<1s 240v AC impulse to >1s contact closure

[martinito]

Hi everybody,

I have been looking for a solution to my problem (below) for some time now, I finally decided to join the forum, hoping someone could help me.

So, here is what I have:

A device controlling a 240v latching relay, producing an

• implulse of 240v AC on two wires during less than 1 second to open the relay, and
• implulse of 240v AC on two other wires during less than 1 second to close the relay

But I would like to replace the latching relay by two regular relays in order to have

• the first relay closed during slightly more than one second when the orignial relay was closing
• the second relay closed during slightly more than one second when the orignial relay was opening

For this application I was thinking about using a rectifier to swich from 240v AC to 240V DC (or four adapted diods), a capacitor to hold the charge a bit longer and finally a 12v DC relay in series with a resistor that would be 19x the resistance of the relay coil (voltage divider).

Could someone help me make this work ?
How could I make it safe... working with 240v AC is quite dangerous.

I will also need to build 10 similar circuits and cost will definitely be an important factor !

Finally, I will give some explanation of the application:
I'm automating motorised curtain tracks to integrate them in my home automation system. The track opening (or closing) swich is a momentary contact between two wires (one +5V and one GND).

The controlling device would be a modified X**broken link removed**.

In advance, thank you for your help,
Martin

 

[martinito]

I am going to have to make this circuit soon...
Could anybody tell me if this approach is reasonable?

Many thanks,
Martin

 

[ccurtis]

Your requirements are not clear to me. Are you simply seeking to duplicate the normal function of the 240 VAC two-coil latching relay, where the two coils are alternately energized, with two separate 240 VAC non-latching relays? Or is there more to it than that?

 

[Willbe]

Yes, post a truth table with tentative schematic.

 

[martinito]

Hi guys,

Thanks for the advice, I spent some time drawing the diagram yesterday (since I have no tool to do it I used powerpoint ):
**broken link removed**

I would like to use it twice since it is supposed to be used to trigger a remote and that I have 2x 240VAC impulse (<1sec) inputs and that I need two contact closures (>1sec) in order to open or close my curtains.

Thanks for having a look at it.

Martin

 

[martinito]

Precision: my main concerns are:

• Is it safe to use a voltage divider to interface a 12VDC relay on mains if properly insulated?
• What type of capacitor and resitors should be used? is 1/8w enough?
• Any component missing?
• Any idea of appropriate values for C1 & R1?

Thanks,
Martin

 

[ccurtis]

It is safe if it is properly fused and either installed in a grounded metal enclosure or double insulated.

What is needed is the minimum time of the impulse. The maximum time is 1 sec, but what is the minimum time? Also, if there is a chance that the impulse will be longer than 1 sec, how long can that possibly be under all possible conditions? Is it possible that the impulse could be applied continuously?

Actual component values depend on the relay. What specific relay do you want to use and what is its voltage and coil resistance. P/N and full specs are most helpful.

Also, how much longer than one second should the relay stay energized? Is 1.00001 second or anything greater than 1 second enough? Just want to be sure.

 

[martinito]

Hi ccurtis,

Thanks for your reply and advices.
I unfortunately don't know how long the 240v impulse lasts, probably less than half a second (but enough to trigger a 240v latching relay) and I have no tool to measure it even if it seems fairly constant. Can I use a trial & error technique with R1?

The output relay contact closure length doesn't really matter, as long as it is above one second (5sec is fine as well). I don't have the specs of the relay yet but I suppose the capacity of the capacitor can be expressed in function of the relay coil resistance + R2. Could you help me with that?

Thanks !
Martin

 

[ccurtis]

Hi ccurtis,

Thanks for your reply and advices.
I unfortunately don't know how long the 240v impulse lasts, probably less than half a second (but enough to trigger a 240v latching relay) and I have no tool to measure it even if it seems fairly constant. Can I use a trial & error technique with R1?

It's a rather unconventional circuit, but what the hey. I would dispense with R1 since it will only limit the minimum impulse time. Short it out.

The output relay contact closure length doesn't really matter, as long as it is above one second (5sec is fine as well). I don't have the specs of the relay yet but I suppose the capacity of the capacitor can be expressed in function of the relay coil resistance + R2. Could you help me with that?

Thanks !
Martin

First you determine from the relay spec sheet what the maximum coil voltage is. Call that voltage Vrmax. Then from the relay spec sheet you determine the relay drop-out voltage (the voltage at which the relay is de-energized). Call that voltage Vrmin. Choose R2 so that the voltage across the relay coil resistance is Vrmax with an initial capacitor voltage of 339 Volts (240×1.414). Add the coil resistance and R2. Call that sum R. Then you calculate the percentage the 339 voltage has to fall so that the voltage across the relay coil resistance is at Vrmin. Call that percentage, Pct. From the standard graph of percent voltage discharge for a capacitor vs. RC time constant (1 RC time constant is 63%, for example) you see how many RC time constants it takes to achieve Pct. From R, determine what C has to be so that for the number of time constants (RC) is greater than 1 sec.

BTW, I did this for one 12 volt coil relay I saw the specs for, and came up with 330 µF. A 330 µF 400 V capacitor is no small device, but it is widely available.

If the impulse voltage is going to be present for more than one second, or very frequently, the several watt power dissipation in R2 needs to be accounted for. To protect against burning up R2, use a fuse with a delay so that it will open in that case, or use a power resistor.

To be safe, and conservative, the diodes should have a minimum reverse breakdown voltage of 400 volts. The forward current rating should about 1 amp. The peak current will be higher, when charging a dead capacitor, but diodes can handle large peak currents. The 1N4005 is a good choice.

You could get by with a significantly smaller valued capacitor if you realize that a relay coil can withstand several times is rated coil voltage for a short period of time. This spec is not given in relay data that I have seen, however. Also, if that fact is to be exploited the impulse time must be limited, or the relay coil will be destoyed, unless some sort of protection is added.

 

[martinito]

Thanks ccurtis,

I'll make my shopping list and try it out..
Your help is really appreciated!

Martin