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I'm a newbie to electronics, but I've come up with a project to cool a
closed electronics cabinet. It uses an ac-dc 12V power (wall wart)
adapter which would power a 500mA load. The adapter is rated for
800mA.
Are there any safety problems with this? Fire hazards? What would
happen if a short occurred?
The adapter is UL listed, but I've been unable to see what criteria
they use for power adapters. Does the UL mark imply any level of
performance or safety?
Thanks,
Ethan
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Author: EeyoreDate: 18:07 28-06-07
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etravenio@hotmail.com wrote:
> I'm a newbie to electronics, but I've come up with a project to cool a
> closed electronics cabinet. It uses an ac-dc 12V power (wall wart)
> adapter which would power a 500mA load. The adapter is rated for
> 800mA.
>
> Are there any safety problems with this? Fire hazards? What would
> happen if a short occurred?
>
> The adapter is UL listed, but I've been unable to see what criteria
> they use for power adapters. Does the UL mark imply any level of
> performance or safety?
It's basically up to you to ensure a short doesn't happpen but in the event it
did such adaptors typically are protected by an internal thermal fuse.
Graham
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Author: David L. JonesDate: 22:53 28-06-07
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On Jun 29, 1:37 am, etrave...@hotmail.com wrote:
> I'm a newbie to electronics, but I've come up with a project to cool a
> closed electronics cabinet. It uses an ac-dc 12V power (wall wart)
> adapter which would power a 500mA load. The adapter is rated for
> 800mA.
>
> Are there any safety problems with this? Fire hazards? What would
> happen if a short occurred?
>
> The adapter is UL listed, but I've been unable to see what criteria
> they use for power adapters. Does the UL mark imply any level of
> performance or safety?
>
> Thanks,
> Ethan
Plugpacks are usually protected with an integral thermal fuse.
But good practice would be to add your own fuse to your application.
You could use an line fuse holder that can be wired in series with
your the plugpack lead.
Dave.
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Author: Ken MoffettDate: 17:12 29-06-07
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"David L. Jones" <altzone@gmail.com> wrote in
news:1183085635.939528.231170@m37g2000prh.googlegroups.com:
> On Jun 29, 1:37 am, etrave...@hotmail.com wrote:
>> I'm a newbie to electronics, but I've come up with a
>> project to cool a closed electronics cabinet. It uses an
>> ac-dc 12V power (wall wart) adapter which would power a
>> 500mA load. The adapter is rated for 800mA.
>>
>> Are there any safety problems with this? Fire hazards?
>> What would happen if a short occurred?
>>
>> The adapter is UL listed, but I've been unable to see what
>> criteria they use for power adapters. Does the UL mark
>> imply any level of performance or safety?
>>
>> Thanks,
>> Ethan
>
> Plugpacks are usually protected with an integral thermal
> fuse. But good practice would be to add your own fuse to
> your application. You could use an line fuse holder that
> can be wired in series with your the plugpack lead.
>
> Dave.
>
Interesting...about them having internal fuses. I've cut
many, many of them open...to get out the small transformers
for various projects...and have never encountered a
fuse...thermal or current...on either side of the
transformen, on the the little PCB's, or in the transformer
windings. I'm talking about the straight
transformer/diodes/capacitor versions...not regulated or
switching. I thought they depended on crappy, easily
saturated transformers for safety limiting.
???
Ken
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Author: Phil AllisonDate: 18:37 29-06-07
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"Ken Moffett"
>>
>> Plugpacks are usually protected with an integral thermal
>> fuse.
>
> Interesting...about them having internal fuses. I've cut
> many, many of them open...to get out the small transformers
> for various projects...and have never encountered a
> fuse...thermal or current...on either side of the
> transformen, on the the little PCB's, or in the transformer
> windings.
** Not all plug pack transformers have a thermal fuse. However, all must
pass a safety test where the output is shorted until the unit fails, without
any hazard to the user.
