Amp ratings of power adapters

[Thomas G. Marshall]

Hugely fundamentally ignorant question.

If I have an electronics device that claims to take a power adapter of 12V
200mA, and I plug in one rated at 12V 800mA, will the device merely draw the
proper 200mA (because the voltages are matched)?

Or is there something about PA's that can somehow force too much current
into a device?

 

[Jamie]

Thomas G. Marshall wrote:

Hugely fundamentally ignorant question.

If I have an electronics device that claims to take a power adapter of 12V
200mA, and I plug in one rated at 12V 800mA, will the device merely draw the
proper 200mA (because the voltages are matched)?

Or is there something about PA's that can somehow force too much current
into a device?



The rating on the supply indicates what it can deliver to the device in

terms of current. All this means is a connected device requesting
current can not ask for any more than 800 ma.
The device connected governs how much current will flow, the supply
only indicates the amount it can give with out damage or shut down to
it self.

Voltage of the supply must be close or exact to what the device requires.
In your case you have a 600 ma reserve or power, so you are fine..

--
"I'd rather have a bottle in front of me than a frontal lobotomy"

http://webpages.charter.net/jamie_5"

 

[Chuck]

On Sun, 20 Jan 2008 15:13:05 GMT, "Thomas G. Marshall"
<tgm2tothe10thpower@replacetextwithnumber.hotmail.com> wrote:

Hugely fundamentally ignorant question.

If I have an electronics device that claims to take a power adapter of 12V
200mA, and I plug in one rated at 12V 800mA, will the device merely draw the
proper 200mA (because the voltages are matched)?

Or is there something about PA's that can somehow force too much current
into a device?

It's not quite that simple.

For regulated adapters, you can reasonably assume the voltage will be
as stated up to the rated current.

Many adapters are unregulated, however, and the voltage will depend on
the attached load. Normally, an unregulated adapter will have the
nameplate voltage under the nameplate load, but the voltage will be
greater under lesser loads.

If the voltage is "too high" then it may force excessive current into
a device, or cause component failure because of the excessive voltage.

Chuck

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[Thomas G. Marshall]

Chuck said something like:

On Sun, 20 Jan 2008 15:13:05 GMT, "Thomas G. Marshall"
tgm2tothe10thpower@replacetextwithnumber.hotmail.com> wrote:


Hugely fundamentally ignorant question.

If I have an electronics device that claims to take a power adapter of
12V
200mA, and I plug in one rated at 12V 800mA, will the device merely draw
the
proper 200mA (because the voltages are matched)?

Or is there something about PA's that can somehow force too much current
into a device?



It's not quite that simple.

For regulated adapters, you can reasonably assume the voltage will be
as stated up to the rated current.

Many adapters are unregulated, however, and the voltage will depend on
the attached load. Normally, an unregulated adapter will have the
nameplate voltage under the nameplate load, but the voltage will be
greater under lesser loads.

If the voltage is "too high" then it may force excessive current into
a device, or cause component failure because of the excessive voltage.

Chuck

If I were to check the nominal voltage with a voltmeter, it would show me
the voltage supplied with essentially no load placed on it, right?




--
Very old classic: Three men check into a hotel: the
room is $25 for the night. They each hand the bellhop
$10 and ask him to bring back the change. When the
bellhop returns with the $5 change, the men figure it's
easiest math to give $1 back to each of them and leave
$2 to the bellhop as a tip. Now each man paid $9 for
a total of $27. The bellhop got $2, that makes $29.
What happened to the last $1?
Answer (rot13): Unir gb or pnershy ubj lbh nqq guvf hc.
Gur guerr zra cnvq $27 gbgny, BHG BS JUVPU $2 jrag gb
gur oryyubc.

 

[Eeyore]

"Thomas G. Marshall" wrote:

Hugely fundamentally ignorant question.

If I have an electronics device that claims to take a power adapter of 12V
200mA, and I plug in one rated at 12V 800mA, will the device merely draw the
proper 200mA (because the voltages are matched)?

