EEVblog Electronics Community Forum
Electronics => Projects, Designs, and Technical Stuff => Topic started by: calzap on December 30, 2014, 05:15:21 am
-
I have a Honeywell thermostat in my house that controls the gas furnace and AC. Model is T8112C1023; made 40th week of 1998 and still sold today. Its program and display are maintained by a couple of AA batteries. We normally heat with wood and haven't used the gas furnace this season. We were going away for a while and wanted to use the gas furnace to keep the house from getting too cold. The display was showing a few, dim garbage characters; so I replaced the batteries. Display became quite legible and showed "BATT LO". I checked the batteries, which were new, and they tested at 1.5+ V. Put them back, still no joy. Unplugged and re-plugged furnace, still no joy. Pressed various buttons, rotated batteries, sill nothing. Finally, I was reduced to reading the manual. Way in the back, it says for display problems, insert the batteries backward for at least 5 seconds to reset the thermostat, then replace them in the correct direction.
I did it; it worked. What kind of design is this? Is reversing the batteries discharging a cap than is retaining some screwed-up settings in memory?
Mike in California
-
You got it, I make it a 1000uF@6.3v. Looks like they geared it up with a resistor so if you put the batteries in backwards you don't run much current through the capacitor, which would harm it.
How do I know? I have a board sitting here... From the late 90s. So I guess if you need some spare parts? :)