open circuit relay coil lockout

[Johnny5]

Not sure what it would be called but I am looking to make a device that when sees continuity between two points it opens a relay coil otherwise relay remains closed when open condition between these two points........(it will lock out when when water is present and will remain locked out until re-set). The power supply is 24 vac Any ideas would be much appreciated.

 

[(*steve*)]

Can current flow through "the 2 points"?

The easiest way is to have a relay held in by current passing through your "2 points" and a pair of NO contacts on the relay. As soon as the circuit opens the relay drops out and cannot get pulled in again. To reset it, you have a momentary contact switch across the NO contacts.

You then place your alarm so it is activated by the closure of the NC contacts.

This is essentially a circuit that latches OFF. It will be triggered by a power loss, but also will not be cancelled by cycling the power.

Note that this circuit will work directly from 24VAC.

If you don't want to have the relay pulled in all the time, you vould rectify the 24VAC and use a transistor to pull the relay in. The "2 points" would keep the transistor turned off. When they open, the transistor turns on, pulling in the relay. The NO contacts would be used to maintain the circuit so the relay can't open again. An NC switch in series with these contacts allows it to be reset. Poser failure would also reset it. (NO contacts would be connected to the alarm)

 

[Johnny5]

Thanks for the reply, current can not pass through the two points, it would be similar to a soda fountain when liquid grounds or closes a circuit when it touches the lever. Ive used the actual module from a soda fountain to make a prototype but my understanding is minimal and duplicating the solid state module is beyond me. If I had a schematic or something to start with I could begin to experiment. I am looking to turn an appliance off when moisture spills out. A/C unit or water softener, could be used in many household appliances.

 

[Lenp]

Two bare bones ideas...

Use a low voltage supply, AC or DC and a relay that is half the supply voltage (6V relay on 12V supply for example). Across the LV power supply wire the relay coil, an open set of relay contacts, and the resistor all in series. Use a momentary switch across the relay contacts. Pressing the switch should cause the relay to pull in and latch. If not, adjust the resistor value until it does. Now if using AC, put a triac MT1, MT2 terminals or if using DC, SCR terminals A, K across the relay coil so when it conducts the relay coil is shunted. This causes the relay to drop out, opening the contacts and turning off the power to the circuit. Sense the water between the SCR Gate / Anode or the TRIAC Gate and MT2 terminals with probes, or a probe on the gate and the other grounded terminal If the circuit false triggers because it is too sensitive add a pair of resistors to form a voltage divider between the gate and the probe. Other contacts on the relay can be used for the control you desire. This design is not too eco-friendly since the relay draws power and the resistor will dissipate heat all the time until triggered.

SO...
Use a low voltage supply, AC or DC. Across the power supply, the relay coil, in series with either. a triac MT1, MT2 if using AC or a scr terminals A, K if using DC so when the scr or triac is triggered the relay coil is energized. If using the scr, the relay should latch and not release until power is removed. If using the triac, a set of N/O relay contacts across the MT1,MT2 terminals will be needed to latch the relay. In either circuit, removing power will unlatch the relay. Sense the water between the scr Gate / Anode or the TRIAC Gate and MT2 terminals with probes, or a probe on the gate and the other grounded terminal If the circuit false triggers because it is too sensitive add a pair of resistors to form a voltage divider between the gate and the probe. Other contacts on the relay can be used for the control you desire. This design is eco-friendly since the relay draws power only when in use.
I would personally use a wall power pack, the scr and a dc relay for simplicity.

Could it be done other ways? Sure! There are alternating and lock/unlock relay and using solid state is even an option but it depends on cost, space and the load power you want to control but I think simplicity is the keyword here.

Questions? feel free!

Len

Footnote:
I once saw a science fair project using an aspirin tablet between two spring contacts to sense water. The aspirin melts, contacts close!

 

[Johnny5]

ah hah its called a wet switch

Thanks Lenp and Steve

This device is readily available at HVAC supply houses $40 dollars gets you something called a wet switch, it does sense wet condition which in this case is something around 9m ohms.

I thought i was going to be a millionare with a new invention yeah right

Would using a transistor and photo transistor and a bridge rectifier to convert to 24 dc be possible not sure how triac configuration works. Thanks

 

[Militoy]

...Footnote:
I once saw a science fair project using an aspirin tablet between two spring contacts to sense water. The aspirin melts, contacts close!

I use sugar cubes on the floor - holding microswitches open, in a burn-in room using water circulating through cold plates and an evap cooling tower. It works well for very little $ - as long as the room is kept clear of rodents! Don't believe the idea will be likely to net me a million, though...