240v Delay Off Timer

As it happens I'm making 1 for my own central heating system, should be ready next week. Eail me if you live in england and are interested.

"off delay" time delay relays area a lot less common than "on delay" relays, because either they need a separate power connection to keep the timer running after de-energization, to an internal energy storage to do the same thing. I think you would be better off to try to use an "on delay" relay with a normally closed contact to run the pump. Then you need a normally closed contact to energize it when the burner shuts down. Of course, this means that the time delay relay will be continuously powered when the burner is off.

But if you Google "off delay" "time delay relay" you might find the one you are thinking about, now.

Off the shelf item.

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schreef in bericht news: snipped-for-privacy@h54g2000cwb.googlegroups.com...

Elektor once published such a circuit. It was placed between the mains and the heating unit and monitored the thermostat. So it could switch off the heating - including the pump - but without loosing its own power. When the requested temp was reached, the thermostat signaled the heating unit which switched off the heater but not the pump. The circuit kept powered the unit for some minutes then switched it off. It kept monitoring the thermostat so when it required heating again, the unit was switched on. An extra feature is a 24h timer which switched on the unit for about 15min when it had not been running for a whole day, so to prevent the pump to get stuck.

It's an old design, maybe over 20 years and not usefull anymore as modern heaters have this provisions already build in.

petrus bitbyter

Since you still need the control coltage to keep the blower riunning this is not an issue. The LV transformer is always powered. The easy way is to use an SSR to drive the blower and a capacitor feeding a darlington for the time delay. We had this discussion a week or so ago.

snipped-for-privacy@hotmail.com wrote in news:1162578378.983244.96830 @h54g2000cwb.googlegroups.com:

These are "Delay-on-break" timers. You would use one of these to control a relay that will handle the voltage and current of your pump.

Ken

View in fixed font:

I'm assuming the thermostat switches 24 volts, not 240. If the timing is not critical, you can run the 24 VAC from the closed contact of the thermostat through a rectifier to charge a cap, and connect that to a comparator. The output of the comparator operates a relay through a driver transistor. When the thermostat calls for the burner, the thermostat contact closes and the burner runs. When the 'stat contact opens. the comparator output continues to hold the relay energized, until the cab discharges below the reference voltage, then the relay drops and the circulator motor shuts off. You need an always on DC supply to power.

24VAC--Bridge---+------+ | | [CAP] [R1] | | 24VAC--Bridge---+-[R2]-+---- Vin to Comparator input | DC Ground ------+-[R3]-+---- Vref to Comparator input | [R4] | + 12 Vcc --------------+

R1 and R2 are a voltage divider to set Vin below + Vcc You want then to be high value to get a large RC. I'd use 680K for R1 and 330K for R2 and 1000 uf for the cap to give you about 11 volts at Vin when the burner is on. That will give you plenty of discharge time.

R3 & R4 set the reference voltage to the comparator. A pot here can be used instead to vary the timing.

The comparator output operates a relay driver, that relay gates the 24 V control voltage to the circulator. See below

Original wiring:

Tstatcontact /

24vac ----o o---------+------------+ | | [Burner] [Circulator] | | 24vac -----------------+------------+

New wiring:

Tstatcontact "10 minute" relay contact / /

24vac ----o o---+------+----o o----+ | | | | [Burner] [Circulator] | | | 24vac --------------+---+------------+ | | Bridge--+ | | | [CAP] | | +-----+ etc

Pick a relay whose contacts can handle the circulator load.

Ed

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JF

Oops - that last diagram was supposed to be:

New wiring: +---------------------+ | | | Tstatcontact | "10 minute" relay contact | / | /

24vac +---o o---+------+ +-----o o | | | | [Burner] [Circulator] | | | 24vac --------------+---+------------+ | | Bridge--+ | | | [CAP] | | +-----+ etc

In my country - and I guess in many others too - it is not allowed to modify the circuit or wiring inside the heater enclosure. You may run into safety and/or assurance trouble if you do.

petrus bitbyter

On 03/11/2006 the venerable snipped-for-privacy@hotmail.com etched in runes:

Go here:

and search for H3DS. It does just what you need. Available from RS Components, Farnell and many others.

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John B

The tstat contact, 24 volt transformer and circulator relay are outside the enclosure. There is no mod to anything inside the enclosure. I can't say whether that applies to 100% of heating units using circulators in the world, but it does not make sense to me that the circulator or its low voltage (control) or high voltage wiring would be inside.

Ed

Well,

Things may differ from place to place. I never saw a heater with transformer, pump or relay outside the enclosure. FAIK it's due to local safety regulations to make sure the gas burner wil not work without a working circulator pump. The elektor article I mentioned before warned explicitely not to change anything inside the heater enclosure. But of course, there will be millions of heaters but the few I ever saw. Besides, newer heaters have these facilities build in already. That's to say, the ones you can buy over here.

petrus bitbyter