Refrigerator Thermostat Replacement

[conachair]

the thermostat on my danfoss bd35/ 101N0210 controller packed up yesterday. And I noticed the fan on the compressor has bit the dust as well.
So rather than order a new one I reckon a much better replacement can be made using a microprocessor with some temperature sensors for about the same money as a replacement.
I had an arduino uno r3 and some temp sensors onboard already so with some breadboard have a basic design up and running switching a transistor on and off at set temps. The temp sensors are great:
DS18B20 Waterproof | eBay
very accurate, cost next to nothing and you can run them off 2 wires paralleled into 1 data pin, each has its own address.

so the plan is...

Sensor top and bottom of the fridge and one on the compressor.
fit a new tiny little fan in the fridge and maybe 2 new ones on the compressor. Control the fans varispeed and the compressor on/off with the arduino depending on the temperatures, initial thoughts were to switch on at 5deg and off at 3deg. Maybe run the fan inside very slowly when the compressor isn't running and speed it up a bit when it is. Though I need to look a bit deeper into running brushless fans using pwm.
And probably fit a little display to show the various temperatures. overall power consumption should be minimal, the arduinos don't pull much anyway and I think underclocking and some other tweets can be brought down to very low power.

Any thoughts/comments/suggestions?

Ta

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[HappyMdRSailor]

I think you're making it more complicated than you need to....

Fully self contained PLC's with a display, 4-8 I/O, 4-20/TC in, relay out, can be had for $50...

Or something as simple as this, except find it in 12-24vdc, not 110...
FOR $17 !!!

Elitech 110V All-Purpose Temperature Controller+ Sensor 2 Relay Output Thermostat Stc-1000 - Hvac Controls - Amazon.com

[conachair]

Originally Posted by HappyMdRSailor

I think you're making it more complicated than you need to....

Fully self contained PLC's with a display, 4-8 I/O, 4-20/TC in, relay out, can be had for $50...

Or something as simple as this, except find it in 12-24vdc, not 110...
FOR $17 !!!

Elitech 110V All-Purpose Temperature Controller+ Sensor 2 Relay Output Thermostat Stc-1000 - Hvac Controls - Amazon.com

Or fully programmable thermostat and varispeeding fan controller, $16.99, just add a sensor or 2 and a couple of transistors.

http://www.amazon.com/SainSmart-ATme...ds=Arduino+uno
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[HappyMdRSailor]

Originally Posted by conachair

Or fully programmable thermostat and varispeeding fan controller, $16.99, just add a sensor or 2 and a couple of transistors.

Amazon.com: SainSmart UNO, ATmega328P *USB CABLE Included* for Arduino: Car Electronics
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I don't disagree... two different mousetraps for the same mouse!

For me, I would go the simpler "fully contained" controller... already enclosed, has digisplay...

Your method is definitely more tweakable and expandable....

[conachair]

Originally Posted by HappyMdRSailor

I don't disagree... two different mousetraps for the same mouse!

For me, I would go the simpler "fully contained" controller... already enclosed, has digisplay...

Your method is definitely more tweakable and expandable....

Up and running. With bits of wire all over the place just to see if it works
And a little data logger writing onto a sd card to see what's going on.

A box of ebay would certainly be easier but fiddling is such fun.

And hopefully I'll be able to trim down the power draw a bit by tweaking the fans.

Anyone know what sort of temperature the compressor would be happiest with? No point having fans running if they aren't needed.

Ta

[a64pilot]

You guy's are way over my head with all this digital electronics stuff, but as a HVAC tech, I can tell you, the cooler you make the condenser, the more efficient and long lived the compressor will be. I bet the small amp draw of the fan is more than offset by the increase of efficiency of a cooler condenser.
I would think maybe the best way to increase efficiency cheaply would be to ensure the fan has access to cool air and the compressor/condenser/fan assembly is not running in an enclosed area without good ventilation.
Of course I do not know the ambient temps of where you are, I'm geared more towards hotter weather way of thinking, if it's cold where you are, maybe you could do without the fan, but how much does the fan draw?

[belizesailor]

+1. The cooler/better ventilation the better. Typical fan draw is negligible and the BD35 controller has a control circuit for the fan so that it only cycles it on when compressor is running.

