Searching \ for '[EE] Power supply question (Class 2 adapters)' in subject line. ()
Make payments with PayPal - it's fast, free and secure! Help us get a faster server
FAQ page: www.piclist.com/techref/power/actodc.htm?key=power
Search entire site for: 'Power supply question (Class 2 adapters)'.

Exact match. Not showing close matches.
PICList Thread
'[EE] Power supply question (Class 2 adapters)'
2005\12\27@210536 by Kenneth Lumia

picon face
I'm in the process of designing a PIC project that
uses a switching regulator to convert 12Vdc from
a "wall wart" supply to 5V.  I'd like to leave out any
fusing if possible.  The wall wart is rated as a
Class 2 device, which I believe also means that
it is "inherently current limiting".  The manufacturer
only provides an output voltage vs. output current
graph up to the rated load (500 mA).  Does anyone
know what happens to a Class 2 power supply
when its rated current is exceeded?  I expect that
current will be limited to a given amount (internal
resistance?), and the voltage will drop, keeping the
power supply from overheating.  Anyone have a
good source of information on the subject (google
didn't help much)?

Thanks,

Ken

spam_OUTklumiaTakeThisOuTspamadelphia.net

2005\12\28@073202 by olin piclist

face picon face
Kenneth Lumia wrote:
> I'm in the process of designing a PIC project that
> uses a switching regulator to convert 12Vdc from
> a "wall wart" supply to 5V.  I'd like to leave out any
> fusing if possible.  The wall wart is rated as a
> Class 2 device, which I believe also means that
> it is "inherently current limiting".  The manufacturer
> only provides an output voltage vs. output current
> graph up to the rated load (500 mA).  Does anyone
> know what happens to a Class 2 power supply
> when its rated current is exceeded?

Try it.  Short the output and see how hot it gets.  Give it at least 30
minutes to let the temperature stabalize.  This isn't conclusive proof it's
OK if it survives, but you'll know the answer for sure if it melts or burns
your house down.

Most "wall wart" supplies can handle a short on the output indefinitely.
I'm not totally sure, but I think this is one of the UL requirements.

Of course the right answer is to CHECK THE DATA SHEET.


******************************************************************
Embed Inc, Littleton Massachusetts, (978) 742-9014.  #1 PIC
consultant in 2004 program year.  http://www.embedinc.com/products

2005\12\28@080048 by Howard Winter

face
flavicon
picon face
Ken,

On Tue, 27 Dec 2005 21:05:31 -0500, Kenneth Lumia wrote:

> I'm in the process of designing a PIC project that
> uses a switching regulator to convert 12Vdc from
> a "wall wart" supply to 5V.  I'd like to leave out any
> fusing if possible.  The wall wart is rated as a
> Class 2 device, which I believe also means that
> it is "inherently current limiting".

I don't know if "Class 2" varies with geography, but over this side of the pond it means it's double
insulated, so doesn't need to be Earthed.  It says nothing about overcurrent protection.

> The manufacturer
> only provides an output voltage vs. output current
> graph up to the rated load (500 mA).  Does anyone
> know what happens to a Class 2 power supply
> when its rated current is exceeded?  

Wallwarts over here tend to have thermal fuses included, in the outer windings of the transformer.  If it
reaches the temperature it's designed for, the fuse opens the circuit, never to return (you bin the wallwart).  
I imagine that the approvals over there wouldn't allow an uprotected device where catching fire is the
response to overload!  See if there's any mention of temperature on the wallwart's spec. plate.

> I expect that
> current will be limited to a given amount (internal
> resistance?), and the voltage will drop, keeping the
> power supply from overheating.

Transformer saturation may provide some overcurrent protection, but the heat this produces may well prove
terminal.

Cheers,


Howard Winter
St.Albans, England


2005\12\28@083625 by Xiaofan Chen

face picon face
On 12/28/05, Howard Winter <.....HDRWKILLspamspam@spam@h2org.demon.co.uk> wrote:
>
> On Tue, 27 Dec 2005 21:05:31 -0500, Kenneth Lumia wrote:
>
> > I'm in the process of designing a PIC project that
> > uses a switching regulator to convert 12Vdc from
> > a "wall wart" supply to 5V.  I'd like to leave out any
> > fusing if possible.  The wall wart is rated as a
> > Class 2 device, which I believe also means that
> > it is "inherently current limiting".
>
> I don't know if "Class 2" varies with geography, but over this side
> of the pond it means it's double insulated, so doesn't need to be
> Earthed.  It says nothing about overcurrent protection.

Actually I believe what Howard means by "Class 2" is actually
called by "<Protection> Class II" and is denoted by a double-walled
square symbol. It means double insulation (with one fault, the
device still has basic insulation) and has quite some other requirements
depending on the product standard.

