Hi all,

I am trying to add a relay de-energise delay to the attached circuit.

The relay is set to be normally energised in steam or in water ES C EW jumper. When the circuit sensing a condition opposite to the normal state, the relay will de-energise. I am try to get a 3 second delay into the relay de-energising.

Any help would be great. The boards are small so space is a premium.

IMG_1699.JPG

 

Where in that circuit is the relay you want to delay?

A simple delay can be made using a MOSFET with a diode, resistor, and capacitor.

 

Thanks for the reply.

If you see between the ES and EW pads, the centre pad feeds the relay on another board. The Pad feeds the 0v to the relay. It's a 12v DPDT relay.

 

If you see between the ES and EW pads, the centre pad feeds the relay on another board. The Pad feeds the 0v to the relay.

Still not clear.
How does the Pad feed 0v?
Does transistor TR1 control the relay?

 

The puds are jumpers, you solder EW pad to centre pad to activate the relay to be normally energised in water and ES to centre for normally energised in steam.

Yes TR1 controls the relay. When 12v is applied to the gate the relay will de-energise.

 

Yes TR1 controls the relay. When 12v is applied to the gate the relay will de-energise.

I don't see how that can happen.
Are you sure TR1 dosen't turn the relay on when voltage is applied to TR1's gate?
Can you draw a simple schematic showing how TR1 controls the relay coil current?

 

Hi,

This one is not as easy as it looks. I have been going through it for a while and it would seem that there are lots of factors that come into play when operating the relay. The only constant is the relay connections themselves. No other single point controls the relay and its settings. It it possible to use the relay coil itself to activate the delay. So as the 0v is disconnected in the circuit, the delay starts.

I know and cant explain it very well but I hope you understand.

 

Can you show how the relay coil is connected?
Otherwise I'm sort of flying blind here.

 

This is a screen capture of the pcb file. The track from the bottom of the BAV20 is missing from the picture. As you can see is is connected directly to 12v+

 

This is a screen capture of the pcb file. The track from the bottom of the BAV20 is missing from the picture.

What is the BAV20?
That picture doesn't help me.
I need to know something very simple.
I need a schematic to see how is the relay is controlled by the circuit.
Otherwise I'm only guessing as to how to solve your problem.

 

On the first drawing you will see the relay connection. PL3 pin 3 goes to PL3 pin 3 on the second board via ribbon cable. Hope this is enough to make sense of it.

 

BAV20 is a low leakage diode

 

If you put the delay circuit directly on the relay it will require a very large capacitance to get a 3 second delay.
Can you measure the coil resistance with an ohmmeter so I can determine the required value?

 

@crutschow
Hi Carl,
This may help.
http://forum.allaboutcircuits.com/posts/1060678/
Eric

 

Does Tr1 fet or Ic1a opamp turn on the relay in post #11 drawing ???

 

If we break down the operation IC1A outputs 12V is the circuit senses water and 0V if it senses steam. So when sensing water the 12V from IC1A pin 7 goes to TR1 gate. This then pulls D7 to ground giving 0V at EW pad and 12V at ES pad. If the circuit senses steam then there is 0V at ES pad and 12V at EW pad.

By bridging the EW pad to Centre pad the circuit to the relay is complete for normally energised in water state. When the circuit senses water there is 0V from EW to centre pad and then to the relay. This energises the relay. When steam is sensed then there is 12V at the EW pd to centre pad. This gives 12V to the relay de-energising it.

By bridging the ES pad to Centre pad the circuit to the relay is complete for normally energised in steam state. When the circuit senses steam there is 0V from ES to centre pad and then to the relay. This energises the relay. When water is sensed then there is 12V at the ES pd to centre pad. This gives 12V to the relay de-energising it.

I hope this explains the operation better. So as you can see there is not one single component that controls the relay operation, but a combination of jumpers are components. The common point is that there is 12V or 0V at the centre pad, 0V when the relay is energised and 12V when de-energised.

 

The best option is to use a 555 monostable timer that fires the relay, that way the relay stays on after the negative pulse goes off, so you can use it in both Steam and Water mode.

Like this circuit, the timer goes high driving the relay on when it receives a Low pulse on pin 2, and stays high until the pin 2 goes high, then the output goes low after the time delay set by the 100k resistor and 1uF capacitor.


You can get a ready-made pcb on ebay.

 

Thanks for the reply. I have been experimenting with 555 chip as I feel this is the best way forward. Just not sure how to implement it.

 

If your time delay accuracy is not critical, here's a simple, two transistor circuit that should do what you want.
It shows about a 3s delay when the input goes high to turn off the relay (relay current goes to zero).
The P-MOSFET can be just about anyone that has a Vgs threshold voltage of at least 2V.
You may have to adjust the value of R2 get the delay you want due to the variation in MOSFET threshold voltages.
The LRelay is the one you have on your board.
V2 simulates the signal from ES or EW.
The collector of Q1 connects to the relay centre pad between ES and EW.

Make sure you add the diode between Q1's collector and +12V to protect the transistor from the relay coil inductive transient.

 

Thankyou very much for the help. I will try this out as soon as I can get the parts after Christmas. Hope you have a great one.