Taming mains voltage

[cbutterworth]

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

[AndrewT]

Stop.
the voltage you have measured is the open circuit voltage of the transformer. It show regulation is about 12% this is normal for an EI type.
The mains voltage is about 7% high when you measured it. That is probably a bit high if your nominal supply is 115Vac, but you are probably adjusted at the transformer for a nominal 120Vac.

Multiply 7% and 12% together (1.07*1.12) and apply that to your rated secondary voltage and you get 299Vac.

When you load your transformer the voltage stored on the smoothing cap will be much lower than square root(2)*299.

Do not put a resistor in the primary. Measure your DC voltage and maybe add a series resistor to the secondary before the main cap to give an RC supply.
If you have cascaded CRC or CLC supply then you have the option to add a small resistor before the first cap and another before the second cap. This method of adjusting the DC voltage gains the advantage of reducing the mains hum/buzz on the DC supply.

[cbutterworth]

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