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

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.

Thanks all.

"Rocky" <RobertGush@xxxxxxxxx> wrote in message
news:373bb65d-28c8-483a-ae9c-8a165480ceea@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
On Apr 26, 10:24 am, "Paul E. Bennett" <Paul_E.Benn...@xxxxxxxxxxxxx>
wrote:
Rodo wrote:
Hi all,

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.
== Rocky ==


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