[GreenKeys] Solid State relays

[scottjohnson1 at cox.net]

A salient point to be made here:  
Most good solid state relays do not use a Triac, they use two SCRs connected in reverse parallel.  The performance 
is much better than that of a triac, especially whan used for phase control at low settings.  It doesn't change the point John made, though.

Scott W7SVJ

---- John Nagle <nagle at animats.com> wrote: 
> On 8/26/2010 9:00 AM, greenkeys-request at mailman.qth.net wrote:
> > Message: 8
> > Date: Thu, 26 Aug 2010 09:59:28 -0400
> > From: "Larry Tighe"<larryradio at att.net>
> > Subject: Re: [GreenKeys] Solid State relays
> > To: "Roy Morgan"<k1lky at earthlink.net>,	<hunybuny at eskimo.com>
> > Cc:greenkeys at mailman.qth.net
> > Message-ID:<5DD73EA1635949CBB1468352014DD1E4 at ljt>
> > Content-Type: text/plain; format=flowed; charset="iso-8859-1";
> > 	reply-type=response
> >
> > Hi Roy,
> >
> > My only question was why it was keying the 3 VDC of the ohmmeter and not the
> > 130 VDC, 60 ma loop.  That had miffed me.  I did receive an explanation,
> > didn't understand it, and so, I guess, it's not a problem now.
> 
>     OK, here's the theory.  See pages 23-24  of
> 
> 	http://www.veech.com/index_files/Triac%20Theory.PDF
> 
> especially figure 2-16.  Note that a triac is a linear device near
> zero current, so to an ohmmeter, it behaves much like a transistor.
> But that's not its useful operating range.  If there's sufficient
> current through the device anode after triggering, above the holding
> current Ih, the device will go into conduction and stay there
> until the anode current through the device drops below Ih.  That's
> why these devices are good AC switches, but not useful for DC.
> 
>      Ih isn't that big; 5 ma is enough to hold most small triacs in
> conduction.  But an ohmmeter is below that.   Try switching a flashlight 
> bulb and a battery with the device and watch it
> trigger and stay in conduction.
> 
> 				John Nagle
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