Latching relays circuit

[AnalogKid]

One more requirement. It has to use no power in the default/off state.

0.000000 A or 0.000010 A?

ak

 

[UKenGB]

0.000000 A or 0.000010 A?

ak

A bit of a moot point. A simple relay circuit in its dormant state will consume 0 power. No current through the coils, no contacts being pulled across, zero power consumed. Then, when 'latched' one or more relays are active which means significantly more current than suggested above. What I don't want is a circuit that requires one or more relays active all the time in either state. There is a common solution to this problem with 2 relays and 2 diodes, but one relay is always active. That's what I want to avoid.

 

[RODALCO]

There exist toggle relays which basically have two coils to change over from one state to the other. I have seen them in electricity prepayment meters. These coils were low voltage coils. Have a loo on eBay.

 

[UKenGB]

There exist toggle relays which basically have two coils to change over from one state to the other. I have seen them in electricity prepayment meters. These coils were low voltage coils. Have a loo on eBay.

Yes, latching relays are quite common, but I refer to my original post:-

"when car is turned off and power to the circuit is removed, it will return to its default state of lights off."

So a latching relay is not suitable here. It's a push toggle circuit required.

 

[AnalogKid]

A bit of a moot point.

Not really. I understand how relays work, and that with them, off is really off. My question is whether or not a few microamps of CMOS static current is acceptable. In some applications, even that small current is not allowed. If your's can handle it, that opens up a large range of possibilities -

IF - a small electronic circuit (as opposed to only relays and wiring) is acceptable. If yes, then 1/2 of a CD4013 flipflop chip plus one MOSFET relay driver can drive any headlight relay you can imagine. Or, the MOSFET can be large enough to switch the headlight current directly with no relay.

ak

 

[UKenGB]

More complex circuitry is of course a possibility and there are plenty of options. Specifically I was looking for the simplest solution, hopefully with just 2 relays. So far, it looks like that's an impossibility.

 

[JimB]

If you are sure that you want a solution which only uses relays, look here:

A variation of something which was posted earlier.

JimB

 

[UKenGB]

Yes indeed. I posted that and mentioned that adding a third relay could be a solution. It is my fallback option, but 2 relays would be better.

In the meantime, I have just received a small circuit from eBay that apparently does what I need and it's very small and at £0.99 is cheaper than even just a single relay. I will test this and if it works, will probably use that.

 

[AnalogKid]

I was looking for the simplest solution, hopefully with just 2 relays. So far, it looks like that's an impossibility.

Probably so, because the output changes state in one direction while the input remains in one state, then has to change in the other direction when the input returns to that same state. Looks like a race condition.

ak

 

[UKenGB]

That circuit above is extremely clever. I just can't use it in this case as the button provides 12v and that cannot be changed - except by the addition of another relay.

So near and yet so far.

 

[eTech]

There is a chip that seem to be a good fit for your application. It's no longer manufactured but there seems to thousands available.
It an Automotive Toggle switch IC U6032B. It cost about 2.00 USD. Also needs a couple of resistors and caps to complete the circuit.
You could connect the push button and relay directly to it.

eT

 

[AnalogKid]

WOW - where was that when I needed it! Talk about the right tool for the job ...

ak

 

[UKenGB]

So, exactly what I and I would think many others need is already 'no longer available'. Brilliant. Not exactly how I imagine 'supply and demand' should work.

Thanks for the info though.

 

[Gaudeamos]

I actually already ordered the first one as I think it does what I want. I guess I'll find out when it gets here and I try it

Cool! Please post a follow-up when you've given it a go.

 

[Gaudeamos]

If you are sure that you want a solution which only uses relays, look here:

Step 2 puts the two relay coils in series on a 12V circuit. Is ~7V enough to energize an automotive relay?

 

[crutschow]

Deleted post.

 

[UKenGB]

Step 2 puts the two relay coils in series on a 12V circuit. Is ~7V enough to energize an automotive relay?

That's a good point. There are 2 things to consider. Firstly, they are only required to HOLD themselves on at this reduced voltage and most relays will hold at half their rated voltage. They actually get the full 12v when being energised. Secondly, they could be any type of relay not just standard automotive relays (which tend to not be DPDT anyway). One could even use 6v relays which would cope with momentary application of 12v when being energised. Or you could add a couple of resistors to limit energising voltage.

So several ways to avoid the problem.

 

[eTech]

So, exactly what I and I would think many others need is already 'no longer available'. Brilliant. Not exactly how I imagine 'supply and demand' should work.

Thanks for the info though.

Hi

Depends on what "no longer available" means. The part is no longer manufactured but there are literally thousands available.
If this is for a production run, probable shouldn't use it. But if it's for a onesy, twosey, kind of thing, I wouldn't be concerned. Just buy a few of them.

eT

 

[eTech]

That's a good point. There are 2 things to consider. Firstly, they are only required to HOLD themselves on at this reduced voltage and most relays will hold at half their rated voltage. They actually get the full 12v when being energised. Secondly, they could be any type of relay not just standard automotive relays (which tend to not be DPDT anyway). One could even use 6v relays which would cope with momentary application of 12v when being energised. Or you could add a couple of resistors to limit energising voltage.

So several ways to avoid the problem.

I worked a lot with relay based systems in my former life and this is what bothered me when I first saw this circuit. While true that the relay will hold at a lower voltage, the voltage fluctuates so much in an automotive electrical system it may drop out unexpectedly at times. I think that half the voltage is stretching it. I'd be more comfortable with at least 3/4 of full voltage.

eT

 

[Gaudeamos]

That circuit above is extremely clever...

It is indeed a clever use of electro-mechanical devices to implement state logic.

However, I'd be concerned on a few fronts:
- voltage division between the two coils, as already mentioned
- momentary impedance of inductor d2 causing corresponding voltage/current sag in d1; if d1 de-energizes (however momentarily), we're back to stage ZERO.
- I wouldn't feel quite right about a switch that requires real power to maintain state - controlling a safety-critical device.

Picture a spontaneous lite's-out as a result of a negative transient...
... induced by, let's say ... panic braking ...
Even with an electronic solution (ie. with improved transient immunity) such a default-off/auto-reset switch may raise safety concerns.