I was given a small, 12-bottle thermoelectric wine fridge from a friend who retrieved it from a metal recycler. It was a nice black countertop unit with a glass door and an LED temperature readout with controls at the top. My friend thought I would probably be interested in the solid-state cooling unit if it were still operational, so I checked it out.
The first thing I tried was to just plug it in and see if anything worked. There were no lights or fan. Maybe the main fuse was open? There wasn't a fuse anywhere on the outside. There were two on the switching power supply board.
I didn't relish digging into someone else's switching power supply if the main fuse was gone, and I decided I would give up if the fuse were missing. However, I found that the fuse had blown on the DC side of the supply. The fuse was soldered into the circuit board cordwood style with heat shrink over it, with one end sticking up in the air.
I dissected the fuse and found that it was a fast blow 3A fuse. I decided it was worth a shot, so I replaced the fuse with a new one. I was careful to not overheat the end of the fuse near the circuit board. The unit worked, but I was still wondering why the fuse blew. Was there was a condition that caused the unit to draw too much current?
I decided to monitor the current drawn by the fridge and discovered that the nominal draw was very close to the current rating of the fuse. Any design engineer knows to use a fuse rated at least 125 percent over the actual current. I installed a four-Amp fuse, and now the unit has worked for quite a while.
I got looking around and noticed that this same model had a lot of failures. I even found an identical broken one on Freecycle -- same problem. My friend and I now both have working wine fridges. All it cost was a bit of time and two new fuses.
This entry was submitted by Jack Gilmore and edited by Rob Spiegel.
Tell us your experiences with Monkey-designed products. Send stories to Rob Spiegel for Made by Monkeys.
I concur with the comment that its important to know the rated current of the supply output as well as its nice to score a "free" vino cooler. I've played around with lots of the peltier thermoelectic cooling (TEC) modules and most work at 12 V with 3 A to 8 A current depending on their surface area and correlated P-N junctions. So they typically require a fan pulling less than a quarter amp and a power supply with 40 to 90 W capacity. What's important is what the switches and wires and ancillary circuitry were built to for current rating and dielectric breakdown. Derating from 3 A fuse to 4 A probably can't hurt you. No one reading these blogs should take away that its okay to double (or more!) fuse rating on any consumer electronics. What may have been a penny fix could result in a serious insurance claim and if the fire inspector finds the wrong fuse, your claim is done. Another consideration is those TEC used in chip coolers, auto coolers and wine coolers are woefully inefficient and limited and when they fail (due to condensation, fan failing or other), they're done. A standard dorm refrigerator uses approx. 300 kwhr per year, keeping food at chilly 40 deg. F with a thermostat. An equivalent TEC cooler will pull double that annual cost, run 100% duty cycle and the best they can do is about 35 degrees below ambient. Not too green, that. For a wine cooler, that's okay but not if you like your beer frosty.
So if the measured current of the broken unit is close to 3A and the unit is using 3A fuse. How do we know what the GOOD load current should be since we do not have the spec of the load? We are assuming that the load is OK, may be it is failing. Looks like more research need to be done before assuming it is bad design.
Replacing a fuse with one having a higher current rating because the original one blew is like installing a penny under a (screw-in) fuse in an old house. It gets the circuit back online, but does not necessarily provide the correct level of over-current protection. Perhaps the power supply had a maximum current capability of 3 A or less. Perhaps the power supply should have had higher current cpability, but was chosen to keep costs low! I am assuming that someone actually thought about the electrical design, but this might be an incorrect assumption on my part.
This story reminds me of experiences I had many years ago with a power supply engineer. He designed and built a 28-VDC, 100-A power supply which fed several curcuits requiring different current levels. One circuit only required about 1 A, so he used small-gauge (AWG 24) wire for the 1 A circuit. I asked him if he was afraid the wire would melt and start a fire if there was a short, and he responded that there was a 1-A fuse in the circuit in case there was a short. What he neglected to consider was that there was 24-gauge wire connecting the power supply to the fuse! If there was a short between the supply and fuse, then there would have been a fire. In this case, the wiring up to the over-current devices had to be capable of 100 A.
