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View Full Version : OT: Black & Decker Thermoelectric (Peltier) refrigerator repair / upgrade



PStechPaul
06-29-2015, 02:38 AM
I just bought one of these little 1.7 cu. ft. fridges for $12 on Craig's List. Only one of the two elements is working and it only cools to 20F below ambient. According to the info I found it is designed to cool to 40F below ambient, so that seems consistent with one element not working. Otherwise it is in good shape and I was surprised how light it is. I plan to use it for Kabin Kamping at the KOA and actually it may work well enough as is if the A/C is holding at 68F and 48F is OK for drinks and food. But it would be nice to have it cool as intended, or possibly even colder to make ice.

It looks like this, but it's white. The CL post is gone and I didn't save the pictures, and I haven't taken pictures of it yet:

http://i.walmartimages.com/i/p/00/89/66/03/00/0089660300249_100X100.jpg http://www.powersalez.com//images/Black_and_Decker_BNA17_Black.jpg

It was sold by Walmart and the reviews were helpful:
http://reviews.walmart.com/1336/11016401/black-decker-1-7-cu-ft-compact-refrigerator-nectarine-reviews/reviews.htm

Here's a little info on what might be wrong:
http://kitchen.manualsonline.com/support/black-decker/refrigerator/ive-got-a-model-bna17-black--decker-thermoelectr-5044798
http://www.fixya.com/support/t6723536-black_amp_decker_bna17b

Here are some specifications:
http://www.2cis.com/black-decker-bna17-nucool-compact-fridge-black-p-4058.html

And I found the manual:
http://data.manualslib.com/pdf3/55/5422/542181-black__decker/bna17.pdf?0186d3a1dbab9f09fe4e59c10124c93c&take=binary (8MB)

It's probably easy enough to fix, and I have some Peltier units I bought a while ago that may even be more powerful. I was wondering if perhaps I could make a freezer using another Peltier unit with an insulated box inside the refrigerator, to drop the temperature another 20F or more, but I don't have much experience with thermoelectrics. If anyone has any experience with this refrigerator or Peltiers in general, any advice will be appreciated. TIA.

macona
06-29-2015, 04:58 AM
I have messed around with them. You just cant put in a "more powerful" peltier and expect it to get colder, you have to make sure you can keep the hot side as cool as possible. If the existing heat sink is too small then you may just sit there wasting power and maybe not get nearly as cool as it would with the right peltier. Just measure the existing peltier's temp and current and you should be able to match it.

PStechPaul
06-29-2015, 12:03 PM
That's why I thought adding another Peltier unit inside the fridge might provide another 20F or so of cooling to make a freezer. I think one of the difficulties with these units is that the thermocouples and the interconnecting conductors are thermally conductive as well, so you have to get rid of the heat on the hot side as efficiently as possible. Possibly the CFM of the fans on the outside heatsink could be increased - and I'm not sure if there are outside fans.

BTW, I found a USB powered refrigerator for $16:
http://www.banggood.com/Mini-USB-LED-PC-Fridge-Refrigerator-Drink-Cans-Food-Cooler-Warmer-ES9P-p-915425.html

http://img.banggood.com/thumb/view/upload/2012/lidanpo/SKU115112 (1).jpg

It's mostly a joke and curiosity but I'm tempted to get one just for fun. They also have a camping/auto fridge for $67 that looks pretty good:
http://www.banggood.com/Car-Auto-Home-Camping-Fridge-Electric-Cool-Box-Cooler-and-Warmer-6LAC41-p-974733.html

http://img.banggood.com/thumb/view/2014/chenqing/04/SKU218962/SKU218962-k.jpg

And they have a complete 72W Peltier unit with heatsinks and fans for about $16:
http://www.banggood.com/Thermoelectric-Peltier-Refrigeration-Cooling-System-Kit-Cooler-Fan-p-936497.html

http://img.banggood.com/thumb/view/2014/xiemeijuan/07/SKU145285/SKU145285b.jpg

If nothing else, it shows how the unit is constructed, and it may even fit my fridge. But I think the problem is one of the power supplies, perhaps the thermistor mentioned in one of the discussions above. Probably an inrush current limiter. Oh, well, time to get off my butt and maybe fix this thing or at least take it apart and look more closely at details...

PStechPaul
07-03-2015, 10:28 PM
Here are more pictures of the refrigerator, showing details of the power supply and thermoelectric units.

http://enginuitysystems.com/pix/electronics/Fridge_2220.jpg

http://enginuitysystems.com/pix/electronics/Fridge_2221.jpg

http://enginuitysystems.com/pix/electronics/Fridge_2224.jpg

http://enginuitysystems.com/pix/electronics/Fridge_2225.jpg

I have not yet tried to test the nonworking module. It seems that the modules are soldered by their wires directly to the 12V supply on the PCB and the fans use connectors. So I think I will need to cut some wires and determine what the problem is. More to follow.

