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

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?

Inductances can store energy for quite long periods of time and even with
the fan disconnetcted, flicking the switch may cause leakages back
through the fans input circuitry.

Is it only the fan that causes the spike or is there other equipment that
causes it too. For all electrical equipment there are now regulations to
limit the emissions the equipment produces. There are also immunity
requirements on all equipment.

When you have eliminated possibilities of direct routes for the
interference you may need to consider less direct routes. Is teh circuit
adequately shielded? Is the circuit layout such that it minimises teh
effect of RF pickup or capacitive pickup from the mains input side?

Answering these sorts of questions may give you enough clues.

--
********************************************************************
Paul E. Bennett...............<email://Paul_E.Bennett@xxxxxxxxxxxxx>
Forth based HIDECS Consultancy
Mob: +44 (0)7811-639972
Tel: +44 (0)1235-811095
Going Forth Safely ..... EBA. www.electric-boat-association.org.uk..
********************************************************************
.

Relevant Pages