# transformer voltage reduction

Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.

#### goodguys

Having difficulty finding a toroid transformer with the correct secondary voltage, so am wanting to mod a transformer to lower the secondary voltage. If i remove turns from the secondary winding what will this do to the voltage will it go up or down.

#### johnego

It will go down of course.
Primary_Voltage/Secondary_Voltage = Primary_Turn/Secondary_Turn
SV=PV*ST/PT-->Lower ST (Secondary Turns) will lower SV (Secondary voltage)

If the power line input has 220V and 240V, you can choose 240V so the output is a bit lower than if you connect to 220V. This is because the 240V has more turns than the 220V.
SV=PV*ST/PT--> Higher PT (Primary turns) will lower SV (Secondary voltage)

#### PANN

DOWN but,

calculate first the ratio between P/S so,
you can estimate the number of the turns need to remove,
turns / volt.
I have done it many times, difficult job because its hard to remove the first E I.
Use a thin knife and be careful dont destroy the core.

#### johnego

I have done it many times, difficult job because its hard to remove the first E I.
Use a thin knife and be careful dont destroy the core.
His is a toroid I believe. The secondary turns are on top of the primary turns. It should be easy unless you want to make it look good

#### zigzagflux

Consider a small little rectifier transformer connected as an autotransformer in bucking polarity. It will take the primary voltage (if 240) and drop it down to where you want it, perhaps 210V. Use that to feed the toroid. The transformer is really small when connected as an auto with a small step down; works great too.