PStechPaul
07-04-2015, 11:36 PM
Well, the fridge is now officially fixed. Here is the Peltier module that was open and not working:

http://enginuitysystems.com/pix/electronics/Fridge_2250.jpg

Fortunately I had a bag full of nearly identical units that I got for about $5 each on eBay a few years ago:

http://enginuitysystems.com/pix/electronics/Fridge_2251_Peltier.jpg

Here it is all connected up and tested:

http://enginuitysystems.com/pix/electronics/Fridge_2252.jpg

The temperature is about 38 degrees F, with an ambient of 68F. I think it is supposed to reach 37F at 70F ambient, but it might be that the temperature control kicked in. Gotta wait for an actual typical warm summer day, unlike today's unusually cool 4th of July.

http://enginuitysystems.com/pix/electronics/Fridge_2254.jpg

I was amazed by how little it takes to get a significant temperature differential on the hot and cold surfaces of these modules. Just 1 volt at 0.5A without a heat sink is quite noticeable and that's only 1/2 watt! At full rated 12 VDC these are 72 watts and there is a warning not to apply voltage without a heat sink, which will result in very fast overheating and destruction. I was quite pleased with what seems to be decent efficiency from the Peltier thermoelectric effect. The efficiency from converting heat to electricity is probably not great, but I figured that I could put the hot side on my wood stove and blow cold outside air across a heat sink on the cold side (keeping it insulated), and perhaps even injecting it into the stove for combustion. Anyway, they are a good deal and fun to play with, and now I have a working portable fridge for less than $20.

Evan
07-06-2015, 12:17 AM
The efficiency isn't just "not great", it's miserable. In a typical application like the coolers you show the ratio of energy used to energy transported is around 3 or 4 to 1. They aren't rated by efficiency since that varies greatly depending on temperature difference, total energy moved as well as rate energy is moved. On the units used in the coolers you must really watch the maximum temperature of the hot side. Get it just a little too hot and it will die. There are units that can run at very high temperatures. I have one (I hope) that is rated up to around 400F without destruction. Normally they cannot stand anything hotter than around boiling or even lower. They are available with different hot side ratings. You must make sure the heat sink is not blocked and the fans are working properly. Also, don't let the sun shine on the heat sinks while operating. That will cook them in a hurry.

Added: Why anyone would make a black cooler is dumbfounding. I guess marketing over engineering strikes again.

PStechPaul
07-06-2015, 02:36 AM
Thanks for the information. It seems like a thermostat on the hot side heat sinks might be advisable. But really, since the Peltier units are only a few dollars, it's probably not worth worrying about. I tried adjusting the temperature control in the unit and it seemed to make the fans run faster but didn't seem to make it run any cooler. I am assuming that it just increased the voltage to the fans as well as the Peltier cells.

Although the efficiency may not be stellar, I am quite pleased with having a refrigerator that runs on just 140 watts or less, and at 18 pounds is easily carried. Running on 12V it should be easy to connect it to the car utility 12V supply (cigarette lighter socket) to keep foods cold while traveling. It may need a small regulator to keep the voltage under 12V and possibly cycle on/off with temperature. It may not work well if the car is parked in the sun or when driving without A/C (which I usually do). So the heat sink thermostats may be needed unless I remember to unplug it in such cases.

Heat pumps such as air conditioners operate with an EER which is in BTUs/watt and a resistance heating unit would have a COP of 1.0 and a SEER of 3.4 based on simple conversion of BTU/hr to watts. A good air conditioning unit may have a SEER of 15 to 30 or 5 to 10 times the unity COP. If the thermoelectric Peltier units are 3 or 4 to 1, they may be only half or 1/4 as efficient as a good heat pump, but for the cost, weight, and simplicity advantages, I'd say they are quite good for such smaller appliances as this. ;)

https://en.wikipedia.org/wiki/Seasonal_energy_efficiency_ratio (SEER)

https://en.wikipedia.org/wiki/Coefficient_of_performance (COP)

https://en.wikipedia.org/wiki/Thermoelectric_cooling (Peltier)

The last link says the maximum differential is about 70C and efficiency is 10-15% compared to 40-60% for Carnot Cycle refrigeration systems. I'm happy with what I have, especially getting it for $12. I think that rates a gloat! :)

