Thermostat + Timer
in [Design]
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From: Jim Thompson on 15 Oct 2009 11:01 On Thu, 15 Oct 2009 15:52:50 +0100, baron <baron.nospam(a)linuxmaniac.nospam.net> wrote: >don <don> Inscribed thus: > >> baron wrote: >>> Hi Guys, >>> >>> I'm doing a hobby project to build a timer that can be used to turn >>> on a relay at a pre-determined time and then turn it off after a >>> delay. I also want to use a temperature sensor to override the relay >>> but only during the time period that is programmed. >>> >>> The idea is to turn on a water heater for a pre-determined period but >>> to use a thermostat to control the temperature during this period. >>> >>> I have suggested using a relay because the heater is rated at 3.5Kw >>> 240/250 volts, 50hz AC. Low voltage relays are available that can >>> handle this load with ease. The heater has a safety cutout built in >>> that should prevent boiling dry. >>> >>> I am aware that I can buy programmable motor driven timers very >>> cheaply, however they are limited in contact loading and I would >>> still need a relay for thermostat control. >>> >>> I thought this might make a nice PIC project. (Ducking) :-) >>> >> >> I have built and programmed a battery backed clock chip and a >> PIC16F628, attached to a RS-232 serial port. >> >> I am sure this can be modified to add a relay driver. >> >> Let me get the docs together sometime this weekend. >> >> don > >Thanks. I'll take any help I can get. Doesn't the hot water heater already have its own thermostat? Thus all you would need would be ON/OFF. Has anyone tallied up power consumption for such an ON/OFF, COOL-DOWN/WARM-UP type of operation? I have recirculating hot water here... big loop around the house and back to the water heater, pushed by a pump (Bosch). So you get instant on hot water. And also big heat losses in the underground loop. Since the water heater is well-insulated, I'm tempted to simply turn off the pump (with a timer) in off hours. ...Jim Thompson -- | James E.Thompson, CTO | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona 85048 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at http://www.analog-innovations.com | 1962 | I love to cook with wine. Sometimes I even put it in the food.
From: Jan Panteltje on 15 Oct 2009 11:07 On a sunny day (Thu, 15 Oct 2009 13:46:48 +0100) it happened baron <baron.nospam(a)linuxmaniac.nospam.net> wrote in <hb75jp$pc8$1(a)news.eternal-september.org>: >Hi Guys, > >I'm doing a hobby project to build a timer that can be used to turn on a >relay at a pre-determined time and then turn it off after a delay. >I also want to use a temperature sensor to override the relay but only >during the time period that is programmed. > >The idea is to turn on a water heater for a pre-determined period but to >use a thermostat to control the temperature during this period. > >I have suggested using a relay because the heater is rated at 3.5Kw >240/250 volts, 50hz AC. Low voltage relays are available that can >handle this load with ease. The heater has a safety cutout built in >that should prevent boiling dry. > >I am aware that I can buy programmable motor driven timers very cheaply, >however they are limited in contact loading and I would still need a >relay for thermostat control. > >I thought this might make a nice PIC project. (Ducking) :-) > >-- >Best Regards: > Baron. You can make a nice zero crossing power switch for 240 V with a MOC3041, driving a TIC263M triac, that one can do 25 A continuous (use heatink). The MOC3041 needs 15 mA, PIC can drive that, the MOC3042 10mA, and the MOC3043 needs only 5mA. Just did one last week. What sort of temp sensor are planning to use? Immersed in the water?
