Taming mains voltage - diyAudio
 Taming mains voltage
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 16th September 2007, 06:42 AM #1 diyAudio Member   Join Date: Oct 2006 Taming mains voltage I am running around 123VAC into a 125-0-125 trafo and a 6.3VAC trafo for an Aikido amplifier. Both these trafos are rated for 115V on the primaries. As a result of the higher input voltages, the 125-0-125 puts out 299VAC primary to primary, and the 6.3VAC trafo 7.03VAC. Based upon current draw, I have come up with the following calculations to estimate what size resistor I would put on the primary side of these trafos to reduce input voltage by 5V (This would ensure that when our mains voltage drops to 120V, the trafos would see 115V and not be underpowered). 125-0-125 VAC: 1. B+ is 312V into 18mA = 5.61W, rounded up to 7W (as an over estimate). 2. Line current would then be 7W / 123VAC = 56mA. 3. Required voltage drop is 5V = 5V / 0.056A = 89.3R 4. Resistor power req. = 5 x 0.056 = 0.28W, which may as well be rounded-up to 2W or 5W resistor. So around a 90R resistor that is rated to 2W or even 5W (this would be overkill). For the filament trafo: 1. Power is 7.03V x 2.4A (four x 6mA) = 16.8W so 20W for a little extra margin. 2. line current 20W / 123VAC = 0.16A 3. Req voltage drop of 5V = 5V / 0.16A = 31.25R 4. Resistor power req = 5 x 0.16 = 0.8W, rounded up to 2W or even 5W. So, a resistor for the filament trafo of 30R rated at 2W or 5W. Does my math seem OK? Also, do I need a resistor on each primary (both transformers have two primaries)? If so, would I use half the total required resistance on each primary? My hope is that by taming my mains voltage, I can reduce the slight buzz and heating that my transformers undergo. It is also a cheap fix, as resistors are cheap. Thanks, Charlie
 16th September 2007, 04:50 PM #3 diyAudio Member   Join Date: Oct 2006 Andrew, You have left me a little confused here. All I am trying to do is to reduce mains input voltage that the transformers see, as they are rated for 115 and get more like 123. After the trafos, my PSU works, with the exception of a slight hum that is due either to some ground loop, or the buzzing of the transformers. I have put in a lot of effort to remove ground loops (maybe not fully successful), but a couple of resistors on the primaries may lower the high voltage seen by the trafos and make them quieter, at least, this is my rationale. Charlie

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