01-17-2012, 07:28 AM
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Join Date: Apr 2005
Location: Nova Scotia,Canada
Originally Posted by KM3F
If anyone is interested in why this conditon exists, here it is.
It depends on where in the AC cycle you last turned the power off.
This can leave the core magnetized.
The next time it is powered up the core presents a very low impedence to the AC input and causes it to draw more than normal current along with charging the filter and pulls on the metal cabinet hard. This amplifies the noise and moves anything that will move and contributes to shortened switch life.
This is why it does not happen every time.
The solution believe it or not is 'soft start' just like an amplifier has.
They are easy to make and install.
A SPST relay with contact rateing of at least 10 amps and a 25 ohm 25 watt resistor wired for a second or less time delay before the full application of AC power to the primary winding, will quiet it down.
That takes care of the start up but sometimes they will hummmmmm like a bugger during standby and get worse under load. It is for the same reason, circulating magnetic currents in the transformer, and can generally be cured by tightening all the bolts and mounting the transformer on a piece of rubber between it and the chassis. The soft start is a good idea on bigger supplies and I have been meaning to install one in my 40 amp Tripplite but keep procrastinating. Several years ago I had to replace the filter capacitor bank and decided that the 100,000 uF can I had would be good. Man that beast thumps like a bugger when I turn it on. Start up current must be through the roof.