Fixing a fridge WITHOUT using an Arduino :-)
Posted by Mark
on 29 Jan, 2017
If your fridge breaks, chances are that you don't have an Arduino, a Dallas temperature sensor, and relays lying around. At least I didn't when mine broke and unfortunately, it was a very hot summer day when it did. So, here's the most simple fix:
Use a timer switch.
Many people might have one already to switch on Christmas lighting in the afternoon and switch it off in the mornings. These timers work quite well to control your fridge as well. As explained in the blog post on using an Arduino to control your fridge, the basic operation of a fridge is to switch on cooling for half an hour or so and then switch it off for the next hour. The proportion of "on" time versus "off" time gives to the actual cooling rate. It turns out that once you found out a good proportion, you won't have to adjust it for weeks, unless the weather really changes (i.e., temperatures drop or rise a lot).
At first, I thought, a digital timer would be great, because I found a cheap model that let me set up to 20 on/off per day exact to the minute. However, it showed that this fine granularity is not needed. Instead, a analogue timer is much easier to adjust, although it might only allow to set on/off for 20 minutes slots.
There are only few things to take care off:
- Once you switched the fridge off, always keep it off for more than 15 minutes.
This is to let the pressures readjust so that the compressor can start more easily.
In case you switch the compressor on after a too short period, it might not be able
to start and block (and in worst case get damaged). So, make off time slots longer
than 15 minutes - always.
- Once you switched the fridge on, let it run for more than 10 minutes. The first minutes
will not show any cooling effect at all, as the compressor first needs to build up
pressure. So, make on time slots longer than 10 minutes ;-)
- The physical system has a large time delay: First, the compressor needs to work some
time until the cooling parts inside the fridge really get cool. Then, slowly, air will
cool down and convection starts, letting the cold air sink to the bottom of the fridge
and bringing warmer air up to the cooling elements. But it will take even more time for
the cold air to cool the actual food that you stored in your fridge, in turn heating
up the air again, making it travel upwards to the cooling elements and so on. As a
result, you will notice, that after you start the compressor, it will take about 10 to
15 minutes where the fridge only gets warmer, until finally, your temperature sensor
'sees' that there is any effect. After a couple of minutes, the decrease in
temperature will reach a steady rate. Then, after you stopped the compressor,
temperatures will keep decreasing for a further 10 minutes or so, as the convection
process is still functional as long as the cooling parts are cooler than the air inside
the fridge.
- Similar physics apply when you open and close the fridge door: This will rapidly
increase the temperature inside because you swap cold air with warm air. However, even
if the compressor is not switched on, convection and heat dissipation from air to
stored (cold) food will let the temperature gradually fall again. Of course, storing
warm food will do the opposite: it will let the temperature rise slowly, because the
air heats up.
Sounds complicated but in practice it's pretty simple. First, put on the compressor constantly, until your (recently broken) fridge reaches the target temperature. Then install the timer and set it to 50 % on 50 % off. This could be one hour on, one hour off, one hour on, ..., and see where this gets you. We found that waiting for half a day gives you some idea on which direction you should take: more cooling or less. After three days, we had a fridge that worked pretty well, while I was waiting for the Arduino to arrive :-)
SUSTAINABILITY
TINKERING
ARDUINO