> I'm talking about the straight
> transformer/diodes/capacitor versions...not regulated or
> switching.
** Yep.
> I thought they depended on crappy, easily
> saturated transformers for safety limiting.
** Some transformers are "inherently short circuit proof " so will not heat
the windings enough to burn even if shorted indefinitely.
Others simply burn and go open - but because of the construction method,
there is no risk of the primary and secondary becoming connected.
........ Phil
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Author: EeyoreDate: 19:14 29-06-07
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Ken Moffett wrote:
> Interesting...about them having internal fuses. I've cut
> many, many of them open...to get out the small transformers
> for various projects...and have never encountered a
> fuse...thermal or current...on either side of the
> transformen, on the the little PCB's, or in the transformer
> windings.
A thermal fuse would be 'buried' in the windings. Did you cut the transformer
open ? Externally you wouldn't be able to distinguish a blown thermal fuse from
a 'blown' transformer winding.
Graham
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Author: Michael BlackDate: 19:49 29-06-07
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Ken Moffett (KLMoffett@comcast.net) writes:
> "David L. Jones" <altzone@gmail.com> wrote in
> news:1183085635.939528.231170@m37g2000prh.googlegroups.com:
>
>> On Jun 29, 1:37 am, etrave...@hotmail.com wrote:
>>> I'm a newbie to electronics, but I've come up with a
>>> project to cool a closed electronics cabinet. It uses an
>>> ac-dc 12V power (wall wart) adapter which would power a
>>> 500mA load. The adapter is rated for 800mA.
>>>
>>> Are there any safety problems with this? Fire hazards?
>>> What would happen if a short occurred?
>>>
>>> The adapter is UL listed, but I've been unable to see what
>>> criteria they use for power adapters. Does the UL mark
>>> imply any level of performance or safety?
>>>
>>> Thanks,
>>> Ethan
>>
>> Plugpacks are usually protected with an integral thermal
>> fuse. But good practice would be to add your own fuse to
>> your application. You could use an line fuse holder that
>> can be wired in series with your the plugpack lead.
>>
>> Dave.
>>
>
> Interesting...about them having internal fuses. I've cut
> many, many of them open...to get out the small transformers
> for various projects...and have never encountered a
> fuse...thermal or current...on either side of the
> transformen, on the the little PCB's, or in the transformer
> windings. I'm talking about the straight
> transformer/diodes/capacitor versions...not regulated or
> switching. I thought they depended on crappy, easily
> saturated transformers for safety limiting.
>
I don't know where all this talk of "thermal fuses" comes from. After
all, a regular fuse has always sufficed to protect things.
The only reason something gets too hot is because too much current
is going through it, and a regular fuse can stop that.
I was under the impression that a lot of those low current transformers
were wound so that the primary would break if too much current went
through them, and that was the "fuse".
On the other hand, I had an old Sony ac adaptor that did get shorted,
and nothing happened so it got really hot, enough to scortch the surface
it was resting on. It was, though, one dating from about 1971, so one
hopes such things don't happen nowadays.
Michael
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Author: Phil AllisonDate: 20:04 29-06-07
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"Michael Black"
>>
> I don't know where all this talk of "thermal fuses" comes from.
** Seems YOU don't know a whole bunch of things.
>After all, a regular fuse has always sufficed to protect things.
** Far from the truth.
> The only reason something gets too hot is because too much current
> is going through it, and a regular fuse can stop that.
** There are many circumstance where a wire fuse is not able to do that
reliably or accurately enough.
Happens when the maximum safe operating current and the unsafe or overload
current values are similar.
Happens when the overload current is only 50mA.
Happens whenever the fuse is accessible to a user.
> I was under the impression that a lot of those low current transformers
> were wound so that the primary would break if too much current went
> through them, and that was the "fuse".
** You labour under many silly misconceptions, then.
> On the other hand, I had an old Sony ac adaptor that did get shorted,
> and nothing happened so it got really hot, enough to scortch the surface
> it was resting on.
