Need help desiging something simple.

betwixt said:

It could be a simple as fitting a capacitor across the speed control voltage but without schematics it's impossible for us to confirm that.

A good starting point is to look at part numbers in the PWM controller. The gadgets on Ebay are usually as simple as they can be so they use the schematic provided on the parts manufacturers data sheets. If you can identify some of the parts, even if there is no 'official' schematic it will give clues about how to modify it.

Brian.

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betwixt said:

Looks like a simple NE555 oscillator with the control adjusting the charge and discharge current ratio. There is a good explanation of how it works here:
https://www.instructables.com/id/555-Timer/step7/555-Timer-Astable-Mode-Duty-Cycle/

If I'm right, the potentiomter is used as both RA and RB so as one increases in value the other decreases. It looks like there are some fixed resistors in series to prevent the value being reduced to zero (short circuited). Unfortunately, this means there are alternating signals on both sides of the potentiometer so a simple capacitor to momentarily 'pull' the control one way will not work.

It should still be possible to do but the method depends on how the output transistor is connected. If you can trace a schematic from pin 3 of the IC to the motor connector it would help greatly.

Brian.

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betwixt said:

Thanks, that helps.

In the lower photograph the 'transistor' on the left (behind the power input connector) is actually a voltage regulator, probably type 7812 and the one on the right is an NPN power transistor or possibly a power MOSFET.

The schematic is as I expected and described in the link in my previous message. Unfortunatley the NE555 doesn't lend itself to easy control of the PWM by applying a voltage, it needs the component values around it to change, hence the potentiometer. There is a trick you can try though and it doesn't need any modification to the board at all. You need diode (1N4001 through 1N4007 will work) a 10K resistor and another capacitor. The capacitor value depends on the power requirenment of the fan which I have no information about but I would suggest 470uF as a starting value for experiments. Increasing the value will give a bigger 'kick' to the fan when the power is turned on.

Wire it like this:

The cathode of the diode (banded end) goes to the positive end of the new capacitor. The anode end of the diode goes to the PWM output connection, the one nearest the mounting screw in the corner of the board. The negative end of the capacitor goes to the negative supply input connection. The 10K resistor is wired across the capacitor.

This is how it works: When power is first applied the capacitor will be discharged and will draw current through the fan and new diode until it has charged up to supply voltage. This extra burst of current is what 'kick starts' the fan. The diode prevents the new capacitor discharging back into the PWM transistor when it conducts so the process isn't continuously repeated. The new resistor is there to slowly discharge the capacitor so it can produce a fresh kick start after a few seconds of the power being removed and reapplied.

Try it and see.

Brian.

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xReM1x said:

How do i wire the fan?

I use fan model PFC1212DE. it draw 4.80A at the start.

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betwixt said:

The fan connects across the 'motor' pins on the board.

The 'kick start' diode and capacitor connect between the positive supply to the board and the positive side of the fan.

Looking at the data sheet it appears there are several version of this fan. If you have the type with red/black/blue and yellow wires the control board you have is not suitable. On those versions the controller is built into the fan itself and all you have to do is provide a low power PWM signal.

Brian.

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