From: ChrisQ on 15 Oct 2009 11:19 Jan Panteltje wrote: > > You can make a nice zero crossing power switch for 240 V with a MOC3041, > driving a TIC263M triac, that one can do 25 A continuous (use heatink). > The MOC3041 needs 15 mA, PIC can drive that, the MOC3042 10mA, and the MOC3043 needs only 5mA. > Just did one last week. > What sort of temp sensor are planning to use? > Immersed in the water? > Alternatively, you could buy a 25a solid state relay with ttl compatable isolated control input for around $10 from Ebay. Did exactly that for a small lab heater a year or so ago, driven from a omron temp controller with a pt100 sensor. Don't actually need the 0.1 degree accuracy, but it works just fine... Regards, Chris
From: Martin Brown on 15 Oct 2009 11:38 baron wrote: > don <don> Inscribed thus: > >> baron wrote: >>> Hi Guys, >>> >>> I'm doing a hobby project to build a timer that can be used to turn >>> on a relay at a pre-determined time and then turn it off after a >>> delay. I also want to use a temperature sensor to override the relay >>> but only during the time period that is programmed. >>> >>> The idea is to turn on a water heater for a pre-determined period but >>> to use a thermostat to control the temperature during this period. Is this a domestic hot water heater to 60-70C stock or a boiler to 100C? Most electric immersion heaters usually have some thermostat built in. >>> I have suggested using a relay because the heater is rated at 3.5Kw >>> 240/250 volts, 50hz AC. Low voltage relays are available that can >>> handle this load with ease. The heater has a safety cutout built in >>> that should prevent boiling dry. >>> >>> I am aware that I can buy programmable motor driven timers very >>> cheaply, however they are limited in contact loading and I would >>> still need a relay for thermostat control. >>> >>> I thought this might make a nice PIC project. (Ducking) :-) You can buy CH timers that could be easily subverted to doing this job but check the contact ratings carefully. >>> >> I have built and programmed a battery backed clock chip and a >> PIC16F628, attached to a RS-232 serial port. >> >> I am sure this can be modified to add a relay driver. >> >> Let me get the docs together sometime this weekend. >> >> don > > Thanks. I'll take any help I can get. But you would probably be a *lot* better off wrapping 10" of fibreglass insulation around it (preferably the easy handle stuff in a plastic tube with a mirror finish on the outside), or the mirror finish bubble wrap if space is a problem. The water in the low loss hot tank in our Village Hall is still warm enough to be useful the next morning. Uninsulated it would go cold overnight. Regards, Martin Brown
From: baron on 15 Oct 2009 11:47
John Fields Inscribed thus: > On Thu, 15 Oct 2009 13:46:48 +0100, baron > <baron.nospam(a)linuxmaniac.nospam.net> wrote: > >>Hi Guys, >> >>I'm doing a hobby project to build a timer that can be used to turn on >>a relay at a pre-determined time and then turn it off after a delay. >>I also want to use a temperature sensor to override the relay but only >>during the time period that is programmed. >> >>The idea is to turn on a water heater for a pre-determined period but >>to use a thermostat to control the temperature during this period. >> >>I have suggested using a relay because the heater is rated at 3.5Kw >>240/250 volts, 50hz AC. Low voltage relays are available that can >>handle this load with ease. The heater has a safety cutout built in >>that should prevent boiling dry. >> >>I am aware that I can buy programmable motor driven timers very >>cheaply, however they are limited in contact loading and I would still >>need a relay for thermostat control. >> >>I thought this might make a nice PIC project. (Ducking) :-) > > --- > View in Courier: > > > +----------+ > | C O------|-----+ > | | NC | | > | > |<--O--|--+ | > +----------+ | | > TSTAT | | > | | > | | > +-------------|--|-----------------+ > | TIMER | | | > | +------+ | | | > AC>--+-| |----+ +---+ | > | | | NO | > | | [COIL]- - -| <--O > | | | | > AC>--+-| |-----------+ OC > | +------+ | > | [HEATER] > | | > +-----------------------------+ Thanks John. Your circuit is exactly what I had in mind. Its the timer and temperature sensing circuits that I want to create electronic versions of. The timer only needs to be accurate to a few minutes but needs to be settable for any span within a 24 hour period. This is why I thought that a PIC, with its programmability might be suitable. I have a rigid rod thermostat probe, but its way too coarse between its on/off switching points. Something around 12 - 18C between them. I would hope to obtain a couple of degrees of hysteresis over say 50 to 85C range. I was thinking diode or thermistor clamped to the bath to measure the temperature more accurately. -- Best Regards: Baron. |