Simple circuit to latch a single pole relay?

You could connect the contact of the relay in a way that makes it turn on its own coil, but then you may not be able to use the relay contacts for your intended load, depending on whether the load runs from the same supply as the coil.

You could use a relay with more sets of contacts, and then use one set of contacts as above.

You could use a SCR (Thyristor) to control the current to the relay coil. That won't turn off until the power supply is removed. Then the feature won't be optional though.

You could add another (PNP?) transistor that makes a circuit similar to a SCR, but if it is a separate transistor you can add a switch so that the latching feature is optional. Beware that separate transistors have low base current ratings so that if you build the circuit of a SCR out of a PNP and NPN transistor, it might have a quite limited current rating. Perhaps you can figure out a way to add base resistors in a way that protects the transistors without stopping it from doing what you want.

You could use a 555 timer chip. That contains a flipflop that can be set or cleared using two comparators, and also has a separate reset input. You could also use a flipflop chip.

Getting it to start up in the correct state might not be easy if you use a logic chip or 555 timer.

And of course these days everyone will suggest using a microcontroller.

rchived notes or a fair amount of googling, but suspect it will prove embar rassingly obvious.

xtend its versatility by optionally latching it on when triggered, staying on regardless of any other inputs, until power is removed several hours lat er.

le monostable with a long period at least as long as ?several hours ?, and triggering the relay with that mono. But I?m sure th ere must be a simpler way?

un in that?

You could use a set/reset latch.

NAND gates work just as well as NOR gates. If you want a packaged part you can D-type flips-flops in any logic family you fancy, but that is a more el aborate part than you want.

--
Bill Sloman, Sydney

On a sunny day (Fri, 14 Aug 2020 06:18:07 -0700 (PDT)) it happened snipped-for-privacy@gmail.com wrote in :

Thyristor

You could use a small SCR, or make one out of a couple of handy transistors. And a cap somewhere so it doesn't trigger on noise spikes.

Merge the probably-existing driver transistor into the new latch, so the latch feature actually adds one transistor.

--

John Larkin         Highland Technology, Inc 

Science teaches us to doubt. 

  Claude Bernard

rchived notes or a fair amount of googling, but suspect it will prove embar rassingly obvious.

xtend its versatility by optionally latching it on when triggered, staying on regardless of any other inputs, until power is removed several hours lat er.

le monostable with a long period at least as long as ?several hours ?, and triggering the relay with that mono. But I?m sure th ere must be a simpler way?

un in that?

You can use a very simple circuit with just two transistors. One will need to be sized to carry the current of the relay coil and have appropriate pr otections when opening the current path. A P and N pair (either bipolar or FET) can turn both transistors off in the relay off state so as to draw vi rtually no power. If some leakage current is no big deal then a pair of 2N

7002 FETs will do a bang up job wired in a ring. The relay coil is the loa d for one and a resistor for the other. Pull down on the "other" gate and the relay will be activated.

You don't say what the relay coil current is, but the 2N7002 will handle so me 100s of mA. Since it is full on the voltage will be low and so the powe r.

Can be much more simple than that.

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  Rick C. 

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What voltage is the contacts switching? And when you say 'until power is removed several hours later' do you mean from the coil or the contacts or both?

Thanks all, some great suggestions amongst those replies. I?ll play with several on the bench. If I can squeeze a switch and an SCR into the e xisting case that?s a candidate. But even better would be a reliabl e toggle (Set/Reset, F/F) before the (untouched) relay unit. I?d ha ve to be confident it would always set predictably (fairly easy I recall) a nd, more important, not get toggled again spuriously.

And a upcoming contender is to add it to my existing working Arduino projec t. With no microcontroller skills I?m finding it hard to get into a software frame of mind and this would be a timely learning exercise. I wou ld input the trigger to input pin D5, and The first time it goes high for > say 10ms, output a permanent high to pin D6 to activate the relay.

