Choosing a filter cap for power supply

[CSB]

Hi,
I'm beginning in electronics and have been trying projects that almost
all required different voltages, so I thought I'd build a PowerSupply unit
from scratch, instead of using batteries. So here's what the rectifier
part might look like:

_____________

--------| ~ + |----------------x--------> To 78xx v-regs

+9VAC | bridge | |
| rect. | |

--------| ~ - |---x----------- | -------> To 79xx v-regs

-9VAC ----------- | |
| |+

-------| --- ? µF ---

0VAC | --- C1 --- C2
(CT) | |+ | ? µF
_|_ _|_ _|_
- - -

(assuming that running a few v-regs in parallel works?)

What kind of caps should I use for c1 and c2? Or is there a formula to
find the values? And is there anything else I should add in that part
(reverse-biased diodes to ground for each side, &c.)?


Any information/source of info is very welcome.
Thanks,
CSB

 

[John Popelish]

Hi,
I'm beginning in electronics and have been trying projects that almost
all required different voltages, so I thought I'd build a PowerSupply unit
from scratch, instead of using batteries. So here's what the rectifier
part might look like:

_____________
+9VAC | bridge | |
| rect. | |
-9VAC ----------- | |
| |+
0VAC | --- C1 --- C2
(CT) | |+ | ? µF
_|_ _|_ _|_
- - -

(assuming that running a few v-regs in parallel works?)

What kind of caps should I use for c1 and c2? Or is there a formula to
find the values? And is there anything else I should add in that part
(reverse-biased diodes to ground for each side, &c.)?

Any information/source of info is very welcome.
Thanks,
CSB

My rule of thumb for low voltage supplies is 4700 uf per amp out.
According to the graph on page 2 of this tutorial:
http://www.eetasia.com/ARTICLES/2000APR/2000APR15_AMD_PD_AN.PDF
a 1 amp load on a 5000 uf cap gives a .8 volt peak to peak ripple
voltage.

 

[Dave]

Go to your local Public Library (assuming you are in the US) and ask for the
Reader's Guide to Periodical Liturature. Look up "Power supplies" in the
'75 edition (or maybe '77, can't remember for sure. You may have to look
for it.) There was an article in Popular Electronics on designing and
building your own power supplies that covered nearly all bases. It tells
you how to calculate the value needed for the filter cap. I have the
article around here somewhere (looked it up a few months ago) and will give
you a better date if I can.

Dave
[email protected]