How to calculate amp for a power supply?
Please pass this on.

Dear cousin,

The power supply is converting power from one voltage to another. The relationship between power, voltage and current is given by the formula \$ P = VI \$, where P is power in watts (W), V is voltage (V) and I is current (amperes, amps or A).

If we step down the voltage from 230 V to 9 V, a factor of about 25, then we would expect the current on the high voltage side to be 1/25 of that on the low-voltage side. The label says it will supply 900 mA (0.9 A) so input current should be about \$ \frac {900}{25} = 36~mA \$.

The manufacturers have specified that it could draw up to 100 mA on the input (from your wall socket). That means that one or more of the following is true:


The power supply is very inefficient (giving out only 36% of what it takes in).
The manufacturer has recorded the switch-on surge current. This would be very brief.
They've just picked the nearest standard fuse value above their required input.


Who knows?

Signed,

Transistor.