# What would you call this?

Discussion in 'Electronic Basics' started by Dave, Mar 13, 2008.

1. ### DaveGuest

I am in need of a solid state relay that latches upon reception of a
positive-going pulse, and holds that state until the power to it is removed.
Actually something that will provide either a logic 1 or a logic 0 for
another device upon reception of said pulse, and hold that state until power
is removed. What would that be called, and where should I look for it? Can
it build it out of OR/AND gates, like a Johnson Counter? Or can it just
buy it like I can buy a CD4017? (I think that's a 0-9 counter...)

Thanks for any help.

Dave

Transistor?

3. ### JamieGuest

RS flip flop.
or,
if you're trying to power up something with a control lock relay..
you can use a double pole relay. 1 pole is used to power the device for
example and the other is used to latch it self on.

But I don't know what you're trying to control?

http://webpages.charter.net/jamie_5"

4. ### DaveGuest

Well, a transistor will change if the input to it changes. I'm looking for
something that locks on until the power to it is cut.

Thanks,

Dave

5. ### DaveGuest

Hey Jamie,

An RS flip-flop. Will have to check that out. I am already working with
4027 JK flip-flops, so maybe I am missing the obvious...

What I am trying to control is a 74HC4053, and whether it connects the A and
B outputs to a logic 0 (a0/b0) or a logic 1 (a1/b1). Basically, whether it
connects the J and K of a 4027 JK flip flop to ground or 5V. This would
lock it into it's current state, or allow it to continue responding to the
clock input.

Thanks for the input. Much appreciated.

Dave

6. ### Michael BlackGuest

An SCR. Apply a control voltage, the "diode" conducts and stays in
conduction until power is removed from the "diode". The control voltage
can disappear as soon as it triggers the SCR, the SCR will continue
to conduct until voltage is removed from the "diode" portion.

If I recall properly, you can synthesize them with a pair of transistors.

Michael

7. ### DaveGuest

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

8. ### VaractorGuest

Look at this one shot:
http://www.discovercircuits.com/DJ-Circuits/4013oneshots.htm

If you want it to accept just one trigger pulse only try a JK flip
flop with k=0 and your trigger to J. If it powers up in an undermined
state (I'm not sure if this is the case), hold k high temporarily with
a capacitor to power and a resistor to ground, the power on reset time

At least that's my first guess.

Cheers

9. ### HammyGuest

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 Micro/Web Data/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.

10. ### DaveGuest

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

11. ### SamGuest

You can achieve that with a double pole relay, wiring the second switch
into the coils path.

12. ### ehsjrGuest

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

13. ### HammyGuest

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.

14. ### DaveGuest

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

Thanks,

Dave

15. ### ehsjrGuest

Hi Hammy,

Well, that entry that has the words "initiating current"
does not mean you *must* have 20 mA AK current. What it
says is that *if* you had 20 mA AK current (and 7V) during
the gate pulse, the device is guaranteed to turn on and
hold at 5 mA (at 25C). It is sort of like a car manufacturer
guaranteeing that the car you just bought will do at least
20 miles per hour. The SCR will latch on with AK
"initiating current" way below 20 mA.

Do a test to see: put the 2N5060 in an LED circuit with
a limiting resistor that guarantees sufficient holding
current, and also guarantees well below 20 mA:

+12---+---[1K]---[LED]---+
| _|_
[47K] \ /
| ---
| _|_ /|
+------o o-------+ |
|
Gnd ----------------------+

The pushbutton will send a ~255 uA pulse to the gate,
and the 1K will limit the current to roughly 10 mA. You
can substitute a 5K pot for the 1K to see how low you
can make the AK current and still trigger the SCR. Find
the point where it just starts to trigger, then measure
the voltage across the pot & the pot resistance, and
compute the current.

What is far more important is the length of Dave's
pulse. If it is too short, an SCR "solution" is not
a solution at all. That may be influencing your
observations, as well. You mentioned you were working
with 0 crossing, so you are talking about small slices
of time. The ap note referenced below discusses dv/dt,
and a whole bunch of other stuff related to SCRs:

http://www.littelfuse.com/data/en/Product_Catalogs/PowerThyristorApplicationNotes.pdf

(Watch out for line wrap in the above.)
There's more complexity to an SCR than meets the eye!

Ed

16. ### Bob MonsenGuest

You could build your own flip-flop out of two NPN transistors. Actually a
'flop', because you'll never 'flip' it...

5V ------------------o------.
| |
.-. .-.
| | | |
| | | |
10k '-' '-' 10k
| |
| |
.------------)------o------------------- out
| o------)------------.
IN | ___ |/ \| ___ |
---->|-o-|___|-o--| |--|___|---'
100k | |> <| 100k
--- | |
--- '--o---'
1nF | |
| .-.
| | | 1k
| | |
| '-'
GND | |
-----------------o-------'
(created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)

Just use a positive voltage spike on IN, and OUT will go from LOW to HIGH.

You may not need the 1k resistor.

Regards,
Bob Monsen

17. ### HammyGuest

I don't use SCR's that often.What I usually do is use NPN and PNP
single die like the MBT3946 bias the NPN that bias's the PNP and
latches the NPN.Cheap and small.

I had some(2N5060) left over in my parts kit from school so I tried
one in a Piezo buzzer cct. with some LED's. I found it wouldnt latch
adequate 5V from an " AND" gate 20mA source/sink through a small RC
low pass filter. No problem fireing it but it wouldnt latch without an

Heres the schematic

http://i30.tinypic.com/s3n4hj.png

really a Piezo buzzer,to lazy to make a proper part in capture.

Oddily enough SCR's arent given that much coverage both in my school
text's as well as the majority of other books I've found on the
net.The appnote link you provided is exellent thank you.

I'm considering the latch I mentioned earlier for the zero crossing
detector, because it gives you the options of resetting without
removing power.

18. ### Michael BlackGuest

But what kind of current is the 2N5060 good for? I'm too lazy to look.
One reason I suggested the original poster "synthesize" an scr is because
it seems easier to find high current SCRs, indeed that tends to be
where they are used, but for the original purpose it would be overkill.
I wasn't thinking of their usefulness at low currents, but rather that
their size would be more than desired.

A device intended for lower current handling would presumably be more
inclined to handle lower current levels. After all, if you normally
want an SCR because it can switch high current levels, you aren't
to the load you want to handle.

Michael

19. ### ehsjrGuest

Having seen the circuit, I'm not surprised. Try the simple
circuit I posted. Then see if you can figure out what the
essential difference is. Then, modify your circuit and enjoy
it triggering and holding at well below 20 mA. The discovery
is worth the effort!

Ed

20. ### ehsjrGuest

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.
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