As of summer 2010 the 1.5 year old thermostat on our Woods all-fridge fridge started failing and replacement parts were no longer available. So I revisited my old idea of building a thermostat - and ran across the Mt. Best site about a chest freezer to fridge conversion (linked from WikiPedia). An upright freezer was available for free - and so the dream of converting a freezer to a fridge became a project.
Here are links to the important sections below:
Much has been said about how much energy is lost by opening a fridge door. Air has low mass and so the energy loss is low. At issue, I believe, is that cooling works by cooling the coils, which then cool the air, which then cools the fridge/freezer contents. As modern fridges and freezers have a 50% duty cycle (running 1/2 of the time) opening the door at this critial time maximizes energy loss because the cooled air has not cooled the fridge contents. In the case of our upright freezer - there is a signif. mass in the shelves / coils and the compressor only runs for <10 minutes (not 20 to 40 min/hour) as our fridge does.
An attempt to quantify the energy lost by opening an upright fridge was done by comparing night time energy use to daytime energy use. Both intervals were 12 hours. There are caveats - ambient temperatures are higher in the daytime and during the daytime warm items are put into the fridge. We minimized putting warm items into the fridge during the 5 days of data collection. Ambient temperature is very important. Ambient can be 20C to 29C in the summer and during humid weather condensation will leach energy out of the fridge. Fridge temperature is about 5C - so comparing a fridge in the garage (ambient 16C - temperature difference 11C) to a fridge in the house (ambient 25C - temperature difference 20C) without correction for the temperature difference will introduce huge errors.
The ballpark estimate for energy lost to opening the fridge door is 1/3 of the total energy use - for an EnerStar 1993 all-fridge fridge in a household where the door is not opened needlessly or held open for long periods of time.
|Fridge||Energy Guide |
|Measured Energy Use |
as a fridge
|Est. Energy Use |
|Vestfrost SE255 |
|237kWh/yr as freezer||103Wh/day (claimed) |
|250Wh/day||Power draw only for a short period after power up.|
From Mt Best Chest Fridge
|Woods 2004 upright freezer |
|420kWh/yr as freezer |
427 kWh/yr over 4 days 19C ambient
|350Wh/day (128kWh/yr) |
|437Wh/day||The door is rarely opened, measured over 4 days|
|Woods 2001 chest freezer |
|351kWh/yr as freezer||305Wh/day |
|385Wh/day||measured over 4 days|
Claims of 1/15 the energy draw (Mt Best) I can not substantiate in any way!! My measurements indicate a 70% (chest fridge) to 60% (converted upright freezer) reduction in energy use, for a similar volume, can be easily achieved. In the Mt Best freezer conversion (PDF document) energy use, including power-up, during 24 hours was 103Wh with ambient temp between 15C and 21C, fridge temp 4C to 7C, compressor running 90 seconds per hour.
Note: They use a superinsulated Vestfrost SE255 freezer (237 kWh/yr, 239L, ~8 cu-ft.) Our appliances include an upright 2004 Woods freezer (17 cu-ft, 420 kWh/yr), Woods 2001 chest freezer (12 cu-ft, 384 kWh/yr) and our Woods 1993 fridge (17 cu-ft, 430 kWh/yr).
They claim 103 Wh/day running 90 sec or 2 min/hour. That is 36.5 kWh/yr or 0.15 of the rated energy use
(as a freezer). However, one would expect 1/3 of the energy use based upon the
temperature difference (delta-T 15C vs 40C); so I'm unsure how how a magical doubling of the efficiency happened.
The reported runtime of 90 seconds seems low (empty fridge?). Initial current draw is high until the
compressor builds up pressure and starts cooling - so very short run times are inefficient.
My run times are around 5 minutes with a 4C temperature swing on the 17 cu-ft
Woods upright and 1.5 bushles of apples inside. Given the Vestfrost is 1/2 the energy use that would imply 2.5 min. runtimes.
Note: runtimes would be determined by the mass inside the fridge. But more mass means longer run times, spaced out further in time.