** That can still happen.
....... Phil
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Author: EeyoreDate: 21:24 29-06-07
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Michael Black wrote:
> I don't know where all this talk of "thermal fuses" comes from.
That only illustrates how little you know about product safety.
Graham
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Author: EeyoreDate: 21:25 29-06-07
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Michael Black wrote:
> On the other hand, I had an old Sony ac adaptor that did get shorted,
> and nothing happened so it got really hot, enough to scortch the surface
> it was resting on. It was, though, one dating from about 1971, so one
> hopes such things don't happen nowadays.
They have thermal fuses.
Graham
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Author: JasenDate: 08:27 30-06-07
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On 2007-06-29, Michael Black <et472@FreeNet.Carleton.CA> wrote:
> I don't know where all this talk of "thermal fuses" comes from. After
> all, a regular fuse has always sufficed to protect things.
a thermal fuse blows before the varnish ignites.
--
Bye.
Jasen
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Author: Michael A. TerrellDate: 11:58 30-06-07
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Eeyore wrote:
>
> A thermal fuse would be 'buried' in the windings. Did you cut the transformer
> open ? Externally you wouldn't be able to distinguish a blown thermal fuse from
> a 'blown' transformer winding.
Really? I've seen a lot of the thermal fuses brought out to separate
terminals. One lead of the primary was soldered to one of the terminals,
and the fuse could be replaced, or jumpered for testing to determine if
the unit was worth repairing. Some transformers even have a notch in
the nylon or plastic bobbin to make it easy to remove the fuse and
replace it.
--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.
Michael A. Terrell
Central Florida
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Author: EeyoreDate: 15:29 30-06-07
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"Michael A. Terrell" wrote:
> Eeyore wrote:
> >
> > A thermal fuse would be 'buried' in the windings. Did you cut the transformer
> > open ? Externally you wouldn't be able to distinguish a blown thermal fuse
from
> > a 'blown' transformer winding.
>
> Really? I've seen a lot of the thermal fuses brought out to separate
> terminals. One lead of the primary was soldered to one of the terminals,
> and the fuse could be replaced, or jumpered for testing to determine if
> the unit was worth repairing. Some transformers even have a notch in
> the nylon or plastic bobbin to make it easy to remove the fuse and
> replace it.
The only transformers I've seen with the leads brought out were ones with resetting
thermal switches. These are a bit more upmarket.
I've also come across transformers with buried self-resetting thermal switches too.
Graham
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Author: EeyoreDate: 15:31 30-06-07
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Jasen wrote:
> Michael Black <et472@FreeNet.Carleton.CA> wrote:
>
> > I don't know where all this talk of "thermal fuses" comes from.
After
> > all, a regular fuse has always sufficed to protect things.
>
> a thermal fuse blows before the varnish ignites.
Or melts, causing a potentially dangerous fault condition. It's all about
temperature rise. Most of the common insulations are rated for no more than 130C
and the transformer winding itself won't fail open at those temps.
Graham
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Author: ehsjrDate: 21:56 02-07-07
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Ken Moffett wrote:
> "David L. Jones" <altzone@gmail.com> wrote in
> news:1183085635.939528.231170@m37g2000prh.googlegroups.com:
>
>
>>On Jun 29, 1:37 am, etrave...@hotmail.com wrote:
>>
>>>I'm a newbie to electronics, but I've come up with a
>>>project to cool a closed electronics cabinet. It uses an
>>>ac-dc 12V power (wall wart) adapter which would power a
>>>500mA load. The adapter is rated for 800mA.
>>>
>>>Are there any safety problems with this? Fire hazards?
>>>What would happen if a short occurred?
>>>
>>>The adapter is UL listed, but I've been unable to see what
>>>criteria they use for power adapters. Does the UL mark
>>>imply any level of performance or safety?
