I've a wall wart which is rated 12 V DC at 1 amp output
The router is (I think) expecting 12 VCD at .5 amp.
And I correct in assuming that the wall wart will not "over supply"
the device - that its rating is essentially the max amperage it will
output, while the router is the amount it will draw to function?
That seems to make sense, to me. At this hour of the morning.
Typically, a device "pulls" what it needs for a lack of a better way of
describing it. For instance, a battery capable of powering the space
shuttle at 12V could also start my motorcycle, but not the other way around.
"Joe AutoDrill" on Tue, 26 Apr 2011 14:28:03
-0400 typed in rec.crafts.metalworking the following:
Ah, so "one way of thinking about it"is that - yes, the wall wart
can supply up to 1 amp at 12 VDC, but the router is only going to use
half an Amp.
Now to see if I let the magic smoke out. (I've been told by
Electrical Engineers, that electrical items run on smoke, and letting
the smoke out is what causes them to fail.)
pyotr filipivich fired this volley in
But the other way of thinking about it is that some Wal Warts are not
regulated. If not, and you do not draw something near to their ratings,
they may damage the powered equipment by supplying too high a voltage.
The little lightweight switching supplies are almost always regulated,
but the heavier transformer-based units are often just a transformer,
bridge rectifier, and filter capacitor; Their outputs can vary 50% from
the rating at full load.
Yes, more than likely. Make sure that the supply is regulated (just
stick a voltmeter across it without a load and read the voltage if
you're not sure- it should read 12V within a fraction of a volt).
If it weighs only a few ounces, it's a switching supply and they're
virtually always regulated.
It's instructive to see what happens to a wall-wart when you abuse it. If
you short the output of one, most of them will fry themselves in an instant
from the current overload.
I've done that for entertainment when I no longer have use for them. I have
too many in my junk box as it is. I have two of them that did not. The
transformers in them must be so pathetically underbuilt that they just
buzzed. Most of them are somewhat self-limiting by virtue of extreme
transformer inefficiency, but most in my limited experience will not
tolerate a direct short.
Regarding the motor, if you supply too much voltage, you probably will get a
surprise regarding that 1/2 Amp.
That's because they are almost always with Underwriters Labs as being a
"current limiting device" and as such they are actually designed to fry in
an overcurrent situation--and the main reason they are so prevalent these
days is exactly because this...IE, in many situations it allows
manufacturers to circumvent agency certification so long as the device is
sold along with a "listed power supply"....essentially making it faster,
easier and less costly for manufacturers to bring new electronic items to
That said, beware of "counterfiet UL stickers", items imported from
Communist China being notorious for this particular consumer fraud.
Boy, Ed, you sure are easily entertained.
Many of these things have fusible links under the first layer of tape
in the transformer, others have "polyfuse" type circuit interruptors.
Would you prefer, perhaps, that they supply unlimited current and just
catch fire when you short them?
BTW, regarding the original question: Yes, a properly working device
will draw only what current it needs from your 12V supply (assuming
it's putting out something near 12V), but the designers sometimes rely
on the overcurrent protection built into the supply rather than
spending the extra $0.25 to put a fuse in the router. So, if you had a
fault situation in the device and it was connected to a larger than
expected supply, you *could* have a fire. In reality, it's unlikely;
at the currents your talking about (0.5A vs 1.0A) it's virtually
impossible, but I've seen installations which have large numbers of
consumer-grade devices connected to a single large supply, and that
sort of thing could be a problem.
More or less correct. However I'm betting that the router doesn't even
need 12 volts. Probably that has a LOT of overhead in it. Most
electronics out there use around 5-6 volts. The extra allows for voltage
sags, spikes due to other items starting up.
If you opened the router the first items in the power feed will likely
be a couple of capacitors and a couple of regulators. Likely a 7805 type
with maybe a 7905 if it also has a negative rail.
Most routers that I've seen use a switching power supply to knock down
an input voltage (5 or 12VDC) to 3.3VDC and lower for most of the
Here's a typical one-chip switch (not router)
138 has the current requirements..
up to 22mA typical at 3.3V = 73mW
up to 330 or so mA typical at 2.5V = 825mW
up to 150mA typical at 1.5V = 225mW
plus more for power supply losses, LEDs etc.
Ignoramus7326 fired this volley in
That may have been true in the past, but more and more we're seeing the
switching variety, now that they've included the switching transistors on
the control chips.
Usually, now, you'll see electronics powered by switchers, and DC motor-
driven appliances by the dumb type. But with the cost and manual labor
associated with assembling a transformer, I expect more will become
switchers, even for the motor devices.
"Steve W." on Tue, 26 Apr 2011 18:13:33 -0400
typed >> I've a wall wart which is rated 12 V DC at 1 amp output
"Probably" - but all I know is what I can decipher off the
>...that has a LOT of overhead in it. Most
>electronics out there use around 5-6 volts. The extra allows for voltage
>sags, spikes due to other items starting up.
>If you opened the router the first items in the power feed will likely
>be a couple of capacitors and a couple of regulators. Likely a 7805 type
>with maybe a 7905 if it also has a negative rail.
The 12V 1A rating is the maximum it's rated for. For example, it's common
for a 120V circuit breaker to be rated for 15A, that doesn't mean your clock
will draw 15A, it just the most you can draw from that circuit. So, a 12V
1A supply can supply 12V at UP TO 1A, 0.001A should be fine as long as there
isn't a minimum rating on the wall wart. Your wall wart should be good for
supplying 12V from 0 to 1A. Using 0.5A, no problem, trying to use 2A will
give you problems.
Supply verses demand.
The wart can supply an amp.
The router only draws 1/2 amp.
Live is good.
If the draw approaches the supply's capability things get if-ier.
At some point the draw will pull the voltage down below spec.
At some other point that will really suck.
Think of voltage as the Pressure.
Current is the flow based on the pressure and the resistance to that
In other words - the voltage sets the current flow on a load.
The router uses 12v and has a resistance (simple terms here) that
draws 1/2 an amp - so the wall wort runs in idle.
you might have a faster starting speed - that might not be good.
It might not matter. Start current is higher than run current.
The wart can handle the higher currents when starting and when loaded.