Determining the max wattage of a low-voltage transformer?

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?
Thank you.
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Ken wrote:

testing for some reason.
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Ken wrote:

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.
lee
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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

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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".
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Mark Lloyd wrote:

If it has "AULT" marked on it, DESTROY it, a well known fire hazard! (was used to provide lamp voltage to "princess" and "trimline" phones)
Also, the typical design of small transformers is for 4% regulation. Apply 120VAC to primary Read no-load secondary voltage Load secondary (with resistive load) until secondary voltage drops 4%. Calculate or read the secondary current, that should be a safe rating.
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
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If it did, I would have destroyed it more than 20 years ago when I heard about that problem.

I never had one of those phones (the transformer was left by a previous resident). I did have a lighted dial phone, but it was line-powered and lit up only when "off hook".

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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?
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Ken wrote:

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 amps.
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.
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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 replace.
plus new ones are over heat protected. old ones could short and cause fires
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