Re: Diac / Diode Confusion ?

From: John Fields (jfields_at_austininstruments.com)
Date: 01/19/05 

Date: Wed, 19 Jan 2005 13:59:55 -0600

On Wed, 19 Jan 2005 13:51:49 -0600, "Trudeau" <Trudeau@123.com> wrote:

>On the following link it says in part the following....
>
>Most diacs are used for pulse generation, probably in the order of amps, and
>are designed as such. I think the published ratings for static (DC) use are
>a bit on the consevative side, take a 1N4148 sized diac and say 100 to 200
>mW dissipation and about 2V across the diac we are left with 50 to 100 mA,
>more than enough!
>
>http://www.hut.fi/Misc/Electronics/circuits/tele_privacy.html
>
>My question is, Is this web site confused. 

---
No. Notice they were referring to a  1N4148 _sized_ diode.  That is, a
Diac in the same size package as a 1N4148.
---
>When I looking up a 1N4148 it is
>NOT a Diac, it is a Diode. Please bear with me as I'm a novice. 
>A Diac goes in one direction and will start conducting electricity 
>at a certain voltage. I Get that.
---
No, you don't.  Not quite, anyway.  A Diac is bidirectional.
http://www.littelfuse.com/cgi-bin/r.cgi/prod_series.html?LFSESSION=Stkas7zMnz&SeriesID=605
---
>A diode works in one direction also but from that point I'm a little unsure
>what the difference is. Can anyone explain this in simple terms? 
---
A diac is a full-wave or bidirectional thyristor. Whenever the
amplitude of applied voltage exceeds the breakover voltage rating of
the diac, it switches from blocking- to conducting-state.
---
>Does the Diode also limt voltage eg. a 5v Diode limits voltage to well .. 5V.
---
A reverse-biased Zener diode forced into conduction will cause the
voltage across its junction to be limited to the value specified in
the data *** for that diode with the specified current flowing
through the diode. Forced to conduct in the forward direction, the
voltage across the diode, and most silicon junction diodes, will be
substantially less than one volt,
-- 
John Fields
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