Is 125 degrees F too hot for a plug-in transformer??

edweatherNovember 18, 2011

To make a long story short, I am using a plug-in 120V to 24V 20VA transformer to power something that requires 24V 24VA. Is that why the transformer is running hot? In other words, if the device needs 24VA, is it trying to draw more than the 20VA transformer will supply, therefore making it hot? Is this ok? I'm assuming that there is some kind of fuse inside the UL approved transformer to prevent fire. I put a thermometer on it and it shows 125 degrees. Am I better off by upgrading to a 40VA or 50VA transformer? Will that run cooler? Thanks in advance.

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bus_driver

Most such transformers do have a feature that fails if the temperature gets too high. The failure, if it happens, is permanent. Not a replaceable fuse. No smoke, no noise when it happens. I do not think that 125 deg is excessive. But loading the transformer like that does drop the output voltage below specifications. Is the load adequately powered at present? Is absence of failure critical? If not, I would use it until it fails and then use the 40VA unit.

    Bookmark   November 18, 2011 at 7:42PM
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edweather

Thanks........the load is adequately powered at present, and absence of failure is not critical.

I was just a little nervous because I could actually smell the hot transformer (maybe because it was new), and it felt really hot to the touch. I have no problem using the transformer 'til failure. Thanks again.

    Bookmark   November 18, 2011 at 8:33PM
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inox

I used the wrong transformer (which supplied too little voltage) to a Verizon DSL modem, and Verizon explained to me that the modem, seeing too little voltage, drew too much current. It was fried in seconds. Fortunately, a FiOS installation was able to be scheduled to make up for the mistake.

    Bookmark   November 18, 2011 at 9:54PM
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brickeyee

Some loads will pull extra current if the voltage is low (and then burn out) while others simply will not operate.

An overloaded transformer will produce lower voltage, and this can lead to the transformer failing or the load failing.
It is not usually all that hard to find the correct rating of 'wall wart' type supplies.
You can go larger than needed (in V-A or amps, not voltage), but should avoid smaller

Look around DigiKey

    Bookmark   November 19, 2011 at 9:22AM
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edweather

Since I'm pretty much electric ignorant, I thought I was doing a good thing by undersizing the power supply by a few VA's, not wanting to ruin the device that I was powering. But as usual, when I'm learning about something, I figured out that it's exactly opposite, and I should have gone with more VA's, not less. The voltage is correct....that much I did know. So for the mean time I'll just live with a warm transformer and continue to shop for a bigger one. Thanks all for the responses. Ed

    Bookmark   November 19, 2011 at 2:05PM
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brickeyee

"undersizing the power supply by a few VA's,"

As long as the voltage is correct the device will not draw more current than it needs.

volt-Amps are like watts.

You should not use a larger wattage bulb in a fixture than it is marked for, and you should not overload a transformer with more VAs than it is designed to deliver.
Both can result in overheating ad damage.
As long as the voltage is correct and of the correct type (AC or DC) a device will not pull more current than it needs to operate.

    Bookmark   November 19, 2011 at 3:05PM
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edweather

Thanks brickeyee...& others.

OK........right now I AM overloading the 20VA transformer with more VA's (24VA) than it is designed to deliver. I'm assuming that's why it's running warm.

So.....if I went with a transformer that supplied, let's say, 40VA, then the 24VA device that I'm running would be able to draw it's 24VA easily and the transformer would run cool as a cucumber.....right?

((A 24VA transformer would be perfect, but I accidently threw out the one I had, and the common available replacement sizes available are 20VA, and 40VA.))

    Bookmark   November 19, 2011 at 7:31PM
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dennisgli

A 24VA transformer would be perfect

Why would a 24VA be perfect??? Buy the 40VA - that's what you want.

    Bookmark   November 20, 2011 at 8:45AM
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brickeyee

"So.....if I went with a transformer that supplied, let's say, 40VA, then the 24VA device that I'm running would be able to draw it's 24VA easily and the transformer would run cool as a cucumber.....right? "

yes.

And have a better chance of producing its designed output voltage.

    Bookmark   November 20, 2011 at 10:01AM
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edweather

OK.....beautiful.......40VA it is! The only reason I said that 24VA would be perfect is that's what it originally came with.

Thanks again. Great forum.

    Bookmark   November 20, 2011 at 1:53PM
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brickeyee

The entire power in your house is fed from a large pole mounted transformer (usually at least 7,200 V on the primary side with a 240 V center tapped secondary).

Just as with the power in your house, a load pulls power based on its impedance and the supplied voltage.
The final rating of the pole transformer V-A is VERY large.
It often is supplying multiple service drops at 100 to 200 amps EACH.

Just as with your small wall wall wart transformer it supplies the current demanded while limiting the output voltage to a narrow range.
Just like you wall wart the voltage does drop as the loan increases, but the POCO takes pains to make sure the transformers are large enough that the voltage you see at the drops stays within allowed limits (typically about +/- 10% of nominal).

If you actually measure the voltage in your house a couple times during the day you can often see the variation as people use more power during the day than at night typically.

    Bookmark   November 22, 2011 at 10:03AM
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edweather

I upgraded to a 40VA transformer, and it's runs awesome. It barely gets warm! No more scary hot plastic smell! Thanks forum.

    Bookmark   December 15, 2011 at 10:38PM
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