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I have a pump (6-12V 4-5W) that i want to power up with a generator (9V 1.5A).

I've calculate the current needed with : I = 5W/8V = 0.55A. Next i've calculated the resistance needed : R = 9V / 0.55A = 16 Ohm Finally i've take a 22 Ohm resistor that i plugged in serial.

Despite all of this the pump doesn't work.. But if i just power up the pump directly to the generator without a resistor it works. Could you explain me why my method with the resistance doesn't work, and why it works without it ? Thanks!

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  • \$\begingroup\$ Why add a serial resistor? \$\endgroup\$ – Solar Mike May 2 '18 at 10:40
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Motors aren't simple resistive loads. The current they take depends on the load applied to them. In addition, when stalled and that includes when the motor is first started, the current will be much higher than normal.

Your 22 ohm resistor is limiting the start-up current to the point where the motor hasn't enough power to get moving.

If you're running a 6-12V motor off a 9V supply, then you don't even need a resistor. You are already in the middle of the specified voltage range.

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  • \$\begingroup\$ Perfect, thanks! Could you define me what is a resistive load (with some examples) ? The behavior of a stm32 card is it the same that a motor or do i have to put a resistance ? \$\endgroup\$ – foxem May 2 '18 at 13:34
  • \$\begingroup\$ @foxem At the risk of stating the obvious, a plain resistor is resistive. That includes most heating elements. Motors aren't, because the current they draw depends on the load you put on them, not just the voltage you apply. Ohm's law only applies to resistors. Anything with semiconductors won't be resistive either. An STM32 board will need a proper regulated power supply, not a resistor. \$\endgroup\$ – Simon B May 2 '18 at 16:16

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