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I have a 5V fan that needs to be controlled by a 12V PWM output (the PWM is there to control the fan's RPM).

Can I use an LDO to step down the 12V PWM to 5V? I'd connect the 12V PWM as the voltage input for the LDO, and the voltage output of the LDO would be connected to the fan.

Since this is running of mains and the fan is tiny (20x20x6, 0.4W) I don't care about efficiency. I just want to know if it will work without damaging anything and if the LDO is fast enough that the PWM will carry through to the fan.

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    \$\begingroup\$ What is the PWM frequency? Did you account for the capacitors that linear regulators need on input and output to be stable? \$\endgroup\$
    – Justme
    Sep 11, 2021 at 23:39
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    \$\begingroup\$ From 12 to 5V LDO is not essential. 7805 is good enough to convert voltage. And simple transistor switch after. \$\endgroup\$
    – user263983
    Sep 12, 2021 at 0:28
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    \$\begingroup\$ When you say "controlled by 12V PWM", do you mean as a supply voltage for 5V fan? Or as an input to the the PWM speed control pin? \$\endgroup\$
    – DKNguyen
    Sep 12, 2021 at 0:34

4 Answers 4

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LDO is not suitable for this purpose at all. It has some regulation time, so you cannot expect fast response.

Use follower below rather made with some power transistor like BD...

6v2 zener is for output protection only.

enter image description here

Blue is input pwm, green the output enter image description here

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You can filter the PWM to supply the LDO and that's it.

schematic

simulate this circuit – Schematic created using CircuitLab
A couple of warnings:

The 12V PWM is PWM in order to change the fan speed, if you manage to place the LDO, this feature is lost.
Make sure the PWM output is cappable of supplying enough power.

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  • \$\begingroup\$ Sugguestion on choosing D1? \$\endgroup\$
    – Travis Su
    Sep 25, 2022 at 7:27
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A lot will depend on the frequency of the PWM and the LDO you select.

You will have to deal with various issues:

  1. All LDOs have specific requirements for input/output capacitance requirements so you want to pick one that needs the minimum amount of total capacitance. The reason is that you want the LDO to full turn off when the signal is off during the PWM cycle. If there is a lot of capacitance, the LDO will simply become a filter and present a much steadier signal than you want.
  2. Some LDOs have other 'features' such as slow start that ramp up the output once the input exceeds a threshold. Another feature that may be desirable is that low-voltage cutoff, where the LDO will simply turn off power if the input signal drops below a threshold. In such a case, such a feature is your friend as it will allow the PWM to somewhat pass through to the 5V.
  3. A feature that you could take advantage of is an enable input. If you feed the 12V into the LDO but also into the Enable pin (without capacitance), then the PWM will be presented to the enable input and enable/disable the regulator at the desired frequency. Barring the frequency being high enough that the enable input can't respond to it, this is your best bet.

Depending on the features of the LDO and the PWM frequency, the approach can go anywhere from fully working to not working.

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I'd suggest you don't need any active devices for the application at all.

A simple series 6.8v Zener diode capable of handling the linear power dissipation would seem to be enough.

D1 is rated to 5W. R1 simply provides a load current if you disconnect the fan and ensures the output is always 5.3V maximum. D2 is optional (lot's of folks believe you need a catching diode across a brushless fan, you do not), put it in if you are one of the believers in it's need.

schematic

simulate this circuit – Schematic created using CircuitLab

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