After box is chilled, a BD35, including fan, should be drawing about 4 AMPs or less.

If you are looking for more efficiency then looking at soft start and variable compressor speed options is going to be more effective (that and great box insulation). The Danfoss site has loads of technical info on their compressors and controllers.

[mikeguyver]

Wow !! Let's see how complicated we can make this and still have it work.....The more crap you have, the more it can ( and will ) screw up...
KISS.......... Keep It Simple Stupid !

Fire Away..!

[MarkJ]

You are lucky if you can fit a new sensor in your fridge.
Mine is built in and stuffed and unless I destroy the whole kitchen I cant replace it.

[conachair]

Originally Posted by belizesailor

+1. The cooler/better ventilation the better. Typical fan draw is negligible and the BD35 controller has a control circuit for the fan so that it only cycles it on when compressor is running.

After box is chilled, a BD35, including fan, should be drawing about 4 AMPs or less.

If you are looking for more efficiency then looking at soft start and variable compressor speed options is going to be more effective (that and great box insulation). The Danfoss site has loads of technical info on their compressors and controllers.

More like 2.5a or less for mine. But no actual data on the danfoss site for efficiency /temp of the compressor that I could find anyway.

How do you soft start the controller? The thermostat is basically just a switch, with added resistance to control speed as far as I can see.
There was some data about higher speeds drawing more Ah overall.
At least with a microprocessor in there it's easy to measure all these kinds of things.



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[conachair]

Originally Posted by MarkJ

You are lucky if you can fit a new sensor in your fridge.
Mine is built in and stuffed and unless I destroy the whole kitchen I cant replace it.

There will be a drill involved.......
Just need to get a little cable in there.

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[conachair]

Originally Posted by mikeguyver

Wow !! Let's see how complicated we can make this and still have it work.....The more crap you have, the more it can ( and will ) screw up...
KISS.......... Keep It Simple Stupid !

Fire Away..!

There original thermostat is probably more complex than what's going in there now.
And it broke.

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[Richard Kollmann]

There are facts you should be aware of when tampering with a Danfoss BD variable speed compressor performance. Compressor speed and under the right conditions energy consumption is controlled by regulating amperage in thermostat eloectrical circuit. When this circuit sees 5 milliamps of current compressor will run at 2,000 rpm and when amperage is reduced to 1 milliamp compressor will run at max rpm of 3,500 rpm.

Danfoss BD compressor systems depend or thermostat cycling hysteresis (cycle off differential) to reduce overpowering of evaporator and its ability to absorb enough heat from box to match compressor’s output.

When a thermostat is used to monitor and control refrigerator box temperature instead of evaporator temperature on these thermo unbalanced components daily system coefficient of performance is going to be low. Most box temperatures can tolerate an evaporator temperature swing of as much as 20 degrees F and still limit box temperature to a 2 degree temperature variance.

If you want to review the different boat refrigeration thermostats and their operation see Tech Tip #6 on My web site. I checked a freezer yesterday that some technician over three years ago over charge unit with refrigerant trying to lower box temperature. New owner for three years had to live with a 26 degree freezer evaporator when original problem was freezer had a refrigerator temperature range thermostat. I Lowered refrigerant volume, bypassed thermostat and evaporator temperature dropped with in minutes from +24 degrees to -10 F.

[mikeguyver]

even if the stat died, isn't the cable still there ?

[conachair]

Originally Posted by Richard Kollmann

[FONT=Times New Roman][SIZE=3]There are facts you s........

Thanks Richard, I've spent quite a bit of time on your site, it's very informative

On the points you raised...

Regarding the compressor speed, as far as I can make out the is a "one shot" alteration, add resistors in series to the thermostat and you can lower the current and thus increase the revs. Do you think there might be any benefit in dynamically controlling the compressor revs with regard to the ambient temperature ?

Do you think it's better to control the on/off cycles by measuring the evaporator plate temp alone?

Thanks again for taking an interest, I think despite being a complex area with the low cost of accurate microprocessors and temperature sen the sors it's well worth a look to see what you can do yourself. Even just for the fun of finding out?




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