Adjusting Transformer Voltage with an Autotransformer

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Sometimes a project will require a transformer with a secondary voltage that is difficult to get or perhaps there is a transformer in your junk parts box that is a little too high or too low. Some equipment, especially vintage tube gear, is best operated at voltages lower than currently provided by the power company. An autotransformer is a simple and inexpensive way to correct for these problems.

So where do we get an autotransformer. Easy! Any conventional transformer can be externally connected as an autotransformer.

Let’s use a 6.3 volt filament transformer as an example.

The two solid dots on the leads indicate the relative phase of the signals. Leads with the dots are in phase with each other. Most of the time, transformers do not display these dot markings. Manufacturer connection diagrams usually do. If the dots are not available, we can easily work around that.

The percentage of secondary voltage to primary determines how much the voltage can be changed on the subject transformer. In this case the percentage is (6.3/120) x 100 = 5.25%. A 6.3V transformer can then alter the secondary voltage of any transformer by +/- 5.25%. If the primary of the 6.3V transformer was 240V, then the degree of change would be 2.6%. If the autotransformer is used to alter the power line voltage, then its secondary voltage changes the line voltage by the same amount as the secondary rating, in this case, 6.3V.

If the desired change is to reduce the line voltage, connect the autotransformer in this fashion:

The secondary voltage is out of phase with the primary and its voltage subtracts from the primary.

Many times, connection diagrams are not available. No problem. Just connect one of the secondary leads to the primary and measure the voltage between the other two leads. If the voltage goes down, then you got it right. If the voltage goes up, then just reverse the secondary leads. If a voltage increase in desired, then connect for an increase in voltage.

The autotransformer must have a secondary current rating at least equal to the primary current rating of the subject transformer. Simply, a subject transformer with a primary current draw of 5 amps should be used with an autotransformer with a secondary current rating of 5 amps or more.

When an autotransformer is used to boost voltage over a subject transformers design rating, limit the total VA of the subject transformer to its specified rating. That is, if you boost the voltage 10%, lower the subject transformers secondary current rating by 10%. Transformer VA is equal to (voltage x current) of all of the secondary windings.

Don’t boost any transformer’s voltage by more than 20%.

Conversely, if the subject transformer’s primary voltage is reduced, do not increase the current rating of its secondary. Total VA is then reduced by the percentage of voltage reduction.

Always connect the autotransformer after the power switch and fuse of the equipment it is installed in.