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so i wanted to reduce the speed of my water pump but i feared that the 7-8V might be to low to start it up. So i thought starting it at 12V and then switching it to 7-8V should do the trick. Main question will this wiring work? I am a complete novice and rarely solder or wire things.

My main concern is that once i fire it up at 12V the 0V going to the Step Down controller might damage or even fry it somehow. Also will a regular pc power supply be okay with this.

I deliberatly picked a step down controller because i did not want to use a PWM as i hear all bad thing about using with my pump. I also thought if there was a way to maybe switch automaticaly after 5-10 secounds have passed but i could not find any lead to how to do that.

If anyone has better ideas i am open to change everything about it.

Step down converter is rated for 75W. The chip seems to be a xl4015e. The pump is rated for 18 Watts at 12V but heard that at the start-up it can be more (30W?).

Thanks in advance!

enter image description here

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  • \$\begingroup\$ what is the model number of the step down? \$\endgroup\$ – BeB00 Oct 26 '17 at 3:26
  • \$\begingroup\$ I guess this would depend on the current inrush of your motor and if your step down can handle the current. \$\endgroup\$ – Ron Beyer Oct 26 '17 at 3:30
  • \$\begingroup\$ I dont really know the model number and i am not sure if there even is one. But here are the specs ebay.com/itm/… \$\endgroup\$ – WeRoRa Oct 26 '17 at 3:35
  • \$\begingroup\$ Assuming your DC-to-DC converter is isolated, you also need to connect together the black wires on either side of it. \$\endgroup\$ – DoxyLover Oct 26 '17 at 5:38
  • \$\begingroup\$ @DoxyLover These kind of DCDC converters are not isolated, the -IN and -OUT are shorted. I know it is not isolated since there's no transformer on the board and there's also no shunt resistor (which would not mean isolation but separate grounds). \$\endgroup\$ – Bimpelrekkie Oct 26 '17 at 6:08
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It's unlikely that your step down would care about not having a load (although we'll need the part number to verify this).

What may be an issue is that when the pump is spinning, the moment you switch it to 8V, it may try to push too much current into the step-down. I would still say this is fairly unlikely, and since this is pretty unlikely to damage your pump or your power supply, the only risk is your $4 step down, which isn't a huge risk.

The actual chip is that big black thing next to the copper coil, below the display. That will have a series of numbers and letters on it. If you post those numbers/letters here, the answers will be much more specific and helpful.

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