What would you call this?

[Dave]

"ehsjr" <ehsjr@bellatlantic.net> wrote in message
news:ykGDj.5726$sw3.1186@trnddc06...

Dave wrote:
"Hammy" <spamme@hotmail.com> wrote in message
news:q9gst3lk7687n8jg31luig2837v8mlhjiv@4ax.com...

On Mon, 17 Mar 2008 04:22:08 GMT, ehsjr <ehsjr@bellatlantic.net
wrote:


Dave wrote:

"Hammy" <spamme@hotmail.com> wrote in message
news:ju5qt39ouvqussmr45b48uhkt2cjbngdic@4ax.com...


On Fri, 14 Mar 2008 08:26:28 -0500, "Dave" <db5151@hotmail.com> wrote:




Aaahhh. Yes, I remember this from school. A circuit that stays on
even
you
try to turn it off... That's what I need. An SCR. Will look into
that.
*Thank you, Michael.*

Much appreciated.

Dave


The thing about an SCR is it needs an intiateing current even the
2n5060 needs about 20mA .I found it to be higher on the ones I have
anyway.Then it will hold on as little as 5mA.If all thats okay then
fine.

I have a similiar dilema only for a zero crossing detector.I want it
to latch when the detector senses the zero crossing. I'm looking at
something like this.

http://rocky.digikey.com/weblib/ST%20Micro/Web%20Data/74V1T77.pdf

Cheap and tiny low power consumption.If you look you can probably find
cheaper and lower power versions.

Search for D-type transparent latches.

Or depending on your application and what your willing to spend there
are latching relays a little pricey though.


Aaah, yes. Very interesting, but I think what I am going to do is
build the
equivalant of an SCR out of transistors, as my electronics textbook
describes. This will allow me (I think) to select the parameters I
want and
tailer them to my exact needs. Or at least that's the present idea.
Thank
you for this pointer however. I may yet end up using it.

Take it easy...

Dave



Gate current is no problem, at all. You can get sensitive gate
SCRs that will trigger well below 1 mA. For example, the 2N5060
mentioned in Hammy's post triggers at .2 mA (not 20 mA!)

By all means, experiment with making your own from transistors.
You can have fun, and learn that way. But there is no problem
in getting one that will trigger at low current.

Ed

Hi Ed

I didnt say that the 2n5060 needs 20mA gate trigger .What I said is
that it needs an intial 20mA AK current.This is written in the
datasheet.From my experience with them they usually need a bit more.

Maybe I should have been clearer the intiating current I'm refering to
is the Anode Cathode current,not gate current.You have to look for it
in the datasheet but its there.


Hmmm. What about a sensitive gate that triggers at (lets say) .2mA, with
an AK current of similar proportions?

Not likely to find an SCR with a 200 uA holding current.
The one we are discussing - 2N5060 - needs a minimum of
5 mA to hold.

What I am wanting to do is switch a couple of leads from +5V (very low
current) to ground (essentially NO current) on a JK flip-flop, so that
the clock is thereafter ignored and the Q and not-Q outputs remain
unchanged. Any suggestions? Oh, and I need to do this with solid-state
devices, not a coil-of-wire relay.

You can use an SCR as long as you provide a path that
will allow it to hold, like a resistor from SCR anode
to the + rail.

+5 -------[1K]-----------+----> To flip-flop
_|_
\ /
---
/|
Signal---[1K]-----+ |
|
Gnd ----------------------+


But if you are already using flip flops,
is there a reason not to use another ff?

Ed


Thanks,

Dave

Thanks for the input, and the advice RE: an SCR unlikely to hold at 200 uA.
I believe I will actually have 5 mA holding current, which is a lot more
than I originally thought I would have, and the SCR I am thinking of using
fits that mold (the NTE5400).

'preciate it.

Dave