My thermostat runs on batteries but it has an LCD, similar to (http://smile.amazon.com/Honeywell-RTH2300B1012-5-2-Programmable-Thermostat/dp/B007BHLUWM/ref=sr_1_1?ie=UTF8&qid=1425045609&sr=8-1&keywords=thermostat) but I thought any type of display (LCD) would suck up the battery. Am I missing something, is this a special type of display?
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1\$\begingroup\$ It's the backlight that sucks the power. If your thermostat has a backlight at all, you'll see that it is very low power and is only activated for a few seconds at a time. \$\endgroup\$– Dave TweedFeb 27, 2015 at 16:02
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LCDs (liquid crystal displays) require very little power to operate. The liquid crystal looks mostly like a small capacitor. To keep the liquid crystal in the dark state, a few volts of AC at a few kHz or so must be applied across this capacitor. That's not a lot of power.
This is exactly why you see LCD readouts and displays in low power battery operated devices to often. Think of a wristwatch. Those run for a year or more on a tiny battery.
answered Feb 27, 2015 at 14:07
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\$\begingroup\$ I have a wristwatch from the late 90s, it is still running... (on the same battery) \$\endgroup\$– PlasmaHHFeb 27, 2015 at 14:19
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\$\begingroup\$ Also, whenever the thermostat is not demanding heat (or cooling), it can use the open-circuit voltage across the contacts to supplement the battery power. Since we're only talking about microamps, the leakage current is not enough to activate the HVAC. \$\endgroup\$ Feb 27, 2015 at 15:57
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\$\begingroup\$ @Ryan: Huh, what? Nobody said anything about bad or good, what the parameters of OK versus not OK are, or even what that would apply to. Your question makes no sense whatsoever. \$\endgroup\$ Feb 27, 2015 at 16:44