Basically that's the idea.

Or is there something about PA's that can somehow force too much current
into a device?

See other posters' warnings about unregulated adaptors having a higher voltage
when only partially loaded.

Graham

 

[Baron]

Thomas G. Marshall wrote:

Chuck said something like:
On Sun, 20 Jan 2008 15:13:05 GMT, "Thomas G. Marshall"
tgm2tothe10thpower@replacetextwithnumber.hotmail.com> wrote:


Hugely fundamentally ignorant question.

If I have an electronics device that claims to take a power adapter
of 12V
200mA, and I plug in one rated at 12V 800mA, will the device merely
draw the
proper 200mA (because the voltages are matched)?

Or is there something about PA's that can somehow force too much
current into a device?



It's not quite that simple.

For regulated adapters, you can reasonably assume the voltage will be
as stated up to the rated current.

Many adapters are unregulated, however, and the voltage will depend
on the attached load. Normally, an unregulated adapter will have the
nameplate voltage under the nameplate load, but the voltage will be
greater under lesser loads.

If the voltage is "too high" then it may force excessive current into
a device, or cause component failure because of the excessive
voltage.

Chuck


If I were to check the nominal voltage with a voltmeter, it would show
me the voltage supplied with essentially no load placed on it, right?

Right.

--
Best Regards:
Baron.

 

[PeterD]

On Sun, 20 Jan 2008 15:13:05 GMT, "Thomas G. Marshall"
<tgm2tothe10thpower@replacetextwithnumber.hotmail.com> wrote:

Hugely fundamentally ignorant question.

If I have an electronics device that claims to take a power adapter of 12V
200mA, and I plug in one rated at 12V 800mA, will the device merely draw the
proper 200mA (because the voltages are matched)?

Or is there something about PA's that can somehow force too much current
into a device?

It may work, and it may not... Some crappy designs deliver the rated
voltage only at (or near) the rated current, and as current drops,
output voltage rises. As such a lightly loaded unit may in fact cause
damage because of overvoltage.

Realize that these cheap wall warts are *not* regulated, and the
output voltage is approxmate at best in most cases.

 

[Tim Wescott]

On Sun, 20 Jan 2008 16:22:00 +0000, Thomas G. Marshall wrote:

Chuck said something like:
On Sun, 20 Jan 2008 15:13:05 GMT, "Thomas G. Marshall"
tgm2tothe10thpower@replacetextwithnumber.hotmail.com> wrote:


Hugely fundamentally ignorant question.

If I have an electronics device that claims to take a power adapter of
12V
200mA, and I plug in one rated at 12V 800mA, will the device merely
draw the
proper 200mA (because the voltages are matched)?

Or is there something about PA's that can somehow force too much
current into a device?



It's not quite that simple.

For regulated adapters, you can reasonably assume the voltage will be
as stated up to the rated current.

Many adapters are unregulated, however, and the voltage will depend on
the attached load. Normally, an unregulated adapter will have the
nameplate voltage under the nameplate load, but the voltage will be
greater under lesser loads.

If the voltage is "too high" then it may force excessive current into a
device, or cause component failure because of the excessive voltage.

Chuck


If I were to check the nominal voltage with a voltmeter, it would show
me the voltage supplied with essentially no load placed on it, right?

"Nominal" means "named", and you check it on the name plate. It is
nothing more or less than the promise you're getting from the thing's
manufacturer.

You can check the _actual_ no-load voltage at no load with a voltmeter,
just by measuring what's on the end of the wire when the thing is plugged
into the wall but isn't plugged into the device you want to power.

--
Tim Wescott
Control systems and communications consulting
http://www.wescottdesign.com

Need to learn how to apply control theory in your embedded system?
"Applied Control Theory for Embedded Systems" by Tim Wescott
Elsevier/Newnes, http://www.wescottdesign.com/actfes/actfes.html

 

[XTL]

On Jan 20, 5:13 pm, "Thomas G. Marshall"
<tgm2tothe10thpo...@replacetextwithnumber.hotmail.com> wrote:

Hugely fundamentally ignorant question.
A very frequently asked one, as well. And very frequently answered.

