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Physical Reason For Reduced Current In Step Down Transformer 
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#19
Apr314, 11:51 AM

P: 10

How is Ohm's law being satisfied? I=11.5/2.75=4A. But you're getting 40A. I'm confused.



#20
Apr314, 01:30 PM

P: 642

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. 


#21
Apr314, 02:12 PM

Sci Advisor
PF Gold
P: 3,664

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. 


#22
Apr514, 09:27 AM

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 105 work just as well as a 2010?
Thanks again for the advice, Luke 


#23
Apr514, 12:29 PM

Sci Advisor
PF Gold
P: 3,664

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 ampturns. Good luck old jim 


#24
Apr514, 01:47 PM

P: 642

http://www.giangrandi.ch/electronics/trafo/trafo.shtml This is about the size of transformer you will need for full power operation: http://www.swgr.com/store/drytypel...FdKGfgodlwADA 


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