Our previous upright converted freezer failed due to rusting thru of the coils due to condensation.
Canadian NRCan Energy Efficiency Ratings: http://oee.nrcan.gc.ca/pml-lmp/index.cfm?action=app.formHandler&nr=1#searchResults
Going shopping revealed that we had several options:
Gas Thermostat - FAIL
I have two fridge gas thermostats and one is far to short while the Ranco is a good 12cm short. Neither worked well. The Ranco could reach to touch the cooling coils and that resulted in the fridge cycling on for about 5 minutes and then off for one and then back on again. Every other location resulted in my pulling the power because the temperature was aproaching freezing. Info about Ranco can be found here: http://www.invensyscontrolseurope.com/rancoproduct.aspx?id=3108 The K50 and K14 are for fridges and various models turn on at between +2 and +12C and run until +5 to -18C
Electronic Thermostat - PROBLEMS
I mechanically attached the thermister using a nylon holddown to the metal plate covering the coils at the back of the fridge. Tweaking of the electronic circuit was necessary. Initially, as configured for our previous freezer conversion the runtime was around 60+ minutes! Overnight, with 30C ambient temp. the compressor comes one once every 6 hours for 30 minutes and my goal is to reduce that to 10 minutes every 2 hours.
Energy use, with the STC-1000 thermostat, set at 6.0C +/- 1.2C, without the door being opened, at 19.9C ambient from Sept 1 8:25am to Sept 7 1:40pm resulted in 0.574 kWh/day or 209 kWh/yr
Energy use, with the STC-1000 thermostat, set at 5.0C +/- 3C, without the door being opened, at 24C ambient from Aug 18 to Aug 23 over 5 1/3 days was 3.14 kWh or 0.589 kWh/day or 215 kWh/yr at 24C or an estimated 180 kWh/yr at 21C
Energy use, with my own electronic thermostat, and nobody opening the door, at 22C ambient, from 9:15am Wed Aug 7, 2013 to 2pm Mon Aug 12, was 2.15 kWh, 13:40:26 time running over 5 days 5 hours which gives: 413 Wh/day or 150 kWh/yr turning on and running for aprox 18 min. at a time.
Converting this freezer was impossible with the two refrigerator gas thermostats I had. An electronic thermostat works very well.
STC-1000 110V temperature controller is 0.8W power draw, awkward to program but otherwise looks excellent.
The defrost controller is mounted in the lower right and unplugging it cuts all power to the compressor as the neutral goes THRU the circuit. However, this means that by switching the neutral at this access point (via an electronic thermostat means that the door light works and the wiring is very simple). One simply needs the correct terminal spades to plug into the available connector.
Energy use from Sun July 21 to Monday July 28th was 0.534 kWh/day or 195 kWh/yr at aprox. 22C ambient or 185 kWh/yr at 21C ambient. This is not as good as our 9 year old converted upright freezer but it's 1/2 the energy use of the best all-fridge we could have bought for a saving of about 200 kWh/yr or about $28/yr (11c/kWh + 3c/kWh for Bullfrog green power).
Energy use data is in a table below.
Whirlpool EV181NZTQ auto df, 635 kWh/yr, 17.7 cf Fridge twin EL88TRRWQ 17.7 cf thermo W10424991 Fan only 3.9W with good power meter or 6W with EM100 startup 230W 165W at 3 min 26.9C 185W 10 min 20.4C 180W 36 min 5.7C 0.1 kWh or 167W average setup to turn on at 4.3C on at 5.9C - temp controller 2.2W DC wall wart
Caveat! The position of the thermostat bulb is critial in a freezer converted to a fridge!! In the conversions I've done the thermostat bulb MUST be touching the coldest coils. As those coils will drop to <-10C the thermal mass determines the
run-time of the compressor (how long it stays on). This MUST be less than 5 minutes.
The coldest shelf in a freezer will generate most of the condensation (it requires a drip tray or only hold items under it that can tolerate being dripped on) and it
drops to <-20C. Food sitting on that tray will be frozen. We use strips from plastic
election signs to space food above the shelf.