Others gave real answers. I agree with the correct ones

If I have an electronics device that claims to take a power adapter of 12V
200mA, and I plug in one rated at 12V 800mA, will the device merely draw the
proper 200mA (because the voltages are matched)?

Or is there something about PA's that can somehow force too much current
into a device?

Actually, I believe. One kind of adapter that might be able to put
more current into a device (by raising the voltage, of course) than is
"asked for" is a constant current supply (battery charger for variable
number of cells). But it's very unlikely that you have one. The plate
would probably have to be wrong, too.

 

[Thomas G. Marshall]

Tim Wescott said something like:

On Sun, 20 Jan 2008 16:22:00 +0000, Thomas G. Marshall wrote:

Chuck said something like:
On Sun, 20 Jan 2008 15:13:05 GMT, "Thomas G. Marshall"
tgm2tothe10thpower@replacetextwithnumber.hotmail.com> wrote:


Hugely fundamentally ignorant question.

If I have an electronics device that claims to take a power
adapter of 12V
200mA, and I plug in one rated at 12V 800mA, will the device merely
draw the
proper 200mA (because the voltages are matched)?

Or is there something about PA's that can somehow force too much
current into a device?



It's not quite that simple.

For regulated adapters, you can reasonably assume the voltage will
be as stated up to the rated current.

Many adapters are unregulated, however, and the voltage will depend
on the attached load. Normally, an unregulated adapter will have the
nameplate voltage under the nameplate load, but the voltage will be
greater under lesser loads.

If the voltage is "too high" then it may force excessive current
into a device, or cause component failure because of the excessive
voltage.

Chuck


If I were to check the nominal voltage with a voltmeter, it would
show me the voltage supplied with essentially no load placed on it,
right?

"Nominal" means "named", and you check it on the name plate. It is
nothing more or less than the promise you're getting from the thing's
manufacturer.

(???) Ok....I'm checking the named voltage as read from the name plate.
I'm not sure how I misstated this. There is a voltage it is supposed to be,
and I'm checking that supposed-to-be voltage.

You raise interesting (non contentious honest) questions within me noggin.
I've heard that voltmeters are not equipped to properly measure a battery's
worth because "it will measure the nominal voltage only" which is of course
the no-load voltage which also happens to be the named voltage written on
the side of the thing. Not sure if the two concepts are yet colloquially
synonymous no-load and nominal....I'll trust you on this. But there is an
acronym NV.....what would that possibly stand for if the definition were
merely satisfied by naming something as "V"?

You can check the _actual_ no-load voltage at no load with a
voltmeter, just by measuring what's on the end of the wire when the
thing is plugged into the wall but isn't plugged into the device you
want to power.

Ok, thanks.

 

[Peter Bennett]

On Tue, 22 Jan 2008 21:40:10 GMT, "Thomas G. Marshall"
<tgm2tothe10thpower@replacetextwithnumber.hotmail.com> wrote:

Tim Wescott said something like:
On Sun, 20 Jan 2008 16:22:00 +0000, Thomas G. Marshall wrote:



If I were to check the nominal voltage with a voltmeter, it would
show me the voltage supplied with essentially no load placed on it,
right?

"Nominal" means "named", and you check it on the name plate. It is
nothing more or less than the promise you're getting from the thing's
manufacturer.

(???) Ok....I'm checking the named voltage as read from the name plate.
I'm not sure how I misstated this. There is a voltage it is supposed to be,
and I'm checking that supposed-to-be voltage.

For an unregulated power supply, like most wall-warts, the nominal
voltage is the voltage it will deliver with some load - probably
somewhere near maximum currents. With no load other than a meter, the
voltage will probably be somewhat higher.

