Re: AC spike go pass XFRM and regulator to reset CPU!!??

Rodo wrote: *** and top-posted - fixed ***
"Rocky" <RobertGush@xxxxxxxxx> wrote
"Paul E. Bennett" <Paul_E.Benn...@xxxxxxxxxxxxx> wrote:
Rodo wrote:

I have a device that is powered by 240VAC (also works from
120VAC). The AC drives the primary of a transformer (with a
fuse and a MOV). There is also a fan connected to the 240VAC
with a switch. The secondary of the transformer has a bridge,
a large electrolytic cap (1000uF/25v) and an LM7805 regulator.
There is a smaller cap (10uF/10v and 01uF/25v) at the output
of the LM7805. Then the 5vdc connects to a PIC.

Almost every time I flip the fan switch a spike shows up at
the output of the regulator. This is the part that I mostly
do not get: even if I disconnect the fan and I flip the
switch... the spike shows up ... and it is a bit larger in
amplitude. I grabbed a picture of the spike ... you can >>> see it at:http://mysite.verizon.net/rodo/ds0000.bmp

The spike is obviously not always exactly the same. Sometimes
is a bigger, or smaller more or less oscillations. But the
general timming is the same. By this I mean I do not have to
change the scope's setting to see it.

I tried to: add caps, common mode choke, remove ground (from
circuit and scope), add larger cap at the output of LM7805,
etc., to find why is there a spike when there is no load (I
removed the fan remember). The ultimate thing is that I need
to avoid the spike from occuring (with or without load)
because it is reseting the CPU.

Could someone enlighten me or point me in the right direction
please?

IS the fan part of the equipment or is it a desk fan?

Rodo: You might find that a very similar spike on the 0v line.
i.e. Just a loop in the scope input to ground. The reason you
see the spike is probably because of the EMI filtering around
the on/off switch.

I was thinking about a ground loop with the scope but the
circuit is being powered by a 110VAC to 220 VAC transformer. I
know the grounds are not connected because I checked. I wonder
if the transformer (110-to-220) is not really isolated. I'll
have to check this next week.

Someone asked if the fan is part of the equipment. No, it is a
totally disconnected from the circuit.

Just a quick thought - have you checked the switch in its various
positions with an ohmmeter? Have you any other beasts connected to
switch terminals? If so, maybe the switch configuration is make
before break, rather than the more common break before make.

Please do not top-post. Your answer belongs after (or intermixed
with) the quoted material to which you reply, after snipping all
irrelevant material. I fixed this one. See the following links:

<http://www.catb.org/~esr/faqs/smart-questions.html>
<http://www.caliburn.nl/topposting.html>
<http://www.netmeister.org/news/learn2quote.html>
<http://cfaj.freeshell.org/google/> (taming google)

--
[mail]: Chuck F (cbfalconer at maineline dot net)
[page]: <http://cbfalconer.home.att.net>
Try the download section.


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