Automatic Triggering of a Water Pump with 12 volts followed by 7 volts operating voltage

[DBCOOPER]

Hi,

I've just registered to ask this specific question. I have a watercooled pc, the waterpump (3 pin by the way, no pwm) only starts up with 12v. Since the fan controller sockets on my motherboard are defect, the pump runs continiously on 12v, which is annoyingly noisy.

I need a circuit which supplies the pump initially with 12v for 1-2 seconds on system startup and then automatically varies the voltage down to 7 volts.

The pump is rated 12v@ 0.5 Amps.

I can solder and can read basic schematics but noting too complex.

Thank you in advance.

 

[AnalogKid]

Without getting into PWM and all of that borscht, the simple approach is to put a resistor in series with the pump to set the low speed, and then short out the resistor during startup. This can be done with a small power MOSFET or bipolar transistor. The equivalent resistance of the pump is 12 V / 0.5 A = 24 ohms. 5/12ths of that is 10 ohms. This is a starting point for the series resistor value, but in reality it probably will have to be a bit smaller to leave 7 V across the pump. So, 8.2, 9.1, or 10 ohms, 5 watts. If you want to go this route I'll whip up a schematic. Note that the resistor will be dissipating something over 2 W continuously.

ak

 

[DBCOOPER]

Without getting into PWM and all of that borscht, the simple approach is to put a resistor in series with the pump to set the low speed, and then short out the resistor during startup. This can be done with a small power MOSFET or bipolar transistor. The equivalent resistance of the pump is 12 V / 0.5 A = 24 ohms. 5/12ths of that is 10 ohms. This is a starting point for the series resistor value, but in reality it probably will have to be a bit smaller to leave 7 V across the pump. So, 8.2, 9.1, or 10 ohms, 5 watts. If you want to go this route I'll whip up a schematic. Note that the resistor will be dissipating something over 2 W continuously.

ak

Sounds like something I'd be capable of with my "soldering" skills. I'm planing to connect a small pcb breaboard with your suggested circuit to a 12 volt output and put a 3 pin connector on to it to connect my pump. I'd be glad if you could give a schematic with readily available components.

 

[Alec_t]

waterpump (3 pin by the way, no pwm)

Assuming two of the pump pins are for 12V and ground, what is the third pin for if not for PWM?

 

[DBCOOPER]

Assuming two of the pump pins are for 12V and ground, what is the third pin for if not for PWM?

A far as I know, the yellow cable is the tachometric signal (sense), the motherboard senses the rpm of the fan/pump over this pin.

 

[Alec_t]

Here's another option, using a 555 to generate a PWM signal whose duty cycle drops over a ~1 sec period after power-up. This signal switches a MOSFET to control the pump motor, dropping its effective supply voltage from 12V to ~7V (adjustable with the pot) over that period. The advantage of using PWM is that there is no wasted power in a dropper resistor that would otherwise be used.

The switching frequency is adjustable by changing the value of R3 or C3.
PWM can conflict with the internal switching in some BLDC motors, so this circuit may or may not be workable with your particular motor.