The temperature control will be finicky. On ours if it's all the way to the warm end the fridge will maintain 6C and cycle on for 5 minutes every 30 minutes to an hour.
Note that very short runtimes (compressor only on for 1 minute or less) are inefficient and long run times (>5 minutes) will start freezing contents in the fridge.
There are several options to controlling the temperature:
Electronic thermostats can allow precise control of the temperature and an adjustable temperature swing and often working for heating and cooling. Note that positioning a gas thermostate accurately can also result in low temperature swings.
My electronic control was based upon the Mt. Best design with modifications to use parts I had kicking around. As I wasn't about to trust my design in the long run I investigating using gas thermostats. The main issue with my thermostat was that while the fridge used an average of 15W of power, my thermostat required 5W extra!! Proper selection of the transformer for the thermostat is critical to reducing the power draw of the thermostat to <2W and possibly <1W.
Note: Oct 2011 Thanks to a reader who spent the money to do a chest freezer conversion I'm 100% convinced that to convert a chest freezer one must use an electronic thermostat. It isn't possible to locate the gas bulb near the coldest coils.
Note: Dec 2010 I've purchased a Ranco VC1 (fridge) thermostat for the upright freezer and it works well if the gas tube is correctly placed. I had to locate it partially on the 2nd coil/shelf from the top and there it runs the compressor 4 to 5 min. max. The Ranco VF3 thermostat is designed for freezers and both were $33 each.
Note: Nov 2010 I moved the (failing) gas thermostat from our fridge to our upright freezer. It almost fit physically and the temp knob did not match but it worked well AFTER I re-located the end of the tube. It was originally the 3rd cooling coils from the top of the freezer and the top 2 get the coldest and have nearly all of the condensation. The thermostat would seeminly never shut off on the 3rd tray (and things were being frozen) so I moved it to the 2nd cooling coil and then it worked well keeping the freezer at 7C (that's about all that thermostat was doing in our fridge).
Gas thermostats have a larger temperature swing and a crude temperature control.
Note that pretty well any gas thermostat will work to convert a freezer to a fridge; but it must be mountable and have the necessary electrical connections.
It can be hard trying to find energy efficient appliances. Avoid auto-defrost models and upright freezers and always look at the EnerGuide ratings. Below are some ratings for fridges I have and others for comparison.
Why does a converted freezer use 40% of the power draw of a fridge?
|Whirlpool 2013 upright 17cf freezer |
with STC-1000 elec. thermostat.
|Date||Sep 1 to 7th||Fridge door not opened|
|Temperature||19.9C avg, 6.0C - 7.2C fridge|
|Average Power use||574Wh/day|
|Average Power use||413Wh/day or 150 kWh/yr||22C ambient, my custom thermostat - high temp. swing (Aug 7 2013)|
|Total Power use||4.14kWh in 7 days 5.25 hours|
|Woods 2004 upright 17cf freezer |
with std. gas fridge thermostat.
|Date||Dec 7 to 14th|
|Temperature||18C night, 20C daytime, 6C fridge|
|Average Day Power use||331Wh/day|| std. dev. 39|
7.5% higher than nighttime
|Average Night Power use||308Wh/day||std. dev. 34|
|Average Power use||320Wh/day or 13W|
|Total Power use||3.02kWh in 21:10:22 over > 10 days||143W average running power|
|Converted Woods 2004 upright 17cf freezer |
with electronic thermostat
|Power draw while running||151W (average)||1.51kWh in 10:00:56 over 4.06days|
|Runtime per day||2.46h/day|
|Average Power use||15W|
|Door thickness||2.25" thick||Only 30% better insulated than a fridge!|
|Woods 1993 17cf fridge|
|Power draw while running||106W (average)||3.79kWh in 35:40;41 over 4.06days|
|Runtime per day||8.78h/day|
|Average Power use||39W|
|Door thickness||1.75" thick|
Here is data from the '04 Woods upright 17 cu-ft freezer converted to a fridge and the '93 Woods 17 cu-ft all Fridge.
