a doubt about a circuit

[Mooly]

That's good, so whats next ?

[Spiny]

[simonwai999]

hi

thanks
the next thing i am going to make is a 5vdc power supply


i wanna use a transformer this time
since i just got a new one
it is a 220vAC primary and 9-0-9 secondary
20VA power transformer

some say the filter cap is like the bigger the better
but i saw some circuits
they only use 47u or 10u
i am going to use a 2200uf cap for the filtering

is it too much?
also i will use 7805 to get 5vdc
at the output of 7805
there is a filtering cap too but it is supposed to be a smaller one
also i will put a bypass cap 100nf parallel with the output cap

here is a circuit i found from the net
I do have a question about it
do you guys know why there is a 1k resistor at the output ?
what is used it for?

any suggestions , please kindly let me know
thanks
regards

[Spiny]

with a 9-0-9 secondary use a bi-phase rectifier for 5V - gif attached - as this will give a secondary rectified voltage of about 12v (9V *1.414 -0.7v diode drop). so the 7805 only has to drop 7V at your design current. You may and probably should use a heatsink on the 7805. You can calculate the heat produced by the voltage drop and the current passed, 1/2amp will give 3.5watts to dissapate for example.

As to the final resistance, this is to provide a load when no circuit is attached. I usually connect a LED and resistor here as a power on indicator and to provide a load if the circuit is disconnected.

The capacitors will do, only a small one is needed on the output of the 7805 as the DC is regulated by the chip. Large input capacitors may need heavy duty diodes to cope with the surge, You may need a 3amp diode (1N5401 etc.) rather than a 1N4001 here.

[simonwai999]

hi pal
thanks so much for your detailed explanations
i understand it now

soon i will make this power supply
regards
simon

[simonwai999]

hi folks
i do have a question about transformer
for example the primary of my power supply is 220
and the secondary is 9-0-9
and it says 20VA
How do i know the output current of this transformer?
is it 20/ 9 or 20/(9*1.414) or 20/(9*1.414-0.7 )?
Thanks in advance

[AndrewT]

Hi,
your 20VA 9-0-9Vac transformer has a maximum continuous output of 20/[9+9]=1.1Aac.
If you use this to feed a capacitor input filter to get a +-12Vdc supply the maximum continuous output is approximately half the AC output i.e. ~550mAdc.
If you run the transformer at it's maximum rating of 550mA it will run hot, right up at the maximum that the manufacturer designed it for. This internal temperature will be about 130degC.
I suggest the maximum continuous demand be limited to about 50% of rating i.e. 280mAdc continuous.

[simonwai999]

hi pal

very detailed explanations! cool!
I understand it all now
Thanks
regards simon

[simonwai999]

hi folks
today i hooked up my power supply and tested
found out something diff from Andrew said
i used 9-0 of the secondary to connect a bridge rectifier and a filter cap
to get 12vdc for a portable tv and connect a current meter
in series of it
but the meter showed 0.7A
i even ran it for 2 hours ( the tv was working well for 2 hrs too)

and i touched the metal part of the power transformer
it was not very hot
i wonder why
Does anyone know why?
please kindly help me
Simon

[Spiny]

AndrewT was describing a +12 and -12 V supply, you have made 1/2 of this so should be well within the transformer (.7 *12 watts =8.4watts)
The bi-phase wiring using 2 diodes could provide over 1 amp at 12V before transformer heating becomes a problem.