The relay will switch on lamps a user-settable period after I leave my shed workshop, and I?ll switch the relay off the following morning.

I was going to suggest a SCR but as you only have 5V the 1-2V that a SCR wastes could be a bit onerous

assuming you are using the usual NPN circuit to control the relay. use a PNP transistor to detect low voltage on the switched end of the relay coil and then feed extra current to the NPN transistor base.

connect the PNP conllector to the input end of the NPN base resistor, connect PNP emitter to +VE supply and a resistor from PNP base to NPN collector. Add a diode between the input and the the base resistor+PNP collector junction.

resistor 10K +/- order of magnitude, any diode, any cheap PNP.

so this

----+--+-- | | D1 Relay +--+ | | in-------R1--Q1 | --+--

becomes this:

+-----+--+-- | | | | D1 Relay | +--+ Q2-R2--+ | | in--D2-+-R1--Q1 | --+--

--
  Jasen.

Thanks Jason, I?ll try that new idea.

ay with several on the bench. If I can squeeze a switch and an SCR into the existing case that?s a candidate. But even better would be a relia ble toggle (Set/Reset, F/F) before the (untouched) relay unit. I?d have to be confident it would always set predictably (fairly easy I recall) and, more important, not get toggled again spuriously.

ect. With no microcontroller skills I?m finding it hard to get into a software frame of mind and this would be a timely learning exercise. I w ould input the trigger to input pin D5, and The first time it goes high for > say 10ms, output a permanent high to pin D6 to activate the relay.

ed workshop, and I?ll switch the relay off the following morning.

The SCR can false trigger and also turn off on transients. CMOS logic devi ces are very high input impedance and so require parts to make them resist transients. I think you'll find a pair of transistors work very reliably. The more reliable circuit is probably a two pole relay with one pole provi ding latching. But you've already ruled that out. I wouldn't bother with an SCR because transistors are already in my parts bin and so readily avail able anywhere.

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  Rick C. 

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Try this maybe.

--

John Larkin         Highland Technology, Inc 

Science teaches us to doubt. 

  Claude Bernard

Thanks for the further suggestions.

The stage before this one under discussion was trickier than I?d ex pected, but I?ve sorted it now. The high input to the latchable rel ay (or alternatively to a flip-flop/toggle/bistable that would eliminate th e need for latching) comes from a small clock/timer. I opened it and solder ed a couple of wires to its piezo transducer. When the set timing period ha s counted down, this repeatedly delivers ?bursts? of pulses for 60 s. (Some of the spikes surprisingly reach around 70V). I?ve converted these into clean pulses of about 0-3V with a some filtering, a zener, then a 3140 configured as a Schmitt.

This works fine as input to my Arduino method. IOW the D10 5V output is now a satisfactory candidate to activate the relay ?permanently? ? as required.

But I?m also going to try both the flip-flop alternative with a 401

3 in D-type configuration (if my pulse rise time proves fast enough), and a lso John?s latch switch.

Correcting typo: ?D10? is my choice of input pin on the 328 . I meant to type ?D11?, the output pin.

John: Did you mean your switch to be wired as you drew it? The NPN would always be inactive when set to your 'OFF' (non-latched) position, instead of operating normally as optionally required.

That aside, although your latching works OK I couldn't get normal mode running. It's only academic now because I wouldn't have been able to add even a miniature SPST switch to the existing case. And I now have my 328 circuit working nicely.

I also tried a bistable method and that too worked fine.

(I abandoned my misguided intention to use an existing (and cased) 'Flip/Flop: 4013' because, that was toggled (not SET) by every pulse from the timer during its 60 second alarm period.

Terry

That's an option. Depends on exactly what you want to do.

Just be careful about using logic gates. Smallish noise spikes will toggle fast logic. That's why I put a cap in my circuit.

--

John Larkin         Highland Technology, Inc 

Science teaches us to doubt. 

  Claude Bernard