|Converted Freezer - 2004 Woods Upright 17 cu-ft|
|Energy Use |
|470||1.62 days||291Wh/day avg, 16C ambient average, early Sept 2010|
|Energy Use |
|Energy Use |
|Time Period||Ambient |
|377||926||1 day||22.5C||Conv. Freezer not being opened, fridge in use|
|352||4:56:44 runtime||889||17:43:10 runtime||2 days||22C||Conv. Freezer 40% of Fridge energy use|
The freezer is rated 1.1kWh/day so I anticipate 1.1/2.5 = 440Wh/day but it used 305Wh/day over 4 days at 19C ambient!!
Note that the 1" foam sheet on the freezer door was removed for this test. When the chest freezer is a freezer the temperature drops like this: 19.5C ambient, 11.5C under 1" R5 foam, -19.2C inside freezer (-17.6 based on thermometer). That means that the freezer door is: (11.5 + 19.2) * 5 / (19.5 - 11.5) = R19
We've decided to upgrade our fridge - by converting an upright freezer to a fridge and selling the fridge.
The options are:
|Fridge Option||Energy Use||Cost||Comments|
|Convert a new chest freezer||est 360/3 = 120 kWh/yr||$600||This idea is a non-starter with my partner.|
|Convert a new upright freezer||est 480 / 3 = 160 kWh/yr||$550 Frigidair FFU1M7HW||All? models have cooling coils as shelves (condensation issue), a fan to coil the heat rejection coils (noise) and many are auto-defrost with electronic thermostats (expensive $800+, energy gobblers 615kWh/yr, can be converted to a fridge??)|
|Convert a freecycled upright freezer||125 kWh/yr||$50 (home made thermostat)||I've improved the homebrew thermostat.|
|Convert a Kijiji upright freezer||est 125 kWh/yr||$350 (typical price)||Never is info provided as to age, EnerGuide ratings - lots of running around.|
|New fridge||320+ kWh/yr||$800|
|Keep the existing 17 yr old fridge||420 kWh/yr||$50 (thermostat)||It's thermostat is failing|
Some issue which we have to deal with:
Wood apparently made a convertable fridge-freezer back in the 1980's (?). What likely killed it was the sensitivity of placing the thermostat. When converting a freezer the position of the gas thermostat bulb is very critical and one then has very little control over the actual temperature. old Woods thermostat Ranco K59 P489190-22-5P24 Amre Cambridge 519-650-2673 - Can match fridge thermostat with another - stock of Johnson: A419 $148 A19 $130 Bills Appliance Repair <$100 to replace thermostat 578-9652 351 Lancaster W (can't find it!) Reliable Parts 935 Frederick 519-570-0559 - No woods parts Waterloo Appliance 519-884-2507 Home Brew Thermostat Tyco relay R10-E1y2 V185 2A coil 185 ohm, 12V aprox 70mA Need relay G6RN-1A DC12V 18mA, 650 ohm rated 8A 250V MOV across relay load? diode on coil side? Solid state relay such as IDEC RSSDN 10A has 20mA max. off-state leakage according to spec sheet - In reality the leakage is ??? They are $24 vs $4 for Omron G6RN relay
Here is the gathered data for determining that daytime energy use is 2/3 of the total energy and opening the fridge door is aprox. 1/3 of the total energy use. This was repeated at 20C and the door energy loss was only 7% (average 403Wh/night, 457Wh/day).
Not all intervals were exactly 12 hours - so the energy use was calculated as Wh/h to allow direct comparison. Note that some oddball daytime calculations were dropped from the average calculation (1.27 Wh/h due to the door being left open!)
