# Solved: Brief pulse of 240vac required from a mains power circuit.



## tmetford (Apr 8, 2008)

How do I obtain a brief pulse of electricity (enough to power the coil on a 240vac relay for only a moment) from a mains power circuit when the power circuit gets turned on (for heating purposes)? 
Purpose.
This brief pulse is intended, in turn, to switch on a lamp that then indicates (from a distance) that the room heater has been powered on, but which then turns the indicator lamp off after a few seconds rather than leaving it switched on unnecessarily.
What Ive done so far.
I have studied various circuits involving the NE555 timer in an attempt to solve the problem. Unfortunately these only seem to work when the signal current that initiates the circuit is a brief pulse and not when theres a constant signal current (as happens when the power circuit remains on as it heats the room). 
The problem.
I have an exercise studio at the bottom of my garden which needs to be warmed up before I go in there. I need a light to come on (for only a brief moment) with the remotely operated power switch so I can see, from my house, that the heater has actually been turned on. As I've said, I dont want that light to remain on after that.
My Request.
I need help in designing a circuit that will do this please.
Thank you in anticipation.
Tim


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## cwwozniak (Nov 29, 2005)

The simplest solution might be an industrial interval time delay relay with a 240VAC rated input and 240 VAC rated contacts. This type of relay activates the contacts for a fixed amount of time when power is first applied and then deactivates after the time period. It resets when power is removed. Fixed and programmable interval models are available. Here is a link to one such programmable product sold by a UK distributor. For this particular model, you would need to either supply a short industrial automation DIN rail to mount it or jerry-rig some other mounting method.

http://uk.rs-online.com/web/p/time-delay-relays/7740339/


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## tmetford (Apr 8, 2008)

Thanks for your advice here. This time delay relay will do what I want which is great if I cannot make a circuit that will do this.
For me, I would like to find out the circuit diagram that enables this behaviour in a low-voltage DC circuit. However I'm not in need of a mains-power rated device, it is simply the switching on of a mains circuit that is, in this case, providing the trigger (not the power) to illuminate the lamp which signals to me that the heating circuit has come on.
Do you have a diagram for a low-voltage DC circuit (e.g. involving a 555 timer chip) that will trigger the illumination of a low voltage lamp for a few seconds and then switch it off, even though the mains-rated 240VAC relay coil that originally completed the circuit to trigger the illumination of the low voltage lamp, continues to remain in the closed (switched on) position?
I hope that I'm being clear enough in my explanation.
Thanks.


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## Paquadez (Jun 9, 2003)

Tim:

I believe what you require is a simple pulse-switched trigger, which enables a mains capable relay.

The trigger is the existence of a 240 V AC current when none previously existed.

However, I note you only require a momentary "On" condition.

I therefore would probably design the core circuit around a 6/12 V DC relay, where the low voltage switching coil was "Slugged" by a suitable capacitor. (Slugging means the C retains a charge and holds the relay closed for a finite period and after this it then drops out. The capacity of the C dictates how long the "slugging" effect pertains).

Rather than a 555, think in terms of a transistor switch to enable the relay.

In order to detect the presence of the 250V AC, why not use a few windings of copper wire coil to detect the EMF?

I designed and made a similar circuit some years back for my home compressor: as the demand pressure switch went Closed Circuit, this turned on the mains motor and simultaneously enabled a 250V mains transformer which tripped the Transistor Switch: this then energised the magnetic clutch and thus compressor pump would turn: except, by slugging the relay a delay was induced in order for the motor to reach maximum revs and then the clutch engaged.

Nice little electronic "unloader" switch!

You require this backwards.



Look for guidance here, for example.

http://www.discovercircuits.com/list.htm

http://www.electronic-circuits-diagrams.com/

http://www.engineersgarage.com/electronic-circuits

http://www.aaroncake.net/circuits/

Thousands out there: and forums too.

*Disclaimer!

Mains voltage electricity is very dangerous!

Unless you are well versed in the risks and the problems, never ever try out projects which are above and beyond your competence and ability: seek professional assistance!
*


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## cwwozniak (Nov 29, 2005)

I found a circuit at the link below that will energize a relay about seven seconds after 12 VDC is applied to the circuit. You could use the circuit as-is if you used a suitable "wall wart" 240VAC to 12 VDC power adapter. The same adapter could power a 12 Volt DC indicator light through the normally closed relay contacts.

http://www.circuitdb.com/?p=211

EDIT: A side benefit of using a commercially made power adapter is that it will reduce the risk of damage or injury by you only having to work with a relatively low 12 Volts DC supply.


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## tmetford (Apr 8, 2008)

Thank you both for a really informative response; much appreciated.
I shall work through the material you've presented and take due note of your timely caution regarding mains voltage!
I have developed respect for the power lying within the mains supply over the years.
Tim


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## cwwozniak (Nov 29, 2005)

You're welcome.


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## Paquadez (Jun 9, 2003)

Bearing in mind my earlier caution and not knowing the OP's skill and knowledge levels when dealing with mains voltage AC, consider the following.

If I were seeking to design a dedicated circuit to accomplish your objectives:

1. I would consider using an Opto-Isolator IC to insulate and separate the mains from the rest of the circuit:

http://en.wikipedia.org/wiki/Opto-isolator

By using a suitable Opto-Isolator and a plain (non-Polarity Sensitive, i.e. not electrolytic Capacitor) to rectify the incoming mains AC, it would be fairly simple to enable a transistor switch to generate your desired pulse.

2. I would also consider using a suitable Thyristor, which are now commonly used for mains circuits such as Lamp Dimmers.

http://www.allaboutcircuits.com/vol_3/chpt_7/6.html

If you evaluate a mains lamp dimmer there is very little inside in terms of passives and discretes.

There are a number of interesting circuit concepts in the second webref to provide ideas.

*Disclaimer!

Mains voltage electricity is very dangerous!

Unless you are well versed in the risks and the problems, never ever try out projects which are above and beyond your competence and ability: seek professional assistance!*


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## cwwozniak (Nov 29, 2005)

Paquadez said:


> consider using an Opto-Isolator IC to insulate and separate the mains from the rest of the circuit:
> 
> I would also consider using a suitable Thyristor, which are now commonly used for mains circuits such as Lamp Dimmers.


Might you think that is a bit of overkill for the intended application? Tim has said that he wishes to momentarily light up a low voltage lamp when he turns on a remotely controlled 240 VAC heater. A basic CE listed AC/DC power adapter could provide both the operating power and high voltage isolation needed for the timer circuit and indicator light. The 12 VDC timer circuit I had previously mentioned could probably be operated by a device such as this:

http://www.maplin.co.uk/p/maplin-12w-switched-mode-acdc-fixed-voltage-12v-power-supply-n12gn

The availability of a switched 240 VAC outlet would need to be addressed.


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## Paquadez (Jun 9, 2003)

That should work, Chuck.

No need for a switched mode PSU: Tim could use a simple plug-in PSU, hard-wired into the supply feed which enables the heater.

Personally, I would also add a pretty beefy electrolytic C to de-couple and smooth the DC output from the PSU.

Something like this is fine:

http://www.ebay.co.uk/itm/New-1000m...In-Power-Supply-With-6-DC-Plugs-/140679972457

Needs a simple - fused- flying power strip wired direct into the heater feed to actuate the switch circuit.


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