Re: Beginner help building 5V power supply


"ehsjr" <ehsjr@xxxxxxxxxxxxxxxx> wrote in message
news:EpPSh.8153$OU1.5321@xxxxxxxxxxx
Joel wrote:

<snip>

I would love to draw the schematic of the circuit (and tried a few
times),
but it wasn't really readable. So the first "mini" question is there a
program that helps me draw schematics using text characters?


Here's one way to draw an ascii schematic without a
program to do it for you:
1) Start every new line of the drawing with a space
2) End every line of the drawing with a non-blank character
3) "Make" symbols you have trouble drawing by using
boxes with text labels
4) Most important - use a fixed font, like Courier.

Here's an example, using your description.


--------- --------- ----
| XFORMER |-| Bridge +|---+----+----+---|7805|---+---> +5
| 24 VAC | | | |+ |+ |+ ---- |
| | | | [C1] [C2] [C3] | [C4]
| | | | | | | | |
| |-| -|---+----+----+------+-----+---> Gnd
--------- ---------

When you use polarized components, you should label
them to indicate the polarity if you draw them as boxes.
See the bridge and C1-C3 as examples.


Another option is to use LTSpice, and supply the ASC text. It is also very
informative if you run the simulation and look at voltages and waveforms.


Regarding other points (which have been answered),
you should add an input cap (.33 uF was mentioned,AIRC)
physicaly close to the 7805. That would add C5 to the
above schematic, from the Vin pin of the 7805 to ground.
Also, your higher than 24V DC was explained, but I'll
add that a higher voltage rated filter capacitor would be
better, as you are very close to the 35V rating. It is
good practice to use electrolytics rated well above the
voltage they will "see". And, for a 5 mA load, you do not
need 3000 uF of filter capacitance.


A good rule of thumb is 2000 uF per amp. The capacitor only needs to hold
the voltage to about 2/3 of the peak, from one peak to the next. Another
way to approximate the capacitor value is using a time constant at the line
frequency. For 60 Hz:

C(uF) = 8300(uSec)/RL. So a 5V 0.005A load is 1000 ohms, and C=8.3 uF.

I may have missed it in the responses, but why not use
a lower input voltage? Wall wart DC supplies are dirt
cheap and often free - you could use one as the input
to your 7805.

Ed



.

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