I am weighing the pros & cons of whether to leave a wall wart (AC -> DC) plugged in all the time or to power it down after use. My concern involves specifically SMPS type AC to DC converters.
Leaving it plugged in:
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Constantly draws current albeit small.
If stays warm, eventually the electrolytic caps in it will dry out & the device will fail.
Removing it after use:
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Alleviates the above problems, BUT.... every time it is plugged in, these devices have an excessive inrush current of 10s of amps, which are extremely stressful on the components.
- Are my assumptions above correct ?
- Can the inrush current be limited by inserting an NTC of about 15 - 20 ohms without affecting the operation too much ?
- My BIGGEST concern is the longevity/reliability of the device. I will be installing one at a location which is very difficult to physically access and therefore I want to insert it & forget it for tens of years. I can install a power switch externally to simulate the 'unplugging' of the device.
Which option would you choose for HIGHEST reliability ?
Thanks
Leaving it plugged in:
------------------------
Constantly draws current albeit small.
If stays warm, eventually the electrolytic caps in it will dry out & the device will fail.
Removing it after use:
-------------------------
Alleviates the above problems, BUT.... every time it is plugged in, these devices have an excessive inrush current of 10s of amps, which are extremely stressful on the components.
- Are my assumptions above correct ?
- Can the inrush current be limited by inserting an NTC of about 15 - 20 ohms without affecting the operation too much ?
- My BIGGEST concern is the longevity/reliability of the device. I will be installing one at a location which is very difficult to physically access and therefore I want to insert it & forget it for tens of years. I can install a power switch externally to simulate the 'unplugging' of the device.
Which option would you choose for HIGHEST reliability ?
Thanks
Highest reliability must be to use a "wall wart" for a considerably higher power than needed. Humidity? Risk of corrosion of the contacts?
Redundancy? You may install double wiring and OR-ing diodes such that if one fails the other is ready to take over.
Tenths of years is long.
Redundancy? You may install double wiring and OR-ing diodes such that if one fails the other is ready to take over.
Tenths of years is long.
For highest reliability, I would use not a wall-wart, but an industrial grade power supply. Best would be to locate the power supply down where you plan to put a switch, and then just run DC to the load. What's the application?
The inrush current should be limited to the bridge and reservoir cap. The 'electronics' will probably be started slowly (duty cycle limited) at start up.
The big problem is not knowing the quality of the design and quality of parts used in a sealed wall wart. An SMPS in itself should be capable of running 24/7 for 15yrs+ without any issues.
Many of the caps deteriorate not just by high temperatures but also by the 'spiky' (fast rate of change of current vs time) of the ripple component they are asked to smooth. Again a good design should account for this, both in the actual design and the parts quality.
I wouldn't foresee any issue with manually switching an SMPS when needed.
A thermistor or resistor shouldn't really be needed, however incorporating such a component would have to be looked at on a case by case basis. In some cases it may be more of a risk as the start up of the SMPS may rely on seeing a smart increase in voltage rather than a slew limited rise. Again it would be SMPS specific.
The big problem is not knowing the quality of the design and quality of parts used in a sealed wall wart. An SMPS in itself should be capable of running 24/7 for 15yrs+ without any issues.
Many of the caps deteriorate not just by high temperatures but also by the 'spiky' (fast rate of change of current vs time) of the ripple component they are asked to smooth. Again a good design should account for this, both in the actual design and the parts quality.
I wouldn't foresee any issue with manually switching an SMPS when needed.
A thermistor or resistor shouldn't really be needed, however incorporating such a component would have to be looked at on a case by case basis. In some cases it may be more of a risk as the start up of the SMPS may rely on seeing a smart increase in voltage rather than a slew limited rise. Again it would be SMPS specific.
Maybe to change to a transformer type, then find a way to use solid caps, and no electrolytics.
I never leave the smps type on without supervision unless it’s a large well proven one for a laptop computer. Have had too many cheap ones go up in flames overnight.
I never leave the smps type on without supervision unless it’s a large well proven one for a laptop computer. Have had too many cheap ones go up in flames overnight.
Class 2 linear transformer wall warts like those used for routers and wireless access points are very reliable and designed to be left on 24/7 for years. Come to think of it, I have never had one go bad.
