hi i my knowledge is limited but ive built a power supply for my amplifier projects as per attachment 1 x 500va transformer with 2 18v seondarys the voltage i need must be less tham 30v dc to drive most amp prodjects the problem is there is no center tap! if i put the secondarys in series i would get a center tap but the voltage becomes 36VAC / 50V DC AFTER RECTIFICATION ! IS THERE A CIRCUIT THAT I CAN USE TO KEEP THE 18V SECONDARYS ???? CREATE A VIRTUAL GROUND MAYBE ( DUNO IF THAT WOULD WORK ) ANY SUGESTIONS ANYBODY THANKS

 

hi carp,
Welcome to AAC.
If you have two 18Vac isolated secondaries, use a full wave bridge on both windings.
That will give 18Vac *1.4 = ~25Vdc peak , less two diode drops, say 24Vdc
E

Bit more info:
So with two 24Vdc isolated sources, one could be +24Vdc and the other -24Vdc
Do you follow that OK.?

 

HI THANKS FOR YOUR INPUT ! 2 sets of 24vdc i get but i still need grounding or maybe ime missing something
(24v dc -) (24v dc +)(ground ??usualy half the voltage ie center tap ?

 

hi,
Have a look at this circuit, ask if a problem.
E

 

hi thanks ! ile have a go at that circuit r2 and r3 are they 48ohms ?? sorry if that sounds silly ??

 

hi,
Those 48R are just to demo the circuit load, in a real project they will your circuit load.

E

 

hi,
Those 48R are just to demo the circuit load, in a real project they will be your circuit load.

E

ok thanks have a virtual beer on me

 

hi,
A point to note, if you use large can electrolytics ensure that the outer case of the cap is not internally connected to the negative terminal.
If so, the clamps holding the capacitor case to the chassis, on both the neg and pos supply capacitors will be shorted.
E

 

If you want a plus and minus DC output with a common ground, then you only need one bridge, connected as shown below (V1 and V2 are the transformer output windings).

You connect the two transformer windings in series with the common connection grounded, and the bridge then acts like a full-wave rectifier for each output.
An addition advantage of this is that the output voltage is reduced by just one diode-drop instead of the two diode-drops using separate bridges.

Note that the transformer windings must be wired with phase shown.
To determine that, measure the AC voltage from point a to point b.
It should be about 36Vac.
If near 0V then reverse one of the windings.

 

hi thanks for your input

 

hi,
A point to note, if you use large can electrolytics ensure that the outer case of the cap is not internally connected to the negative terminal.
If so, the clamps holding the capacitor case to the chassis, on both the neg and pos supply capacitors will be shorted.
E

hi again , you seem switched on !!! can you help me with another problem please if you can ,ive built a amplifier using lm3886 and a separate atx computer pwer supply using a stepup converter to get the voltage up see (attachment) the problem is it plays great from a portable device ie mp3 player but if i plug in my pre amp it shuts off and for the same reason ive disconected the earth at the power supply !!!! ime guessing its another ground/earth issue any ideas ??

 

hi carp,
Do you have a circuit of the step up converter that you could post.?
As you say, it sounds like a grounded problem.
E

 

cant find schematic , only pic

 

hi,
Have a look at this circuit, ask if a problem.
E

hi there !!! in the circuit it does not show the polarity of c1 and c2 could you confirm thanks

 

hi there !!! in the circuit it does not show the polarity of c1 and c2 could you confirm thanks

Look at this marked up image.

 

Look at this marked up image.
View attachment 159937

perfect thanks

 

Look at this marked up image.
View attachment 159937

all done
works fine thanks

 

hi,
A very rough rule of thumb for the cap values, is 1000uF for every 1 Amp of the max load current.
E