Oops! I wrote:

> The right treatment can be given only when the problem is not=20
> correctly diagnosed! =20

It should obviously read:

The right treatment can be given only when the problem is=20
correctly diagnosed! =20


> -----Original Message-----
> From: Bala.Chandar@AVENTIS.COM [mailto:Bala.Chandar@AVENTIS.COM]
> Sent: Monday, September 15, 2003 11:12 AM
> To: PICLIST@MITVMA.MIT.EDU
> Subject: Re: [PIC:] Why is my 12F629 resetting?
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> Russell McMahon wrote:
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> >
> > The "direct offline" capacitor fed power supply is especially=20
> > susceptible to
> > noise spikes.
> > At high frequencies the 0.33uF capacitor is very low=20
> > impedance and noise
> > sees mainly the 220 ohm resistor. The 470 uF capacitor is=20
> probably an
> > electrolytic and will have an ESR of around 0.1 to 1 ohm. This will
> > attenuate noise spikes considerably BUT less so at higher=20
> > frequencies due to
> > lead inductance and any wiring will add to this.
> >=20
> > The output ground lead and the input diode should both=20
> > terminate on the
> > capacitor and it would help to add a 0.1uF ceramic with short=20
> > leads across
> > the 470 uF. Having a series resistor with a second=20
> > electrolytic after it to
> > provide a second stage of filtering also helps. We used to=20
> > use supplies
> > derived from Telecoms 50v battery (very noisy) long ago using=20
> > pure resistive
> > dividers - using two stages of zeners with a resistor between=20
> > them helped a
> > lot.
> >=20
> > Not having the 240V lead in close proximity to the 5v=20
> supply will help
> > although, for noise spikes, capacitive coupling will be=20
> > relatively small
> > compared to the direct path via the 0.33 uF cap.
> >=20
> > As you note - this is a highly dangerous power supply and=20
> > must be regarded
> > as being at 240 VAC at all times.
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> Thanks Russell, for your insight and tips.
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> The 0.1uF ceramic cap that you are suggesting is already there in the
> circuit, soldered across the 470uF electrolytic cap. I tried=20
> a few minor
> layout changes like bypassing certain tracks with short=20
> lengths of wire and
> increasing the value of the elec. cap to 1000uF, but they=20
> didn't seem to take
> me closer to the solution.
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> The right treatment can be given only when the problem is not=20
> correctly
> diagnosed! =20
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> Regards,
> Bala
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> =20
> =20
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