How do you calculate the total mA needed to power 3 9v DC 200 mA devices?
Separate devices should always be hooked up in parallel, and not
series. Therefore, you would combine the total current requirement,
which in this case is 600mA or 0.6A. You would therefore need a 9V
power supply that is capable supplying at least 0.6A.
It depends on whether the devices are connected in series or
parallel or some combination of the two.
The current in a series circuit is the same throughout the
circuit. The current in a parallel circuit is the sum of the
current in each parallel branch
If the three devices are connected in series, the required
current is 200mA.
If the three devices are connected in parallel, the required
current is 600mA
The total power required by a circuit is the sum of the power
consumed by each component. It doesn't matter if the devices are
connected in series or parallel. Power = voltage x current.
If the three devices are in series, the power consumed by one of
them is 9 x .2 = 1.8 watts. The total power consumed is 1.8 x 3 =
If the three devices are connected in parallel, the power
consumed is 9 x .6 = 5.4W
Notice that the power consumed is the same for the series and
parallel combination. The battery (or power supply) must be able to
supply 5.4 watts.