A Basic Electronics Question

[HBarlow]

A year or so ago TDR member "klenger" wrote an interesting post and referred us to his website for instructions on retrofitting a house type 24 volt digital thermostat into an RV to replace the crude bi-metal spring type furnace thermostats that allow wide swings in temperature. I liked the idea and considered installing one in my fifth wheel. The only drawback to klenger's retrofit is the house type digital thermostat requires 24 volts dc which everyone knows is not readily available in an RV. Klenger's solution was to use a small 24 volt appliance battery to power the digital thermostat.



I would like to install a digital thermostat in my trailer also but have never done it because I've been reluctant to use a small 24 volt battery that may expire unexpectedly in the middle of a very cold night.



It would be relatively easy to use the same source as a house, an ac transformer and filter device, but that would only work when the RV has access to a 115 volt power source.



Recently I was checking the electolyte levels in my trailer batteries and got what is probably a goofy idea. Modern RVs are equipped with two 12 volt batteries connected in parallel. Is it electrically possible to "create" a 24 volt dc source by tapping the positive and negative terminals of each RV battery, routing the two 12 volt sources to a terminal board via one or more isolation diodes, then connecting the two sources in series? I know, the two RV batteries are already connected in parallel so the two batteries are electrically one 12 volt battery with greater current capacity.



I'm grinning as I write this because I know some will read this and roll their eyes thinking what kind of dummy would ask such a question.



Would someone with an EE degree or greater knowledge and understanding of electricity and electronics than I have please tell me if this is possible?



Harvey

 

[champspa]

no ee degree here but hvac yes , can't you use one of your batteries to power a inverter of small voltage and then go about your transformer and a digital thermostat as they are a lot more reliable in the application where you might not be level.

 

[ATClark]

I purchased a 12 volt 'digital' thermostat at my local Lowes store. Love the ability to actually set the temperature I wanted and the ability to define the tolerable temperature deadband. The issue with them is that they will not work when the batteries are drawn down, as I discovered in Glacier National Park on a cold night (no hookups in those campsites), even though I had enough power to make the heater run if I manually shorted out the thermostat contacts.

 

[klenger]

Here's the page that Harvey was referring to.

http://www.klenger.net/arctic-fox/hunter-tstat/index.html



The installation DOES NOT NEED any additional battery. The T-stat is powered from internal batteries in the unit.



I reciently installled a Honeywell TH5220D Non-Programmable 2-Stage Heat/Cool Digital Thermostat that can be powered from it's internal battery or from the 12 V RV power. I really like the way this one works because I can use the 2-stage feature to run an electric heater and the furnace. On the cooling side, I use the 2-stage feature to control the AC fan speed. Details can be seen at http://www.klenger.net/arctic-fox/TH5220D_T-Stat/index.html

 

[Gary - K7GLD]

YUP - I replaced the cheap quality T-stat in our 5er with a $24 digital from Walmart - has it's own internal battery, and works GREAT!



Thanks Klenger - I got the info from you as I recall...

 

[TowPro]

I have done several. The thermostat runs on its own internal batteries, which last at least all summer.



I even converted my pickup camper's overhead AC (the type that has controls on the roof piece) to run off one. I just installed a 12 volt relay into the AC unit to open and close the 120v circuit to the compressor, then ran 2 small wires to the thermostat and had it switching 12V to this relay.

 

[HBarlow]

Thanks for several informative replies. Klenger, I guess I didn't understand or maybe understood at the time and forgot that your earlier modification used its own internal batteries. I may have been doubtful of the life expectancy of internal batteries for winter season use other than an occasional weekend of camping.



It sounds like the simplest and cheapest solution is to purchase and install one of the 12 vdc digital thermostats obtained from Lowe's Home Improvement or Wal-Mart as mentioned above. I was not aware that a digital thermostat that operates on 12 vdc was available.



How long will the internal batteries last during winter heating daily use? Does anyone know?



Harvey

 

[Gary - K7GLD]

I was not aware that a digital thermostat that operates on 12 vdc was available.

I forget whether the internal battery is a AA cell, or a 9 volt type - mine last about a year, and I doubt it makes any significant difference whether the T-stat is actually being used or not as far a service life is concerned. Mine is the "Hunter" brand, and the display is always active, displaying current temp - so I imagine the battery drain is pretty constant at all times.



The service life is quite adequate for most users - and more specifically, no need - or advantage - in tapping into the trailer wiring or batteries.

 

[HBarlow]

Gary,



Thanks. A year of service sounds perfectly adequate to me also. I agree . . . if a small replaceable internal battery would last a year, or even six months, it would not be practical or necessary to try to obtain 24 vdc from house batteries.



I was thinking in terms of heavy use and heavy current drain on the thermostat battery during winter heating months. It had not registered in my aging brain that the thermostat doesn't really work any harder or draw significantly more current when the furnace blower is called upon. As you stated, the display is always on and the current draw is basically the same whether the trailer is in use or stored in the backyard.



I'll check Wal-Mart and my local Lowes store for a Hunter 12 vdc digital thermostat.



Harvey

 

[HBarlow]

champspa:



Sorry, I didn't see your reply to my post so didn't reply to your suggestion. I had not thought of your idea of powering an invertor from the RV batteries then using a transformer and filter to rectify and step the power down to 24 vdc. That would work but the hardware required might be a little too expensive and complicated just to allow me to use a digital thermostat.



Harvey

 

[TowPro]

There is no reason to worry about supplying 24v, or even buying a 12v thermostat. All of the less expensive ones I have seen run on 2 AA batteries. the one in my pickup camper had batteries in it that were 15 months old when I sold it last week, and it still worked fine. I just keep a spair set of batteries in the cabinet in case they die.



Most of the digital thermostats are designed so you can change the batteries (if you do it quick) and even the clock stays set!



try one, they go for less then $20. 00 at places like walmart.



I even modify them. I add a small SPST switch to the bottom so I can switch the AC fan from high to low.

 

[HBarlow]

Update:



Well, I had to go to Wal-Mart for something else today so I went to the aisle with thermostats and found probably exactly the model Gary described. It is a Hunter, price was like $19. 47, and I bought one. Info on the package states the thermostat is intended for 24 vdc applications but that it will also work with 750 milliamp applications. I'll install it when I return to Indiana next week.



I wonder, is operation in a 12 vdc system the reason for the situation described by ATClark above who found that his wouldn't work when battery voltage got low.