Re: (ET) new relay in - now no reverse

[Michael S Briggs <msbriggs alberti unh edu>]

On Sun, 6 Aug 2006, RJ Kanary wrote:

>              Pages 5-2 and 5-3  of the Homeowner's Service manual of the 
> cover the basic theory  of the control system for the AA- E-15 and its less 
> endowed stepchild, the E-12 S.

Ahhhhh, I usually just skip right past those pages to the schematics. :)

>        One of the things that is unique about an SCR is that once it is on, 
> it is ON. Remove the gate current, and it is STILL on.You have to interrupt 
> current flow to the anode the make it unlatch.{More sophisticated gated 
> devices came later to address this shortcoming.}
>          This is what the RTN circuit does for the PTO and the 'L' 
> contactor.

Ah, I was wondering why they needed a separate circuit for returning to 
neutral. :) That clears many things up, thanks.
	I decided I need to learn more about SCRs, so found this set of 
pages useful, if anyone else wants to do some reading also:
http://www.tpub.com/neets/book7/26c.htm

>           As far as using a bipolar transistor to do the 
> job.................... Why reinvent the wheel? <VBG>

Because the wheel is broken? ;)
	Actually, since I didn't realize the SCR stays closed without 
needing a gate current, as you mentioned above, it seemed to me that 
operationally a PNP transistor would be functionally equivalent. Now I 
know they're not. :)

>         In (tongue in cheek) view of your other misadventures, maybe we 
> should just stick to getting this unit to back up without making sacrificial 
> smoke. <VVBG>

I deserve that. :) I've had a rather careless/clumsy streak lately (my 
wife would probably say that that streak has been going on for years :) ).

>         BTW, have you found out if those relays have clamping diodes in them 
> yet ?

Based on my digital multimeter (haven't brought an analog one home from 
work yet - or I suppose I could buy an analog ohmmeter) they don't seem 
to. I'm pretty sure that that first one was just toast to begin with. When 
I was using the ohmmeter to figure out how the relay was wired up (didn't 
come with a schematic, so I needed to figure out which points on the 
external connections were connected where on the enclosed relay), I 
vaguely recall putting the ohmmeter across the two power connections and 
being surprised that it read 0 ohm - for some reason that didn't really 
register with my brain at the time. Again being careless and not paying 
close enough attention. Tisk tisk.
	I have determined though that those surplus relays royally suck. 
The second one I put in had a perfectly fine coil, and would switch back 
and forth fine (with pine 8 and 9 from card 1 temporarily jumpered). But, 
after triggering it once, when it went back to its default untriggered 
position, it would never again make good solid contact with the default 
points. Since opening those sealed relays up can be a pain, I decided to 
just put the original fried relay back in so I could get the lawn mowed, 
just rewiring it so that I only had forward instead of only reverse. Now 
that the lawn is mowed, I can pull out card 1 to inspect it (hopefully 
just a blown trace, and I won't need to bother replacing that SCR anyway), 
and replace the dead relay with a good original from a listmember (thanks 
again Jerry).
	Just in case it's not just a blown trace, I suppose I should find 
a good modern SCR to replace that old one with.... any idea what gate 
current those old ones require to close?

Mike