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...)
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.
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.
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.
If you want it to accept just one trigger pulse only try a JK flip flop with k=3D0 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 will be about RC.
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.
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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.
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.
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? 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.
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:
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:
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(Watch out for line wrap in the above.) There's more complexity to an SCR than meets the eye!
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 reliablily unless I had about 22mA load. The gate single was more then 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 addtional load.
Heres the schematic
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I had to adjust R4 and add R5 for the SCR to latch. M1 IRF034 is 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.
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 concerned about any "minimum" load that would be nearly invisible compared to the load you want to handle.
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!
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