# Step down autotransformer calculations

**If they are called autotransformers, do they hate Decepticons?**

# How to calculate a step down auto transformer.

I actually find these calculations quite fun. The trick to auto transformers is remembering the cardinal rule of transformers, which is, KVA in = KVA out.

# What is an auto transformer?

There are transformers that share a magnetic circuit (the same flux), but do not share an electrical connection. Auto transformers share both an electrical and a magnetic circuit. They are great for stepping up and stepping down small voltages ie. 240- 208 V (or vice versa) or for lighting ballasts.

# How do they work?

Remember that transformers work off of a ratio of volts per turn.

If you tap off a certain amount of turns on a single winding you will get the same percentage of voltage as the percentage of turns you tapped off at.

Once you have the “secondary” voltage, the voltage at the tap, you can calculate the secondary current and power. Using the secondary power you can determine the primary power.

Remember that KVA in = KVA out, so what ever the load is asking for the primary has to give. Once you have the power on the primary you can determine the primary current.

VA divided by voltage is equal to current yo! So now we have the primary current and the secondary current the only thing left to figure out is the common winding current. This is easy breezy. Take the difference of the primary and secondary currents and what you are left with is the common winding current. Booom!!!

# Is that explanation for step up or step down transformers?

The same principles apply to both step up or step down. The only difference is, with step down you take the supply voltage and multiply it by the percentage of the tap off point. With a step up transformer you take the supply voltage and divide it by the percentage of the tap off point. Every thing else is the same.

# HUH????

Look, just watch the video. If you still don’t get it, shoot me an email.

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calebMarch 22, 2016 at 10:05 amThanks much.this post is helpful……

ChadMarch 22, 2016 at 12:34 pmNo worries. I’m glad it helped Caleb