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I have a center tapped transformer (12-0-12) and a 24V fan, in order to get the required 24 V for the fan and also bypass the fan noise, I'm thinking of a circuit like this:

schematic

simulate this circuit – Schematic created using CircuitLab

Is the "Circuit A" immune to the fan noise in above diagram condition?

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    \$\begingroup\$ Nothing is immune 100% but that looks as good as you can make it. \$\endgroup\$
    – Andy aka
    Nov 22, 2020 at 14:58
  • \$\begingroup\$ @Andyaka Thanks, any extra tip greatly appreciated. \$\endgroup\$ Nov 22, 2020 at 16:10
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    \$\begingroup\$ If the fan is 24 volts DC then the rectified and smoothed out from the rectifier feeding the fan will be about 32 volts DC. That could ruin your fan if not rated to handle this voltage. \$\endgroup\$
    – Andy aka
    Nov 22, 2020 at 16:13
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    \$\begingroup\$ It does help reduce noise and ripple voltage. \$\endgroup\$
    – Andy aka
    Nov 23, 2020 at 9:03
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    \$\begingroup\$ It won't be as effective due to the capacitor being charged and discharged cyclically all the time. But that's a generalism and so there may be some circumstances where it's different. Sims are your friend here. \$\endgroup\$
    – Andy aka
    Nov 24, 2020 at 19:08

2 Answers 2

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No, your circuit A is susceptible to electrical noise generated by the fan. However, the amount of noise generated by the fan, and how much it would affect circuit A are not clear. A possibly greater concern is that circuit A is supplied with an unregulated voltage source. Circuit A may implement it's own voltage regulation, or it may not need voltage regulation, but in your schematic, it is subject to significant ripple. The filter capacitors remove some ripple, but a significant fraction will remain. A voltage regulator inserted between the filter capacitors and circuit A will dramatically reduce both power supply ripple, and any noise that might come from the fan.

Inserting a linear voltage regulator, which would be cheap and easy, would involve some voltage drop between the filter capacitors and circuit A. However, if you could tolerate the ripple, you could probably tolerate the voltage drop. Also, if your transformer is rated 24VAC, that refers to rms value, not peak. When 24VAC is rectified, it gives more than 24VDC peak output -- more like 24*1.4=33.6V. That is not the average DC output, nor the lowest given the ripple, but the peak. So, if you need 24VDC output, a linear voltage regulator should not be a problem.

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  • \$\begingroup\$ The question has been edited and it now implements your suggestions \$\endgroup\$
    – Anas Malas
    Jul 9, 2023 at 4:58
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As per another answer, the no-load DC supply is nominally the peak-to-peak voltage minus the two diode drops in the bridge; (24 X 1.414) - (2 x 0.7) = 32.5V. Under load this will reduce in mean value, with ripple increasing substantially, necessitating a 24V voltage regulator for Circuit A.

Fan noise can be reduced by putting in a low-pass filter between it and BR2. A ceramic or similar low-inductance capacitor in parallel with its smoothing electrolytic would be sensible, along with a series R and/or L in one or both supply lines from BR2. I am not sure what the extra diode is doing there, as BR2 already has two in series?

There is a howler in your earthing arrangements. You have both the centre taps and the negative line of Circuit A earthed. But there is a nominal 12V difference between them. You need to choose between 12-0-12 or 0-12-24 throughout.

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