How the step down mini transformers operate in too small current in the primary


For electronic circuits I use step down mini transformers. That can transform the residential 230V AC voltage nearly 14V.
I know that (IV)primary = (IV)secondary. That means the secondary current should much higher than the primary current. I measured the currents in my electronic circuits are in mili range. That means the current in the primary must be in mili range and lower than the secondary current.

if Vprimary = 230V, Vsecondary = 14V then K = 14/230 = 0.061
Now, if I assume that Isecondary = 1Amp then the Iprimary = 1*0.061 = 61mA

That is too small. Moreover, the secondary currents are usually less than 1Amp. In this case the current in the primary should smaller.

It seems somewhat awkward to me that our residential main can supply too tinny current??

Best Answer

A transformer moves power from one location to another, so if you have a transformer supplying, say, 1000 watts to a load from its secondary, then its primary must capture that 1000 watts from the mains and transfer it to the load through the secondary.

Assuming that the 1000 watt load draws 10 amperes from a 100 volt secondary means that with 240 volt mains supplying the primary, the mains current into the primary must be:

$$I=\frac PE = \frac{1000\text{ watts}}{240\text{ volts}} = 4.17\text{ amperes}$$

Interestingly, if the load draws 50 amperes out of a 20 volt secondary, that's still 1000 watts and the primary must still draw 4.17 amperes from the mains, as earlier.

In your case, with a 14 watt load drawing 1 ampere from a 14 volt secondary, the current required into the primary, from the mains, will be:

$$I=\frac PE = \frac{14\text{ watts}}{240\text{ volts}} = 58\text{ milliamperes}$$

so you can see that rather than just being able to to supply a tiny current, the mains will supply whatever current the load requires, up to the mains' limit.