[GreenKeys] Solid State relays

[John Nagle]

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