What the OP mentioned by UL Class 2 power supply may well be totally
different. It may simply means the output of the supply is below 60Vdc
& 8A according to Google search result .

/* Quoted from http://www.elpac.com/resources/faq.html */
3. Q: Class 2 - what is this?
A: "Class 2" is a UL definition for an output which is below 60Vdc
(42.4Vpk) & 8A. Virtually all Elpac power supplies are class 2,
however they are not tested and certified to class 2, and this is
rarely needed by customers.

4. Q: Class II - what is this?
A: " Class II" is a European definition for a 2 pin input WITHOUT
Ground present, and as such the design has to be with Reinforced
Insulation since it cannot rely on Ground for safety. Few Elpac
products are Class II. Examples which are include WM1005 and W4012.
/* End of quotation */

There are other Class II with different meanings in UL standards and other
standards.

Regards,
Xiaofan

2005\12\28@085304 by Kenneth Lumia

picon face
From: "Olin Lathrop" <olin_piclistspamKILLspamembedinc.com>
> Try it.  Short the output and see how hot it gets.  Give it at least 30
> minutes to let the temperature stabalize.  This isn't conclusive proof
> it's
> OK if it survives, but you'll know the answer for sure if it melts or
> burns
> your house down.

Yeah.  I was trying to avoid this.  I had test (regulatory) engineers to
do this at the company I used to work for (as I have a flashback to
the old Dilbert cartoon series character "Bob Bastard, test engineer").

> Most "wall wart" supplies can handle a short on the output indefinitely.
> I'm not totally sure, but I think this is one of the UL requirements.
>
> Of course the right answer is to CHECK THE DATA SHEET.
>

The data sheet is rather sparse. Basically just mechanicals,
performance chart up to the rated output, copy of the ID plate
and a UL file number.  I went to the UL site, looked up the file
number, but other than the device part number matching the file
number, it appears that it is not possible to pull up the actual
test results.  I guess they are proprietary.

Here's a link to the data sheet for those interested.

http://204.202.11.159/tamuracorp/clientuploads/pdfs/engineeringdocs/420AS12050.pdf

ken
.....klumiaKILLspamspam.....adelphia.net


2005\12\28@142257 by Spehro Pefhany

picon face
At 09:05 PM 12/27/2005 -0500, you wrote:
>I'm in the process of designing a PIC project that
>uses a switching regulator to convert 12Vdc from
>a "wall wart" supply to 5V.  I'd like to leave out any
>fusing if possible.  The wall wart is rated as a
>Class 2 device, which I believe also means that
>it is "inherently current limiting".

I believe the phrase is "energy limited".

{Quote hidden}

Hi, Ken:-

The final answer is in the appropriate UL standards for testing.
I think UL508 and/or UL1310. Maybe you have an engineering library nearby
where you can read them.

My understanding is that UL Class 2 supplies are designed to be safe
in the sense of not starting a fire, under short-circuit
and other overload conditions. Of course this is in non-hazardous
locations (which is where you get into the confusingly similar Class I, II
etc.
and division classifications). This safety can be achieved by
crummy regulation- "impedance limited" such as you see some domestic
transformers, a one-time fuse, a safety cutout, a varistor,
or some other method. AFAIUI, there is no guarantee that the
supply will survive even a momentary short, just that it won't be unsafe.
I think some use even self-fusing primary wires that deliberately self-destruct
before the core temperature can rise to the point where it could threaten
the plastic housing. That saves a penny or two. There's an unrelated
European Class 2 that has something to do with insulation, and of course
your Class 2 power supply might have a Class 2 transformer (or not) and
that transformer might have Class F insulation (a temperature rating). ;-)

As on all safety issues, however, I shall defer to the actual relevant
UL, CSA, IEC etc. standards and the NEC/NFPA standards for wiring and
installation.

>Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
EraseMEspeffspam_OUTspamTakeThisOuTinterlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
->> Inexpensive test equipment & parts http://search.ebay.com/_W0QQsassZspeff


2005\12\29@063737 by Howard Winter

face
flavicon
picon face
Xiofan,

On Wed, 28 Dec 2005 21:36:25 +0800, Xiaofan Chen
wrote:

> On 12/28/05, Howard Winter <HDRWspamspam_OUTh2org.demon.co.uk>
wrote:
{Quote hidden}

Yes you're quite right - in my defence I will just say that "Class 2" and "Class II" sound just the same when
you say them!  :-)

Cheers,


Howard Winter
St.Albans, England


More... (looser matching)
- Last day of these posts
- In 2005 , 2006 only
- Today
- New search...