I've never seen a router or modem over here that doesn't use an SMPS. I've never had one fail either.
What's the worst that can happen? It starts a fire that burns down the building and kills everybody inside except you. Protect yourself against that.
A fuse won't stop a fire, as enough heat could develop from 200mA. NTCs can get hot. Bad solder joints and loose connectors can get hot. I tend to switch off cheap unknown stuff before I go to bed....
You could avoid early failure by 'burning in' the PS you have in mind, so at least it's been 'torture tested' to your satisfaction before installation...?
A fuse won't stop a fire, as enough heat could develop from 200mA. NTCs can get hot. Bad solder joints and loose connectors can get hot. I tend to switch off cheap unknown stuff before I go to bed....
You could avoid early failure by 'burning in' the PS you have in mind, so at least it's been 'torture tested' to your satisfaction before installation...?
There's a conflict between your flag, and your location 🙂
To add to the above, proper thought given to layout and installation method should offset most/much of this risk. I usually take a step back and think OK, if what I am installing was replaced by an 1875W power resistor, would it cause a fire? That's realistically the most energy you can get from a 15A breaker.
If it's in a metal box mounted on a brick wall, the answer is no.. if it's located in a combustible location, then do something to make sure that fire won't occur. I always like to rely on passive protection like distance and use of non combustible materials, rather than active protection like fuses and themistors.
Side note: I once had a 200VA transformer develop a short circuit while I was out. It was mounted on a piece of wood. The coils got hot enough to melt together and fill the room with wood smoke, but the branch circuit fuse let go before there was a fire.
We've lived in Marin Co almost two years, and both years forest fires have started once it dries out in Oct-Nov.
Rodents are another cause of damage to wiring.
Sorry everyone, I don't mean to sound like a concern troll, but I heard a recent story: Someone's apartment burned down, and the fire was traced to extension cords run underneath the rugs that had gotten trampled and shorted out.
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At some point though, you have to rely on the UL listing of a piece of equipment. Overloaded extension cord under carpet? not being used in accordance with UL listing... every extension cord says it's for temporary wiring and warns to not run under carpet.
UL listed "wall wart" plugged into correctly installed outlet, and being used within ratings? That's just fine... reliability is another concern, but if it carries the correct approvals and is being used as intended, I wouldn't worry excessively about it in the context of fire hazard.
If you got it grey market from aliexpress, then yeah, you're on your own.. but UL listed and bought from a reputable vendor, it's just fine.
UL listed "wall wart" plugged into correctly installed outlet, and being used within ratings? That's just fine... reliability is another concern, but if it carries the correct approvals and is being used as intended, I wouldn't worry excessively about it in the context of fire hazard.
If you got it grey market from aliexpress, then yeah, you're on your own.. but UL listed and bought from a reputable vendor, it's just fine.
On digikey under wall adapters, one of the filters is 'Approval'. Below, I am adding the combinations supported:
BIS, CB, CCC, CE, cULus, EAC, GS, KC, NOM, PSE, RCM, S-Mark, UkrSEPRO, VCCI
Any idea what those are 🙂 And which one shows longevity/reliability ?
Thanks
BIS, CB, CCC, CE, cULus, EAC, GS, KC, NOM, PSE, RCM, S-Mark, UkrSEPRO, VCCI
Any idea what those are 🙂 And which one shows longevity/reliability ?
Thanks
Safety approvals. It means they can be legally sold and used in specific countries, and have been tested to meet some minimum safety standards. None of it is related to reliability, only safety.
To get something with gauranteed reliability, I suggest to look for an industrial grade power supply, such as those made my Phoenix Contact, and mount it on DIN rail in a suitable enclosure.
Your flag shows US. You want the "cULus" tag to avoid trouble with your fire insurance coverage. UL has become an octopus with many legs, but the original core purpose of UL was to help insurance ('underwriters') companies know what products were safe to insure and what products they should refuse to cover damages from.
UL makes no "reliability" test. They do yank the cord and some other abuse, but a product can be rigged with a good cord clamp and crappy caps.
In Europe a CE tag shows that the product can be marketed throughout the European Union. Again, no reflection on reliability. NOM seems to cover the Mexican market. S-Mark seems to be a tester trademark for CE. CCC is another trademark for China market testing.
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