You raise interesting (non contentious honest) questions within me noggin.
I've heard that voltmeters are not equipped to properly measure a battery's
worth because "it will measure the nominal voltage only" which is of course
the no-load voltage which also happens to be the named voltage written on
the side of the thing.

Batteries are a bit different from power supplies. For a dry cell
battery, the nominal or marked voltage is the voltage delivered by a
fresh battery. A used battery will show somewhat less, even with no
load.

The "nominal" voltage of a lead-acid battery or an automotive
electrical system is a sort of "random number" only roughly related to
the actual voltage. The actual voltage of a fully-charged lead-acid
cell is about 2.2 volts, so the "12 volt" battery in your car should
measure about 13.2 volts when fully charged, and the voltage of the
car's electrical system will go over 14 volts when the engine is
running, and the battery is being charged.



--
Peter Bennett, VE7CEI
peterbb4 (at) interchange.ubc.ca
GPS and NMEA info: http://vancouver-webpages.com/peter
Vancouver Power Squadron: http://vancouver.powersquadron.ca

 

[Thomas G. Marshall]

Peter Bennett said something like:

On Tue, 22 Jan 2008 21:40:10 GMT, "Thomas G. Marshall"
tgm2tothe10thpower@replacetextwithnumber.hotmail.com> wrote:

Tim Wescott said something like:
On Sun, 20 Jan 2008 16:22:00 +0000, Thomas G. Marshall wrote:



If I were to check the nominal voltage with a voltmeter, it would
show me the voltage supplied with essentially no load placed on it,
right?

"Nominal" means "named", and you check it on the name plate. It is
nothing more or less than the promise you're getting from the
thing's manufacturer.

(???) Ok....I'm checking the named voltage as read from the name
plate. I'm not sure how I misstated this. There is a voltage it is
supposed to be, and I'm checking that supposed-to-be voltage.

For an unregulated power supply, like most wall-warts, the nominal
voltage is the voltage it will deliver with some load - probably
somewhere near maximum currents. With no load other than a meter, the
voltage will probably be somewhat higher.

You raise interesting (non contentious honest) questions within me
noggin. I've heard that voltmeters are not equipped to properly
measure a battery's worth because "it will measure the nominal
voltage only" which is of course the no-load voltage which also
happens to be the named voltage written on the side of the thing.

Batteries are a bit different from power supplies. For a dry cell
battery, the nominal or marked voltage is the voltage delivered by a
fresh battery. A used battery will show somewhat less, even with no
load.

The "nominal" voltage of a lead-acid battery or an automotive
electrical system is a sort of "random number" only roughly related to
the actual voltage. The actual voltage of a fully-charged lead-acid
cell is about 2.2 volts, so the "12 volt" battery in your car should
measure about 13.2 volts when fully charged, and the voltage of the
car's electrical system will go over 14 volts when the engine is
running, and the battery is being charged.

Ah, ok, essentially each of the 6 cells in series then I take it?

 

[John Fields]

On Sun, 20 Jan 2008 16:37:03 +0000, Eeyore
<rabbitsfriendsandrelations@hotmail.com> wrote:

"Thomas G. Marshall" wrote:

Hugely fundamentally ignorant question.

If I have an electronics device that claims to take a power adapter of 12V
200mA, and I plug in one rated at 12V 800mA, will the device merely draw the
proper 200mA (because the voltages are matched)?

Basically that's the idea.


Or is there something about PA's that can somehow force too much current
into a device?

See other posters' warnings about unregulated adaptors having a higher voltage
when only partially loaded.

---
Geez, I guess without your helpful "me too" he'd be a little lost
lamb?

 

[John Fields]

On Sun, 20 Jan 2008 15:13:05 GMT, "Thomas G. Marshall"
<tgm2tothe10thpower@replacetextwithnumber.hotmail.com> wrote:

Hugely fundamentally ignorant question.

If I have an electronics device that claims to take a power adapter of 12V
200mA, and I plug in one rated at 12V 800mA, will the device merely draw the
proper 200mA (because the voltages are matched)?

