Thermostat + Timer
in [Design]
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From: Jim Thompson on 15 Oct 2009 16:47 On Thu, 15 Oct 2009 16:34:56 -0400, Phil Hobbs <pcdhSpamMeSenseless(a)electrooptical.net> wrote: [snip] > >One cute way that works with plastic pipe is a MHD sensor: a magnetic >yoke around the pipe, look for a voltage drop perpendicular to B. >Happens only when the water is moving. Is softened city water sufficiently conductive? I earned the money needs beyond my scholarship by technicianing MHD... for the Woodson, Jackson, Melcher bunch in MIT Building 20 ;-) > >Or an acoustic sensor on the pipe--you can probably easily hear it via >the pipe when somebody turns on the tap. Possibly. > >Cheers > >Phil Hobbs ...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: mpm on 15 Oct 2009 19:52 On Oct 15, 8:46 am, baron <baron.nos...(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) :-) > > -- > Best Regards: > Baron. I guess I'm missing something here? Doesn't the water heater already have a thermostat? All you need to do is apply power. When the water heats up, the thermostat opens. And if it doesn't, then the eventually the safety switch (or valve) will. For setting the on/off window (within which the existing thermostat will operate), one off-the-shelf idea might be to use one of those fancy water sprinkler timers and a 240-volt high-current relay. Most give you 4 to 6 watering cycles per day, 7-day programming, etc.. Highly flexible. That said, I do understand the need to "experiment". A PIC with the appropriate precautions (as noted above) should work nicely. Good luck.
From: Martin Riddle on 15 Oct 2009 22:17 "baron" <baron.nospam(a)linuxmaniac.nospam.net> wrote in message news:hb7g70$jul$1(a)news.eternal-september.org... > 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. A 10K(a)25 NTC and a pull up resistor on the 0-5v ADC of a pic will give you around 0.2c resolution. A good stable Vref would help too, Like a LM4040 instead of the supply voltage. Cheers
From: baron on 16 Oct 2009 06:52 Martin Riddle Inscribed thus: > > > "baron" <baron.nospam(a)linuxmaniac.nospam.net> wrote in message > news:hb7g70$jul$1(a)news.eternal-september.org... >> 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. > > A 10K(a)25 NTC and a pull up resistor on the 0-5v ADC of a pic will > give you around 0.2c resolution. > A good stable Vref would help too, Like a LM4040 instead of the supply > voltage. > > Cheers Thats a useful bit of info. I have a number of 10k thermistors I could use. Thanks: -- Best Regards: Baron.
From: MooseFET on 16 Oct 2009 09:40
On Oct 15, 8:01 am, Jim Thompson <To-Email-Use-The-Envelope-I...(a)My- Web-Site.com> wrote: > On Thu, 15 Oct 2009 15:52:50 +0100, baron > > > > <baron.nos...(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? Yes, the math has been done many times by many people. Back when I was in high school, it was used as a math question. It is fairly easy to do the math if you assume that the losses from the water heater are linear with the amount its temperature is above room temperature. > 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. That will work. If your schedule is fairly constant, you can use a programmable timer to do the job. |