Discussion in 'General Electronics Chat' started by worldHello, Oct 21, 2014.

1. ### worldHello Thread Starter New Member

Oct 15, 2014
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Today my professor told me I can flip a step-down transformer to make it a step-up transformer. I'm skeptical ...

I have a 100:1 transformer, and he said I can make it become a 1:100 transformer by flipping it?

Really? Then what's the point of labeling primary and secondary?

2. ### ISB123 Well-Known Member

May 21, 2014
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So that you dont kill yourself.Thats why.Basically primary side has high Voltage and low current while secondary has low voltage but high current.

3. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
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While very important the answer goes beyond purely safety issues.
Firstly, not all transformers are step down with a higher primary and lower secondary voltage. One might have a 110V to 220V transformer so that equipment designed for 220V operation could operate off a 110V supply through the transformer.
The various windings will also have a limit to the voltages which can be applied to any of them without potential damage to the transformer. One could reverse a 110V:24V transformer but the 24V winding (now "primary") would be limited to 24V input. One would obtain 110V on the reversed configuration output side. However, connecting an input of 110V on the reverse configured 24V secondary (now "primary") would be a potential recipe for disaster - probably resulting in the winding overheating and potentially burning out. Hopefully the supply fuse or protective device would "blow" to circumvent the problem.

Last edited: Oct 22, 2014
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4. ### MrChips Moderator

Oct 2, 2009
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In theory, step-up or step-down is determined by the turns ratio of the windings.

In practice, one has to take the inductance of the windings and magnetic flux coupling into account in order to arrive at a practical transformer.

5. ### Vinetou New Member

Sep 5, 2014
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Your teacher is right. Make this little experience: hold the primary wires and ask somebody to touch the secondary wires on the poles of a 1.5V cell. You will get an electric shock.

But bear in mind that energy is not created from nothing. In theory, the power in transformers keeps steady. In your example if you have a current of 1A and a voltage of 100V on the primary, this gives us a power of 100W (P = V/I). Now on the secondary we have 1v (100:1 ratio) and the power must be the same 100W, so the current will be 10mA (I = V/P).

6. ### alfacliff Well-Known Member

Dec 13, 2013
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actually the current available on the secondary would be 100 apmps. one amp at 100 volts on priaty, 1 volt at 100 amps on secondary, same power ( minus a little loss).
the steput voltage when reversing a transformer is limitted by the insulation in the windings.
cliff

Oct 2, 2009
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P = V x I

8. ### Vinetou New Member

Sep 5, 2014
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You're right, MrChips and alfacliff. I'm really sorry for that. I have mixed the equations up. Really sorry.

9. ### Johann AAC Fanatic!

Nov 27, 2006
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In short: The primary is where the supply is connected to and the secondary is where the load is connected to. (As long as the voltages match the windings requirements).