Date Time kWh Wh/h Night Daytime Ambient Wh/h Wh/h Temp (C) Aug 28, 2010 19pm 2.67 Aug 29, 2010 7am 3.11 37 night 0.44 Aug 29, 2010 19pm 3.93 68 day 0.82 28 set thermostat slightly cooler Aug 30, 2010 6am 4.36 39 night 0.43 30 Aug 30, 2010 21pm 5.25 73 day 0.89 30 Aug 31, 2010 6am 5.63 42 night 0.38 30 Aug 31, 2010 18pm 6.45 68 day 0.82 30 door left open after lunch Aug 31, 2010 22pm 6.9 113 day 0.45 1.27 30 Sep 1, 2010 6am 7.17 34 night 0.27 25 Sep 1, 2010 19pm 8.1 120 0.93 31 Sep 2, 2010 6am 8.59 45 night 0.49 25 Sep 2, 2010 18pm 9.18 49 0.59 25 Sep 3, 2010 6am 9.7 43 night 0.52 25 Sep 3, 2010 20pm 10.41 51 0.71 25 Sep 4, 2010 6am 10.8 39 night 0.39 22 Average 0.43 0.79 Wh/h per time period Fridge Energy Star rated 432kWh/yr = 1.2kWh/day, past measurements: 0.77kWh/day in winter, 1.3kWh summer Fridge vs converted Freezer: Freezer Fridge Energy Use Per Period Ambient Comments Date Time kWh Temp(C) kWh/day kWh kWh/day Freezer Fridge Ratio Period Temp (C) Sep 21, 2010 16:00:00 0 Sep 21, 2010 19:30:00 0.05 4.4 0.343 0.13 0.891 0.343 0.891 38% 24.0 Freezer not used yet, full of apples Sep 22, 2010 06:00:00 0.23 6.9 0.411 0.54 0.937 0.411 0.937 44% Night 22.4 Sep 22, 2010 19:00:00 0.42 6.8 0.378 1.09 0.980 0.351 1.015 35% Day 23.1 added MOV, 111W running, ~600W startup Sep 23, 2010 05:50:00 0.57 6.8 0.363 1.42 0.902 0.332 0.731 45% Night 18.6 Sep 23, 2010 18:30:00 0.74 4.6 0.352 1.87 0.889 0.322 0.853 38% Day 23 Sep 24, 2010 06:00:00 0.9 4.8 0.349 2.28 0.882 0.334 0.856 39% Night 22.1 Sep 24, 2010 20:30:00 1.14 6.4 0.358 2.82 0.884 0.397 0.894 44% Day 23.8 Warm day Sep 25, 2010 06:30:00 1.29 7.2 0.359 3.17 0.879 0.360 0.840 43% Night 3.458 Total days test running Sep 25, 2010 12:45:00 1.4 4.7 0.363 3.43 0.887 0.422 0.998 42% 21.9 Fridge power meter hung . no data missed?? Sep 25, 2010 21:00:00 1.51 6.8 0.359 3.79 0.901 0.364 1.047 35% 21.4 4.063 Fridge little used Sep 26, 2010 06:45:00 1.64 5.0 0.356 0.31 0.888 0.320 0.763 42% Night 20.4 Sep 26, 2010 19:00:00 1.79 4.7 0.349 0.7 0.876 0.294 0.764 38% 19.6 Freezer just ran Sep 27, 2010 05:30:00 1.92 5.3 0.345 1.01 0.862 0.297 0.709 42% Night 19.5 Sep 30, 2010 The converted freezer has now become our fridge. Note: the day and night use at about 21C (reported as kWh/day equivalent) is: Freezer: 346 and 342 (day and night use the same) Fridge: 915 and 806 (day use 6.5% higher)
Note: Sept 16, 2013. We've had a case where the coil in the fridge froze up and ice started to form on the metal plate covering it. To deal with this I moved the STC-1000 temperature sensor from the side wall to the top right of the metal plate at the back. Some research indicates that the twin, refigerator, model has a defroster. Trying to get the lower part of the fridge < 6C requires a defroster! In order to defrost ours I've broken out the power for the fan so that I can turn the fan on. Then by turning off the fridge and leaving the fan on for 6 hours the fridge is defrosted.
