroup\$
    – harmonica
    Commented Aug 9, 2022 at 4:41
  • \$\begingroup\$ Just how will you be driving the solenoid from the capacitor? You might be able to just add a resistor between the battery and the capacitor so it keeps it charged up, but will limit the current from the battery when you activate the solenoid for 1 second. \$\endgroup\$
    – PStechPaul
    Commented Aug 9, 2022 at 8:42

1 Answer 1

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This strikes me as easier to solve with two diodes like below. Any capacitor charge on the uController side will not be used by the solenoids and vice versa.

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ Thanks for the answer. Actually, that's not exactly the problem. I don't want the solenoid to use the battery during operation. \$\endgroup\$
    – harmonica
    Commented Aug 9, 2022 at 4:31
  • \$\begingroup\$ You can do it this way with one diode. @harmonica Why don't you want the solenoid to use the battery during operation? Why does it have to be exclusively powered by the capacitor? \$\endgroup\$
    – MOSFET
    Commented Jun 7 at 20:25

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