Filter capacitors are often used in groups, say 4700, 47, and 0.47 uf in pa= rallel.=A0 The larger capacitor values have a lower self-resonant frequency= due to their higher series inductance and series resistance, so a single c= apacitor cannot filter all of the noise present.=A0=A0 I agree with many of= the other comments that a single large filter capacitor is used at the inp= ut of the regulator IC and one or more smaller capacitors are placed at the= output. Another source of hum can come from the rectifier diodes.=A0 If they turn o= ff fast they will generate high frequency noise at harmonics of 60 Hz.=A0 A= typical fix is to put a 0.01 uf ceramic capacitor across each of the diode= s to suppress the turn-off transient.=20 A linear voltage regulator will reduce hum.=A0 The amount of reduction is s= pecified as the Power Supply Rejection Ratio and the units are in decibels.= =A0 That spec assumes that no point in the input waveform drops below the m= inimum regulation voltage of the regulator IC, which means that you need to= have some degree of filtering present on the input.=A0 A raw rectified sig= nal cannot be improved by the regulator.=A0 Regulators can only subtract vo= ltage, not add.=20 You can use a second, lower-voltage regulator to further reduce hum.=A0 Thi= s is done in some communications equipment such as receivers where the main= part of the radio is fed from a conventionally regulated supply, and criti= cal portions such as the VCO are fed by a zener-regulated supply that draws= power from the main regulated rail.=A0 You will also see decoupling resist= ors and capacitors in the zener circuit to further isolate the zener from t= eh main supply rail.=20 Rather than guess or speculate I'd put an oscilloscope on the power supply = and look at the waveforms.=A0 If you don't own an oscilloscope you may be a= ble to find one readily by looking up a ham radio operator in your area.=A0= =A0 Do a google search on "ham radio operators by zip code".=A0=A0=A0 You c= ould also check at nearby electronics schools or colleges (physics or engin= eering departments).=A0=A0 The application is interesting enough that someo= ne there would be willing to help simply to satisfy their own curiosity abo= ut what you are making.=20
| Filter capacitors are often used in groups, s=
ay 4700, 47, and 0.47 uf in parallel. The larger capacitor values hav=
e a lower self-resonant frequency due to their higher series inductance and=
series resistance, so a single capacitor cannot filter all of the noise pr=
esent. I agree with many of the other comments that a single la=
rge filter capacitor is used at the input of the regulator IC and one or mo=
re smaller capacitors are placed at the output. Another source of hu= m can come from the rectifier diodes. If they turn off fast they will= generate high frequency noise at harmonics of 60 Hz. A typical fix i= s to put a 0.01 uf ceramic capacitor across each of the diodes to suppress = the turn-off transient. A linear voltage regulator will reduce hum.= The amount of reduction is specified as the Power Supply Rejection Ratio and the units are in decibels. That spec assumes that no point= in the input waveform drops below the minimum regulation voltage of the re= gulator IC, which means that you need to have some degree of filtering pres= ent on the input. A raw rectified signal cannot be improved by the re= gulator. Regulators can only subtract voltage, not add. You c= an use a second, lower-voltage regulator to further reduce hum. This = is done in some communications equipment such as receivers where the main p= art of the radio is fed from a conventionally regulated supply, and critica= l portions such as the VCO are fed by a zener-regulated supply that draws p= ower from the main regulated rail. You will also see decoupling resis= tors and capacitors in the zener circuit to further isolate the zener from = teh main supply rail. Rather than guess or speculate I'd put an osc= illoscope on the power supply and look at the waveforms. If you don't own an oscilloscope you may be able to find one readily by looki= ng up a ham radio operator in your area. Do a google search on = "ham radio operators by zip code". You could also check a= t nearby electronics schools or colleges (physics or engineering department= s). The application is interesting enough that someone there wo= uld be willing to help simply to satisfy their own curiosity about what you= |