Note: Sept 12, 2013. Temperature measurements with Thermochrons have revealed that while the lower part of the fridge maintains 6C - the middle and up is around 10C to 12C. Also when the compressor turns off it can get quite cold at the bottom as the cold from the coil sinks. A solution would be to make sure that the coil fan stays on for an extra minute after the compressor has run. This was not a problem when the coils were at the top of the fridge. But trying to maintain an constant temperature thruout a volume which is 2x larger than most fridges is a challenge. We now keep foods that require cold lowest.
Energy use, with the STC-1000 thermostat, set at 5C +/- 3C, without the door being opened at 24C ambient from Aug 18 to Aug 23 over 5 1/3 days was 3.14 kWh or 0.589 kWh/day or 215 kWh/yr at 24C or an estimated 180 kWh/yr at 21C
Energy use, with my own electronic thermostat, and nobody opening the door at 22C ambient, from 9:15am Wed Aug 7, 2013 to 2pm Mon Aug 12, was 2.15 kWh, 13:40:26 time running over 5 days 5 hours which gives: 413 Wh/day or 150 kWh/yr turning on and running for aprox 18 min. at a time.
Energy use from Sun July 21 to Monday July 28th was 0.534 kWh/day or 195 kWh/yr at aprox. 22C ambient or 185 kWh/yr at 21C ambient.
My [analog] electronic temperature controller uses 2.2W and runs at 12V using a DC wall blob.
The freezer defrost timer was 2W according to the EM100 power meter and 0.5W according to the more accurate Belkin.
Here is the energy use data for our Whirlpool freezer converted to a fridge:
Power use is measured with an EM100 Power Meter.
|Date||Time||Fridge Temp. |
|Energy Use |
|Tues July 16||5:06pm||gave up on gas thermostat, switched to electronic thermostat|
|5:16 turn on, 5:17 turn off||thermister touch coils at back|
|7:44pm||2.7||0.27||1:28:48||just turned off|
|9:10pm||5.4||30C||still has not turned on|
|Wed July 17||3:30pm||6.0||0.96||5:26:10|
|3:48pm||6.2C||just turned on, changed feedback resistors:|
from 200k over 220k to 200k over 470k
|5:44pm||3.2||1.31||7:22:34||30C||door opened 2 or 3 times|
|Thurs July 18||5:30am||6.3||1.61||9:09:50||30C||0.3kWh/12 hours = 220 kWh/yr at 30C|
|3:40pm||6.6||1.98||11:03:29||turned on when door opened|
|4:15pm||3.5||2.06||11:42:22||it ran 39 minutes!! Feedback resistor changed|
from 200k over 470k to 200k over 1M
|8:30pm||5.9||just turned on|
|Friday July 19||5:30am||5.3||2.34||13:17:22|
|1:15pm||3.8||2.64||15:01:06||recently turned off|
|2:40pm||4.1||2.71||15:21:06||Feedback resistors changed to 200k and 2.7M ohm|
|7:00pm||6.4||2.93||16:37:50||27C||storm front moved in - temp drop to 27C|
|7:36pm||5.3||2.97||16:53:28||fridge just ran|
|Sat July 20||6:15am||5.0||3.13||17:49:10||24.5C||Fridge recently turned off.|
|Sun July 21||5:30am||5.9||3.58||20:31:50||21C|
|Mon July 22||5:00am||5.4||4.09||23:29:52|
|Tues July 23||5:30am||5.1C||4.69||27:08:06|
|Wed July 24||5:30am||5.6||5.33||30:56:48||21.3C|
|5:30pm||6.0||5.55||32:16:40||22.8C||turn-on at 6.2C 6:20pm|
|Thur July 25||5:30am||-||5.76||33:33:52||just turned on|
|Fri July 26||5:50am||6.6||6.20||36:15:00||20.3||just turned on|
|Sat July 27||5:30am||5.3||6.74||39:31:50|
|Sun July 28||6:00am||6.0||7.38||22.1C||moved thermo. to top - 3.2C to 7.7C swing - so top is 2C warmer|
|Mon July 28||5:30am||7.3||7.85||46:19:56||19.9C|
How does the fridge temperature change in different parts of the fridge? This is for the STC-1000 thermostat set to 5C +/- 3C so it turns on a 8C and runs until 5C.
