I had some trouble with setting up a transformer for my low volt garden
lighting. Here's what I learned:
The small (size of a pack of cigarretes) $25 12v transformer is an
ELECTRIC transformer that must be
mounted indoors and is easily damaged by moisture, the large (size of a
shoebox) $100 12v transformer is a MAGNETIC transformer suitable for
mounting outdoors and
more weather resistant.
The reason the small one blew my breaker is because the small wires go
to the 120v
wall plug & the fat wires go to the 12v lighting. The smaller voltage is
more sensitive to voltage drop thus the larger wires. The 120v leads
were really skimpy, less than most electric appliances.
They said the power usage when the transformer is plugged in is trivial
so it's not really necessary to switch the 12v side. The small
electronic type does not have a delayed startup like the large magnetic
one so you could switch the 120v side if you want to save a tiny bit
more energy and don't mind operating everything on one switch.
Having a too-small transformer isn't a big problem, no loss of
efficiency but if you only have say 10 watts of lights on a 150w
transformer it might provide irregular supply however they said a 40w
draw on a 150w transformer was no problem.
Just thought y'all might find this helpful.
The lower voltage (12 volt) wires are larger diameter because they have
higher current in them than the 120 volt wires. Almost 10 times as much
Not necessarily true for inexpensive transformers. Feel the case when
it has been plugged in for a while with no load and see if it's warm.
If it is the current isn't trivial. Most inexpensive transformers will
have fairly high magnetizing currents. This is largely reactive
current, but it's still current and is still going to result in core
losses and resistive losses in the winding...all these losses are real
and cause heating.
but if you only have say 10 watts of lights on a 150w
60 Hz is the abbreviation for 60 Hertz (formerly 60 cycles per second)
the frequency at which the voltage alternates polarity. Magnetics are
Both are electric. Nothing you've said indicates the quality of the
unit. The distinction between "magnetic" and "electric" is not there.
By definition a transformer is magnetic. It is also electric. Energy
from one port is coupled to the other by means of a magnetic field. The
heating of the larger transformer under no-load conditions is because of
the magnetizing current. This heating is frequently significant in a
cheap, not necessarily inexpensive, transformer.
Mmmm there must be some distinction. Can you clarify? All I get is that
my info was wrong but I don't know anything further.
What is the significance of a hertz?
What is the difference between the large and small transformers?
How closely should the wattage match? I may have several fixtures on one
transformer and switch them separately. Is it necessary to have a bunch
of 60w transformers switched on the 120v side?
These are not solid state devices.
Transformers are transformers, big or small. They're stacks of iron
plates with wire wrapped around them, and they're all relying on "magnetic"
induction to transform power from one voltage to another. That's what the
definition of a transformer _is_ (at least within this context).
The only differences between a small interior "wall cube" transformer and
a large exterior-rated transformer is that the former is smaller than the
latter, and the latter is rated for out doors. Also, the former will
_generally_ not be as robust as the latter - mass production/engineering
shortcuts/cost tuning etc. etc. etc.
A "perfect" transformer consumes _no_ power when there's no load being drawn,
and runs perfectly cold even under full load, so the transformer wattage is
irrelevant as long as you have enough.
But nothing is ever perfect.
All things being equal, then, the more excess wattage capacity in the
transformer, the more power you waste. But the effect is usually fairly
small. And between small mass-produced wall cube transformers, and larger
LV lighting purposed transformers, there's bound to be an efficiency
The main issue simply is ensuring that the power transformer has enough
power for the load, and preferably a bit extra. That 60W transformer
driving exactly 60W worth of lightbulbs is, when compared to a 90W
transformer driving 60W of lightbulbs:
(a) going to run a lot hotter
(b) probably have a shorter lifespan
(c) be FAR more likely to trip its integral breaker due to
power glitches/spikes/manufacturing tolerances and other
Should you switch the 12V or the 120V? Well, it's probably better to switch
the 120V AS LONG AS you leave the bulbs on for hours at a time. A bit less
energy wasted in heating the transformer when there's no lights on, but there's
on/off stress on the transformer as well as the bulbs. However, in most
cases LV lighting switching (eg: motion or light detectors) are almost always
on the 12V side - less on/off stress on the transformer.
Tradeoffs all over the place, depending on individual units, installations
and usage habits, so it's impossible to tell ahead of time exactly which one
The only definitive things you should keep in mind is: choose good quality
units, and make sure you have 10-50% more capacity in the transformer than you
need right away. Besides, it makes it easier to add a few lights later.
Chris Lewis, Una confibula non set est
It's not just anyone who gets a Starship Cruiser class named after them.
Thanks! The little wall box is cold when it's not carrying a load. The
big one stays rather warm. It seems there must be some difference in
design the way they described it and given the huge difference in size.
Chris Lewis wrote:
Hertz, a proper noun, is the SI (International System of Units) unit for
frequency named for Heinrich Rudolf Hertz.
The difference is the size, and possibly the amount of power that can be
handled by one vs. the other. There is no such thing as a sold state
transformer. Look at a dictionary for the definition of the word
The "wattage" specified on a transformer is the maximum that can be
handled. Lower loads are no problem.
I may have several fixtures on one
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