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Bucking Transformer
... I am shocked, shocked ...
WARNING: Lethal Voltages
Mains Voltage
120V became the U.S. standard in 1967. 50+ years on, line voltages are still all over the map. The standard line voltage tolerance is ±5% for a range of 114 to 126v.
Many power transformers are rated at 115 [≈109-121] or 117 [≈111-123]. Many manufacturers rate equipment @ 117V for the North American market.
Generally, not a problem for most equipment. However, with antique equipment or faithful retro designs, tolerances may add up to close to the ragged edge and leave very little safety margin. Antique equipment may be particularly vulnerable due to less robust insulation and materials then available.
One solution is a variable transformer. This will allow precise control of the line voltage at the moment it is set. However, line voltages fluctuate continuously. Cheap units can have short lifetimes, poor switches and questionable internal wiring. In all cases, they are not particularly attractive.
Variable transformers can also INCREASE the voltage with potentially disastrous results!
NOT recommended where the unknowing may have access.
Many power transformers are rated at 115 [≈109-121] or 117 [≈111-123]. Many manufacturers rate equipment @ 117V for the North American market.
Generally, not a problem for most equipment. However, with antique equipment or faithful retro designs, tolerances may add up to close to the ragged edge and leave very little safety margin. Antique equipment may be particularly vulnerable due to less robust insulation and materials then available.
One solution is a variable transformer. This will allow precise control of the line voltage at the moment it is set. However, line voltages fluctuate continuously. Cheap units can have short lifetimes, poor switches and questionable internal wiring. In all cases, they are not particularly attractive.
Variable transformers can also INCREASE the voltage with potentially disastrous results!
NOT recommended where the unknowing may have access.
Bucking Transformer
Click thumbnails for a larger image
An alternate solution is a Bucking Transformer. While fixed, it is essentially foolproof.
For an excellent article on Buck and Boost Transformers, see Elliot Sound Products' Bucking (And Boosting) Transformers. Yes, you read right. If you have low consistently low voltage, it is possible to boost to nomimal. Sadly, no hope for Californians when they shut the power off.
It works by wiring the primary and secondary out of phase. The primary opposes the voltage in the secondary. Simple math 122-5 = 117. Using a center tapped transformer gives two choices -5V and -2.5V. See drawing below for a method to determine polarity.
For an excellent article on Buck and Boost Transformers, see Elliot Sound Products' Bucking (And Boosting) Transformers. Yes, you read right. If you have low consistently low voltage, it is possible to boost to nomimal. Sadly, no hope for Californians when they shut the power off.
It works by wiring the primary and secondary out of phase. The primary opposes the voltage in the secondary. Simple math 122-5 = 117. Using a center tapped transformer gives two choices -5V and -2.5V. See drawing below for a method to determine polarity.
The unit shown has a nominal rating of 1500W. Reduction is adjustable: ≈-5V, -2.5v and 0V. Size the secondary for the amount of current you want to supply and the voltage to drop.
My amps draw 5A at idle, about 10A at full power and that for only very brief periods. Hence, 15A secondary. Transformers are rated in VA so 5V @ 15A is 75VA.
My amps draw 5A at idle, about 10A at full power and that for only very brief periods. Hence, 15A secondary. Transformers are rated in VA so 5V @ 15A is 75VA.
IF YOU ARE NOT WELL SKILLED IN READING SCHEMATICS, ELECTRONIC DESIGN AND WORKING WITH LETHAL VOLTAGES DO NOT ATTEMPT! Severe INJURY or DEATH may occur. It may be illegal in your jurisdiction.
Click on the images below for Polarity Determination, Mechanical, Schematic, Wiring and Parts List. Right Click to Save as...
