# Physical Reason For Reduced Current In Step Down Transformer

by jaus tail
Tags: current, physical, reduced, step, transformer
 P: 6 How is Ohm's law being satisfied? I=11.5/2.75=4A. But you're getting 40A. I'm confused.
P: 599
 Quote by jaus tail How is Ohm's law being satisfied? I=11.5/2.75=4A. But you're getting 40A. I'm confused.
That's his problem, he's NOT getting 40A @ 11.5V and even if the heater load resistance was adjusted downward to flow 40A @ 11.5v he still would not have the desired power level in his heater. The fundamental energy factors when dealing with the initial possible requirement of a power transformer is not current, it's desired power ,the resistance/impedance of the load and voltage level desired to power that load. The power and the resistance of the load determine what voltage is needed at that power level (the resistance can then be adjusted to the correct power level at that voltage). Once you have the voltage you calculate the current using the load resistance. From these basic facts combined with the supply power voltage and current capabilities you can begin to design a power transformer to meet your needs. In his case where the required voltage (115) is equal to the utility voltage (115) you don't need a transformer if the utility voltage circuit can also deliver the required amount of current.

Because we often use standard voltages (110/220) on utility circuits we commonly use current flow as a proxy for power as the voltage is assumed constant. This view of current flow (in isolation from other factors) as power is a common source of confusion when looking at circuits for beginners.
PF Gold
P: 3,518
 How does the secondary side affect primary side? From what I'm reading here it has something to do with the magnetic fields created by the windings?
yes, post 7 was my attempt to paint a visual picture of that. Draw yourself a magnetic circuit .

Original poster got it, as evidenced by his post 8.

Whatever current is allowed to flow in secondary will show up in primary also but adjusted by the turns ratio.
 P: 4 Thanks again for all the help, I have one last question though. I will be using 10 gauge wire for the primary side and 6 gauge wire for the secondary side. Considering the size of the transformer core I am working with I will be extremely lucky if I can fit a 20 turn primary and 10 turn secondary in there. I know the ratio of turns is important in determining the circuit values but how critical is the total number of turns? Will a 10-5 work just as well as a 20-10? Thanks again for the advice, Luke
 Sci Advisor PF Gold P: 3,518 A transformer core can only carry so much magnetic flux. Webers is the SI unit of flux. A more practical unit for the home experimenter is "volts per turn". Volts per turn at any given frequency is linear with flux. The bigger the cross section of a core the more flux it can carry, hence more volts per turn. The designers push a core fairly hard because iron is expensive. It'll have no more iron than it needs. Before you remove the primary, thread one turn through the core (maybe ten turns would be better) and find out how many volts per turn it makes. FIRST - if this is a microwave oven transformer (MOT): BE DARN SURE YOU HAVE REMOVED THE HIGH VOLTAGE SECONDARY ! NEVER energize a MOT that has not had its high voltage secondary physically cut off and removed ! MOTs are killing tinkerers. Knowing volts per turn lets you decide how many turns will be required to handle 120 VAC on your primary, and how many turns you need to make desired secondary voltage. As you have already figured out, core window area limits your wire size and amp-turns. Good luck old jim
P: 599
 Quote by ZeroPoint Thanks again for all the help, I have one last question though. I will be using 10 gauge wire for the primary side and 6 gauge wire for the secondary side. Considering the size of the transformer core I am working with I will be extremely lucky if I can fit a 20 turn primary and 10 turn secondary in there. I know the ratio of turns is important in determining the circuit values but how critical is the total number of turns? Will a 10-5 work just as well as a 20-10? Thanks again for the advice, Luke
Simple method of calculating mains frequency closed core power transformers:
http://www.giangrandi.ch/electronics/trafo/trafo.shtml

This is about the size of transformer you will need for full power operation: