I have a low voltage transformer that was in the house when I bought
it. It doesn't doesn't have any exterior markings at all, so I'm not
able to know the wattage rating for the unit by looking at it. Is
there any way to determine the watt output of this unit?
Why do you say it's a low voltage xformer? was it connected
to your house mains? Where'd the output go? Doorbell?
thermostat? Outdoor lights? Or was the thing just laying
unconnected in a closet? by googling on transformer+intended use, you might
come close to the power rating.
Or put slowily increasing load on the output monitoring the
voltage, temperature, and 'hum', you might get an idea.
There are probably markings on it that may be difficult to see. The two most
typical transformers found in a house would be 24 volt heating transformer,
usually 40 watts, and 8-16 volt bell transformers, usually 10 watts
On Fri, 14 Sep 2007 19:11:37 -0400, "RBM" <rbm2(remove
I still have a transformer I found in an old house. It was connected
to the black and yellow wires of the inside phone wiring. It was made
by Western Electric and is marked "105-125V 60Hz sec: 6-8V 1.75VA".
If it has "AULT" marked on it, DESTROY it, a well known fire
hazard! (was used to provide lamp voltage to "princess" and
Also, the typical design of small transformers is for 4%
Apply 120VAC to primary
Read no-load secondary voltage
Load secondary (with resistive load) until secondary voltage
Calculate or read the secondary current, that should be a
You can also use "degree rise" to be sure the transformer is
safe for your load. A thermo couple type thermometer (now
found in cheap digital vom's) probe can be placed inside the
core area. Read the "cold" temp(no power applied), then
operate your transformer and load. If the temp rises 50F, or
over 140F, get a larger or better transformer. The UL & NEC
spec is higher, but I'd rather be conservative, and not
waste money heating a low quality transformer.
The 10% regulation mentioned is ok for low quality
transformers used for short periods of time. But I would do
the temp rise test if its used for long. Also, none of this
is about anything using a switching supply or has a DC output.
-larry / dallas
Not much info to go on!
Assuming it is the type to be connected to 115 volts AC @ 60 hertz?
It's weight (i.e. the amount of metal) may give some indication.
As others have indicated:
1) Something the same weight as small front door bell/chime
transformer can probably handle 6 to 10 watts, but that may be
intermittently (cos. door bells not expected to ring for very long!).
2) If it is an old fashioned (now not to code in some jurisdictions)
shaver transformer they are usually rated at a maximum of 7.5 watts.
Again short term use, cos it typically doesn't take long than say 5
minutes maximum to shave!
3) If it's say a 24 volt transformer for heater thermostat/control
maybe 20 watts? Low current (probably less than an amp for operating
relays etc.) although a fairly continuous duty rating.
4) Might be better to post a description? Where was/is it located? Was/
is it hooked up to anything, if so how wired? Picture?
You could measure the no-load output voltage and load it down until
the voltage drops about 10-20%, but if you don't have a load resistor
you'll need an amp meter as well as a voltage meter, but many
multimeters, including digital ones, won't measure AC amps, only DC
Or you could look at some bare transformers at Radio Shack to see
which ones are closest in size to yours. Multiply their voltage and
amp ratings together, then divide the product by the voltage of your
transformer to get a rough idea of its amp capacity.
probably a door bell transformer and the OP is looking to get chimes.
which take both more current and output voltage
just buy a new matching transformer they are cheap and easy to
plus new ones are over heat protected. old ones could short and cause
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