EEVblog Electronics Community Forum
Electronics => Beginners => Topic started by: little_carlos on April 28, 2016, 04:14:50 am
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Hi, so i made a 3 amp 30 volt psu, every thing was fine, was able to handle 2.8 amps at 30 volts no problem, but these days something weird is happening, when i put a load bigger than 1.8 amps the transformer starts to make a hum, didnt used to do it before, its weird, can some one explain me what is happening? should i replace it?
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A transformer is not a solid block of iron. Instead it is made of several thin, preshaped pieces called laminations., which means that there is always an airgap between those laminations. As the load increases, the transformer has to 'work' harder and the laminations begin to vibrate, producing this hum.
Good transformers are dunked into shellac, which penetrates, or at least should do so, these airgaps which in principle should dampen these vibrations and thusly the hum.
As long as the transformer stays warm, there is no concern. However, if the transformer gets hot you should be thinking of replacing it.
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Excessive hum can be a sign of saturation. Check your power supply for shorts or overloads. There could be a bad solder joint, failing cap, whatever. Check for current draw at the secondary side, with all due cautions.
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Check the rectifier diodes. If one has gone O/C, you'd only be getting half-wave rectification, drawing pulsed DC from the secondary which would cause saturation and hum at a significantly lower current than the normal full-wave rectification which draws a balanced AC current.