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PICList Thread
'220VAC relay, Need SNUBBER help ????'
1998\12\23@215351 by Ricardo Ponte G

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       Hi to everybody out there:


       I«ve read the e-mails about this matter and have some doubts, Can you
help
me with that:


       1.- What is a MOV ???

       2.-  What is a bidireccional tvs ???

       3.- Can be an EMI filter helpful (2 bobbins and one capacitor) ?        
       
       3.- I have read about the choke coil and I don«t know much about it.0
        What Can is the function of a choke coil ?


       Thanks for  you interest in help and Merry Christmas for everybody

1998\12\24@034859 by Gerhard Fiedler

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At 10:49 12/23/98 -0400, Ricardo Ponte G wrote:
>        1.- What is a MOV ???

metal oxide varistor: a resistor which becomes more conductive above a
certain voltage, and is used to suppress voltage peaks

>        2.-  What is a bidireccional tvs ???

a tvs (transient voltage suppressor) is basically a zener diode designed to
withstand high peaks. it becomes bidirectional when two of them are in
series but reversed polarity, and you can buy them integrated like that.

>        3.- Can be an EMI filter helpful (2 bobbins and one capacitor) ?

it depends a lot on your setup. obviously you get something back from the
solenoid to the pic, and it's not easy to tell where exactly you get it
back without seeing how it's all set up. it could be from the solenoid over
the ac power line into your pic's power supply, or through the relais with
which you switch the solenoid, or wires lying around... but if it's close
to the solenoid, it might be helpful (even though i'd add a mov to it).

ge

1998\12\28@101535 by lilel

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Ricardo Ponte wrote

>         Hi to everybody out there:
>
>
>         I've read the e-mails about this matter and have some
>         doubts, Can you help
> me with that:     (much about TVS, MON, COILS and u\other useless complex devi
ces


I've dealt with this problem too.  I added snubbers, capacitors,
ferrites, MOVs, etc. etc. until my circuit looked like the Maginot
Line in France in 1940.  Unfortunately for the French, Hitler simply
walked around the end of the Maginot line.  Unfortunately for your
circuit, all this protection isn't going to help a hoot either,
because the problem is not going to be helped by all this armor.

If your circuit is like mine, firing the relay is dragging down the
supply voltage and causing the PIC to either reset or go LaLa.
First, you need a stiff power supply, that is immune to variations by
the relay's power consumption, and second you need a brownout
protection circuit, either built into the PIC or separate.  I build
stuff all the time with a pic driving a small transistor through a 2k
resistor, the transistor running a 24 volt relay on the collector,
with nothing more than a diode snubber.  The trick is to make the 5v
supply stiff enough to accomodate the variations in 24V (which in my
circuit varies from 48v to 20volts as it is loaded.)   No snubbers,
ferrites, ground rings, kryptonite shielded cages, nuclear fallout
shelters, TVS's, MOVs, or other armor plating.

Do the math at either end of your power supply variations, scope it
and see how low the power supply dips under load, and design it so
the 5 volt supply is still 5 volts under these conditions.


BTW if you are using an emulator, Microchip's bondout chips are
often VERY sensitive to power supply fluctuations, unlike the
production chip.  Spikes and dips which a production PIC will ignore
can freak out an emulator.
-- Lawrence Lile

=> Median Filter Source Code
=> AutoCad blocks for electrical drafting
=> Brownout tester plans
=> Amateurish pictures of my family

at:  http://home1.gte.net/llile/index.htm

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