Sometimes it's really tough to pick the right fuse, and it really is an art form. In addition to all the fuse specifications you have to deal with agency requirements and how the device will respond to surges and faults on the line. I too had a Phillips plasma that kept blowing its fuse for no reason. The failure was amplified by having to remove the heavy appliance from the wall and removing more than a dozen screws to get to the fuse. I changed the fuse to a different manufacturer and that has cured the problem.
Another case of what one would think should be screamingly obvious to the design engineer. A carpentry parallel would be using the wrong size nail or screw. OTOH, my county has an overabundance of construction contractors and a (perhaps corresponding) overabundance of people who use the wrong screw, don't measure correctly, can't choose lumber correctly, and ad nauseum. It's a lot harder to retrofit the wrong nail or screw--thanks for letting us know that a fuse change may be all that's needed in this case.
I don't think that an inrush current surge would exist with a thermoelectric cooler. That just does not make any sense. And, in the output circuit of a regulated switching power supply, I have a hard time imagining any surges big enough to pop a fuse. It seems that most switchers are regulated on the output side. I can easily imagine a cheap fuse failing mechanicaly, I have seen that kind of failure. In that application the fuse would be there to protect from a short circuit, not from an overload, and so the big question would be about what protection the supply needed. Probably the system would have been safe with NO secondary fuse.
It is quite amazing the number of appliance type things that include a fuse that is obviously never intended to be replaced, and have no marking to indicate that they contain a protective fuse.
Fuses are not simple devices. Standards agencies provide a very broad guideline regarding a device's current and voltage ratings. However, differences between "equivalent" fuses from different manufacturers can cause trouble in precision applications.
Fuses can have an AC or DC rating, or both. Each will likely have different levels of capability when it comes to interrupting the current. There is also a very significent difference between a European fuse vs. a North American fuse for small fuses. The different standards required make these two "mutually exclusive" when seeking a replacement. If a European fuse is employed, it is designed to carry 100% rated current and typically at 250 Vac. A similar looking North American fuse (UL) is designed to carry no more than 75% rated current to achieve a satisfactory life and safe operation. These are typically rated 125 Vac.
A North American application can use a European fuse if it is not able to be easily replaced by the consumer. A soldered fuse meets this criterion. The description given of a leaded fuse standing on end (typically referred to as "hairpin lead configuration) with a heat shrink cover is standard.
I have worked in the fuse and protection industry for 30 years with my main responsibility being interfacing with manufacturers employing fuses and their engineers. Much of my time was also dedicated to standards committees covering these components.
From Dell / Intel® New Paradigms in Design Work Scott Hamilton, vertical market strategist for Dell Precision workstations, 5/2/2013 3
Early in my career, I worked as a draftsman and remember the days of drawing on vellum with numbered pencils and Mylar with plastic lead. This was a fun experience in the sense that I ...
I've been using workstations for more than 10 years and love finding ways to get more performance from my system. With demanding professional applications that require more power each ...
A lasting memory from my first job as an engineer in an auto assembly plant is standing on hard concrete at six in the morning, vending-machine coffee clutched in hand, listening to ...
A quick look into the merger of two powerhouse 3D printing OEMs and the new leader in rapid prototyping solutions, Stratasys. The industrial revolution is now led by 3D printing and engineers are given the opportunity to fully maximize their design capabilities, reduce their time-to-market and functionally test prototypes cheaper, faster and easier. Bruce Bradshaw, Director of Marketing in North America, will explore the large product offering and variety of materials that will help CAD designers articulate their product design with actual, physical prototypes. This broadcast will dive deep into technical information including application specific stories from real world customers and their experiences with 3D printing. 3D Printing is
To save this item to your list of favorite Design News content so you can find it later in your Profile page, click the "Save It" button next to the item.
If you found this interesting or useful, please use the links to the services below to share it with other readers. You will need a free account with each service to share an item via that service.
Tweet This
6 