The first 2 columns were done at one point in time and the 2nd two (middle and top temperature) were done later.
Summary: The middle temperature swings from 5 to 8C while the bottom swings from 2.8 to 5.0C and the top swings from 4.2 to 8.1C. So the top is around 3C warmer than the top.
|Middle Temp.||Bottom Temp.|| ||Middle Temp||Top Temp.|
|7.8C||4.1|| ||7.8||6.5C (just put in)|
|7.2||5.0||8.0||8.1 (turned on)|
|5.0||4.1 (turned off)||7.2||6.6|
|6.6||2.8 (door opened)||5.3||4.5|
|7.0||3.1||5.0||4.2 (turned off)|
W10474749 FOSHAN FSTB 62" capillary tube - stock freezer thermostat for the Whirlpool FOSHAN KDF8A 4-30733-012 FSTB 31" capillary - fridge thermostat, way too short to get near the cold coils VC1 K50-P1110 09-51-27S Ranco 47" 120cm capillary VaRifix (specs turns on at -2.5C, off at -8C to on -16C off at -25C) - fridge thermostat used on previous upright freezer and is about 15" too shortComparitor Voltage and Trip Points
For the simple op-amp temperature controller the feedback resistors have a voltage swing which equates to a temperature swing. This needs to be adjusted based upon the freezer. Initially the resistor values were the ones used for a chest freezer and manual defrost upright freezer conversion - but they resulted in runtimes that were too long and temperature swings that were too large. Pin 2: 6.47V 3.0C, 3.60V 4.4C TRIP 3.65V for 200k over 220k feedback resistors Pin 2 is the amplified, and offset, thermister reading. Pin 3 is the feed loop with an offset to prevent quick on-off cycling Fridge Pin Fridge on? Feedback Vcc Temp 2 3 off? Resistors 6.0C 4.04V 5.90V on 200k / 470k 10.43V, 1.5V swing, > 40 min runtime 5.2 4.62 - running 3.5 6.28 4.38 off 3.0 7.12 4.77 off 200k / 1M 850mV swing, 30 min runtime 4.3 6.02 4.77 off 5.9 4.52 5.61 on 4.1 6.03 5.07 off 200k / 2M7 320mV swing, 15 min runtime ~6.1 4.79 5.40 on Fridge Initial turn-on startup 230W, running time, power draw and temperature 165W 3 min 26.9C 185W 10 min 20.4C 180W 36 min 5.7C 0.1kWh energy used
Trying to compare the STC-1000 temperature controller with my home-brew one gives the following data. The STC-1000 thermistor is in the middle back of the fridge, just above the metal plate housing the coil and fan. This area has a larger temperature swing and putting the thermister inside the coil housing resulted in large temperature swings as that area gets very cold while running. My temperature controller thermostat is on the bottom, back side wall.
5:05pm turn on at 8.7C and runs till 3.6C 5:45pm 7.0C (drop of 3.4C in 40 min or 5.1c /hour 7:01pm 2.0C (compressor just turned off) 7:34pm 6.4C 7:50pm 7.1C 8:05pm 7.7C 8:07pm compressor turned on 8:08pm 3.9C 8:42 pm 6.1C This data reveals that the temperature probe for the STC-1000 sees a relatively quick temperature rise after the compressor turns off and then a constant rise in temperature over time. 4:48am 7.8 (open door and it rises to 8.5C) compressor turns on 4:50am 8.2C 4:52am 8.0C 4:57am 6.4C 5:01am 5.0C 5:01am 4.8C compressor turns off 5:06am 5.3C 5:10am 5.6C 5:15am 5.9C 5:24am 6.5C 5:34am 7.0C