Hi all,
A while back I posted a problem I was having with a spike (see original text below). Someone suggested that the spike would be in the ground line as well. So I connected the scope probe and ground to the ground of the circuit and yeap, there it was. I felt rather silly connecting the scope probe and ground together but oh well ... This week I disconnected the probe but left it next to the circuit and I noticed that I also got the spike. So I thought it must be getting in the scope from the AC ground. So ... for some reason I move the probe on top of the scope which sits in a higher shelf and repeated the test. I got no spike. Several tries show the spike if the scope probe is close (1 or 2 inches) to the circuit but it goes away if I place the probe further from it. So it looks like the spike is being generated and transmitted (for lack of a better word) everywhere to the circuit? Is this something that would require more ground to shield the PIC?
Comment/ suggestions welcome .... this is driving me nuts :-(.
Thanks in advance.
---------------- Original message 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:
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?
Thank you
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