Re: Beginner help building 5V power supply


"Randy Day" <randy.day@xxxxxxxx> wrote in message
news:ylLSh.50072$aG1.46707@xxxxxxxxxxxx
Joel wrote:
Hi, I'm new to the group (and fairly new to electronics).

I created a 5V DC power supply to power a PIC microcontroller from a
pre-existing 24V AC source and intermittently the 7805 regulator is
getting
very hot. The 5V section is only drawing .005A continuously. It is
powering
a PIC and several LEDs. The regulator goes through random length cycles
of
being rather cool to periods when it is so hot you can't leave my finger
on
it for more than a second. These cycles happen on there own and the
current
draw is constant.

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?

AACircuit v1.28 beta
www.tech-chat.de


Second question is why would the v-reg go through these cycles, and is
that
OK?

verbal schematic:
24V AC into full wave bridge rectifier (GBL005)
Output of rectifier has 3 1000uF caps (35v) in parallel across output
output feeds into 7805 v-reg (Output Voltage: +5VDC @ 1A Maximum Input
Voltage: 35VDC) (no heat-sink used)
I have a .1uF cap across the 5V output of the v-reg


I noticed that the 24 VAC when rectified with no load was reading as
34VDC
this is pretty close to the max input voltage for the v-reg. Is this OK
and
why would the rectified voltage be HIGHER than the AC voltage.

Alternating current is measured RMS (root mean squared);
24v is more of an average value, not your peak-to-peak
voltage. After filtering, and under low load, yes,
you'll see near-max voltage across the cap.


Thanks


.

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