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Old 14th October 2005, 12:19 PM   #1
Nordic is offline Nordic  South Africa
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Default Capacitors to remove noise from PSU

Hi

I have a transformer with a 3rd secondary winding of 6V at 1A.
I used 4 FR104 diodes to make a rectifier, then connected a PC fan to the rectifier... all working and I'm very impressed with myself.

BUT, when I do connect the rectifier to the transformer, I can hear a slight noise on the amps connected to the other windings of the transformer... sounds a bit like the fan maybe makeing noise...

Is there anyway I can suppress/filter some of the noise using small capacitors or something... I got tonnes lieing around, but I don't just want to go and add random capacitors to my circuit... also not sure where they would have to go...
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Old 14th October 2005, 01:33 PM   #2
AndrewT is offline AndrewT  Scotland
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Hi,
the fan is running on pulsed DC since there is no smoothing on the supply. That may be aggravating the noise problem.
If you add caps after the rectifier the fan will see more voltage and run faster. Is that OK?
To suppress noise on the fan line you can add small caps to the AC side of the rectifier and to the DC side. Any sizes below 100nF should be OK.
You can also add caps to the other PSU supplies but these are after the horse has bolted, better to solve at source if possible.
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Old 14th October 2005, 01:57 PM   #3
Eva is offline Eva  Spain
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Add a storage capacitor after the diodes, and a resistor in series with the fan. Suitable values would be over 100uF and under 100 ohms (use a 2W unit).
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Old 14th October 2005, 04:46 PM   #4
Nordic is offline Nordic  South Africa
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Like this?

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Old 15th October 2005, 10:57 AM   #5
Nordic is offline Nordic  South Africa
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Tried circuit like diagrame I posted above... but it has no effect...
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Old 15th October 2005, 11:18 AM   #6
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Make sure the wires to the fan are twisted together, and run as far away as possible from other, especially signal, wiring in the amp.
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Old 15th October 2005, 03:21 PM   #7
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Nordic,

Eva said to put the resistor in series with the fan. A capacitor in parallel and a resistor in series with the load makes a low pass filter which will help filter out the noise from the fan that creeps onto the AC. Your diagram shows the resistor in parallel, not in series, with the load.

Dick
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Old 15th October 2005, 04:37 PM   #8
Eva is offline Eva  Spain
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I meant a resistor in *series* with the fan, not in paralell.
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Old 16th October 2005, 06:59 AM   #9
Nordic is offline Nordic  South Africa
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HAHA ok, excuse me... I plead ignorance...

Will do what you said!

absolute amateur here, only completed my 3rd gainclone yesterday, and it took like an hour or so...
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Old 17th October 2005, 09:10 AM   #10
Nordic is offline Nordic  South Africa
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Click the image to open in full size.

Connected like this, but now fan doesn't turn... is the resistor supposed to be in seriesor parallel with the capacitor maybe?
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