Or is there something about PA's that can somehow force too much current
into a device?

---
Most small unregulated wall-warts and transformers have secondaries
wound with wire with a resistance which will result in the specified
nominal output voltage being obtained with the specified load
connected to the output and nominal mains voltage applied to the
input.

That means that with high mains and/or lower than specified output
current the output voltage will be higher than nominal and with low
mains and/or higher than the specified output current the output will
be lower than nominal.

The reason for this is that the resistance of the secondary looks like
it's in series with the secondary, so as current into the load
increases the voltage dropped across the secondary resistance also
increases, decreasing the voltage available to the load, like this:

View in Courier:

Vs Vl
/ /
MAINS>---+ +--[Rs]--+
| | |
P||S |
R||E [Rl]
I||C |
| | <--Il |
MAINS>---+ +--------+

Typical regulation of small transformers is about 30% no-load to full
load, so if we assume that's true for the transformer in your
wall-wart, Vs will be 1.3 times Vl, or 15.6V, when Vl is at 12V.

With Il = 800mA, Rl will be equal to:

VL 12V
Rl = ---- = ------ = 15 ohms
Il 0.8A

and Rs will be equal to:

Vs - Vl 15.6V - 12V
Rs = --------- = ------------- = 4.5 ohms
Il 0.8A

So, to simplify things, we now have:


15.6V E1
|
[4.5R] R1
|
+--> E2
|
[15R] R2
|
0V

Your load is going to draw 200mA at 12V, so its resistance under those
conditions will be:

E1 12V
R2 = ---- = ------ = 60 ohms
Il 0.2A

and the circuit now looks like:

15.6V E1
|
[4.5R] R1
|
+--> E2
|
[60R] R2
|
0V

so the current in the circuit will be:

E1 15.6V
Il = --------- = ------------- = 0.242A
R1 + R2 4.5R + 60R

and the voltage across your device will be:

E2 = Il * R2 = 0.242A * 60R = 14.52V

Here's a data sheet for a typical smallish wall-wart which will
illustrate the principle:

http://204.202.11.159/tamuracorp/clientuploads/pdfs/engineeringdocs/830AS12080.pdf

JF

 

[ehsjr]

John Fields wrote:

<lots of good stuff snipped>

Here's a data sheet for a typical smallish wall-wart which will
illustrate the principle:

http://204.202.11.159/tamuracorp/clientuploads/pdfs/engineeringdocs/830AS12080.pdf

JF

_Nice_ link.

 

[John Fields]

On Wed, 23 Jan 2008 23:50:35 GMT, ehsjr <ehsjr@bellatlantic.net>
wrote:

John Fields wrote:

lots of good stuff snipped


Here's a data sheet for a typical smallish wall-wart which will
illustrate the principle:

http://204.202.11.159/tamuracorp/clientuploads/pdfs/engineeringdocs/830AS12080.pdf

JF

_Nice_ link.

---
Yeah, they've got great data sheets!

Little problem with that one though, they've got the width of the
mains prongs dimensioned out as 2.5" instead of 0.25"!

I just emailed them about it, so we'll see what happens. I've done
business with them before and they've always been decent, so I'll bet
they email me back with something like, "Oops... Thanks!"

JF

 

[Jakthehammer]

"Thomas G. Marshall" <tgm2tothe10thpower@replacetextwithnumber.hotmail.com>
wrote in message news:5yJkj.1858$XI6.1836@trndny04...

Hugely fundamentally ignorant question.

If I have an electronics device that claims to take a power adapter of
12V
200mA, and I plug in one rated at 12V 800mA, will the device merely draw
the
proper 200mA (because the voltages are matched)?

Or is there something about PA's that can somehow force too much current
into a device?

Yes IF the power are really what the label said.

How do you know the powers are matched? Not all adapters telling your the
truth. Use a meter, do not trust the cheap wall-adapters made by the
Chinese.

Jack...