>>>
>>>Thanks,
>>>Ethan
>>
>>Plugpacks are usually protected with an integral thermal
>>fuse. But good practice would be to add your own fuse to
>>your application. You could use an line fuse holder that
>>can be wired in series with your the plugpack lead.
>>
>>Dave.
>>
>
>
> Interesting...about them having internal fuses. I've cut
> many, many of them open...to get out the small transformers
> for various projects...and have never encountered a
> fuse...thermal or current...on either side of the
> transformen, on the the little PCB's, or in the transformer
> windings. I'm talking about the straight
> transformer/diodes/capacitor versions...not regulated or
> switching. I thought they depended on crappy, easily
> saturated transformers for safety limiting.
>
> ???
>
> Ken
I just opened one that had failed to get the wire from
it. The fuse was not apparent, until I dissassembled
the laminations to get to the bobbin, and removed the
tape so I could unwind the primary. The fuse was
between layers of tape.
Ed
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Author: Don KlipsteinDate: 23:57 02-07-07
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In article <71iii.8685$pa5.3839@trndny05>, ehsjr wrote:
>Ken Moffett wrote:
>> "David L. Jones" <altzone@gmail.com> wrote in
>> news:1183085635.939528.231170@m37g2000prh.googlegroups.com:
>>
>>>On Jun 29, 1:37 am, etrave...@hotmail.com wrote:
>>>
>>>>I'm a newbie to electronics, but I've come up with a
>>>>project to cool a closed electronics cabinet. It uses an
>>>>ac-dc 12V power (wall wart) adapter which would power a
>>>>500mA load. The adapter is rated for 800mA.
>>>>
>>>>Are there any safety problems with this? Fire hazards?
>>>>What would happen if a short occurred?
>>>>
>>>>The adapter is UL listed, but I've been unable to see what
>>>>criteria they use for power adapters. Does the UL mark
>>>>imply any level of performance or safety?
>>>Plugpacks are usually protected with an integral thermal
>>>fuse. But good practice would be to add your own fuse to
>>>your application. You could use an line fuse holder that
>>>can be wired in series with your the plugpack lead.
>>
>> Interesting...about them having internal fuses. I've cut
>> many, many of them open...to get out the small transformers
>> for various projects...and have never encountered a
>> fuse...thermal or current...on either side of the
>> transformen, on the the little PCB's, or in the transformer
>> windings. I'm talking about the straight
>> transformer/diodes/capacitor versions...not regulated or
>> switching. I thought they depended on crappy, easily
>> saturated transformers for safety limiting.
>> ???
>
>I just opened one that had failed to get the wire from
>it. The fuse was not apparent, until I dissassembled
>the laminations to get to the bobbin, and removed the
>tape so I could unwind the primary. The fuse was
>between layers of tape.
Probably a thermal fuse placed to get blown if the windings overheated.
Power supplies / "wall warts" that are UL listed as a "Class II
transformer" or "class II power supply" are supposed to be reasonably
safe
even if the load malfunctions.
However, I would use some common sense with the load. If the load is a
prototype or an experimental device, I would add a fuse that is actually
tested to reliably blow if the load fails short or encounters its most
probable failure modes that result in increased current draw (component
failure or shorting within the load). With the fuse reliably blowing even
when the Class II wallwart is warmed up and its winding resistances are
higher.
Otherwise only operate the experimental/prototype device when you are
around to unplug it if things start smoking. I know that Class II power
supplies are supposed to be reasonably safe from shorts and overloads, but
I suspect some barely pass certification and some may cut corners compared
to the units that got sent to the certification lab. For example (with a
different device), I have known a fire to start from a fluorescent lamp
ballast overheating when a recently-failed lamp caused a starter to "get
stuck" (fail short).
If you see any fluorescent lamps blinking on and off or steadily glowing
only at the ends, remove the lamp ("bulb") or the starter. If there is
steady end glow, the starter failed short if there is a starter in the
fixture.
- Don Klipstein (don@misty.com)
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