BTW, these are the cells I bought. They may be rated 15V and 8A or 120W, and there are some rated over 200W for about $12-15 each.
http://www.ebay.com/itm/QTY-5-40-mm-Nord-RTV-Sealed-Peltier-Thermoelectric-/370534167552

PStechPaul
07-07-2015, 07:21 PM
I may be guilty of premature gloating. I put some beers in the fridge yesterday and they seemed to get reasonably cold, but when I checked again, the fridge was at about 50F and the ambient was about 72. Later I found that the fans had stopped running. Today I opened the back and when I plugged it in, the fans ran (a bit slowly) for about 10 seconds and then stopped. I read the current on the 12V line and it was only 5 amps, but the voltage was only about 8.5 volts. Then it dropped to 7.5V and 3.5A. I suspected the temperature control, which I think is a thermistor that plugs into the board. It didn't change when I unplugged it, which may not be surprising because a high resistance means it is cold. I read 15k on the connector so I shunted it with 1k but no joy. The supply stays on with just the fans but the voltage stays about 7.5V. So I may need a new 12V supply, like this (http://www.banggood.com/150W-Power-Supply-Driver-For-LED-Strip-Light-DC-12V-12_5A-AC110-220V-p-919522.html), unless I can find and fix what's wrong with this one:

http://enginuitysystems.com/pix/electronics/Fridge_2253.jpg

It very easily could be one of the input capacitors or the diodes. If it is a doubler circuit then a failure would make it a "singler" and with half the input the maximum output would be about half as well. I just don't feel like fooling with it now. It's 85F here (inside) with 74% RH which is pretty miserable, but the sun is getting low and it's cooling off a bit. Maybe that's why I like to work at night and into the "whee" hours.

wmgeorge
07-08-2015, 08:55 AM
FYI we had a new one just about like what you have. Stopped using because it just wasn't cold enough for food, maybe if you like lukewarm / cool drinks. 34- 36 DegF is where you want your potato salad. That is why you got it so cheap....

Evan
07-08-2015, 01:29 PM
The biggest issue with these units is the ambient temperature. You need them the most when it is hot but that is when they work the least well. In cool weather they can easily get to freezing on one with decent insulation. That is another major factor, the quality of the insulation. Something that works well is to paint the entire unit with bright aluminum paint. That alone adds about 1 to 1.5 R factor. If you cover it with reflective aluminum it adds close to R factor 3 to the unit. I have a couple of those type units. I kept the small one for myself and my x has the bigger one. On that one I modified the internal and external heat sinks and the fan to increase heat movement and it lowered inside temp quite a lot. One thing that helps a lot is to place a very small fan inside the unit that is at the end opposite the peltier unit.

Also, the peltier units do not act like diodes, they have no inbuilt fixed voltage drop even though they are P-N devices. They act as a positive coefficient resistor so as the temperature goes up the amount of heat pumped goes down. Any increase in the efficiency of the heat rejecter (heat sink) makes a big difference. The main heat limiting factor is the melting temperature of the solder that holds the array together.

I double checked the Max temp rating and it is 100C on the standard units, so my memory is still working.

PStechPaul
07-08-2015, 06:36 PM
I was able to fix the power supply. It turned out to be one of the 3A rectifiers on the input that may not have been adequately soldered and overheated and lost connection to the PCB, causing half-wave rectification or singler (non-doubler) operation. It also looks like there may have been some flashover with some conductive metal coating on the PCB, which I scraped off. Then I re-soldered the diode and now all seems to be good, although at 80F ambient the inside temperature is only about 42F:

http://enginuitysystems.com/pix/electronics/Fridge_2261.jpg

http://enginuitysystems.com/pix/electronics/Fridge_2262.jpg

Maybe it was caused by a bug. I found a green stinkbug on the door of the fridge. Perhaps it has chameleon properties and changed color to match the PCB. I haven't seen very many in the house lately, and probably during hot weather they live outside and perhaps adapt to the green foliage. Rather a cute creature, actually, but they are supremely annoying and I even had one crawl into my ear where it sprayed its stink and it caused an unpleasant burning sensation. I have an electronic flyswatter that works pretty well, or I just use a vacuum.

http://enginuitysystems.com/pix/electronics/Green_Stink_Bug_2258.jpg

http://enginuitysystems.com/pix/electronics/Green_Stink_Bug_2260.jpg

Evan
07-08-2015, 11:03 PM
That is doing pretty well considering the ambient. The theoretical max difference for a single stage is around 75 F but in operation you are doing well to get around half that. The theoretical presumes it is not actually moving heat, just the max between sides. It will do a lot better at lower ambient.