# Driving servos using microcontroller and current limiting

I am new to electronics and would like to design a PCB that uses an Atmel microcontroller, and powers three servos. I was thinking of designing the PCB with a single power supply.

The servos I have can draw up to 500mA of current each, when fully loaded. If I use a 5V 1.5A supply (VCC), and connect the servos and the microcontroller in parallel to VCC I understand all sources will get 5 volts. But will they each only draw the current they need?

The Atmel microcontroller datasheet says it should only take a max of 200mA, so would I need to place a current limiting resistor? Or do I need to create a current divider circuit to fix the current flows to each unit? But the motor current draws can change depending on the load and it seems like it might be wasteful (?) to have resistors burning power if its not always necessary.

This seems to be a very common type of question.

In general, a power supply is rated for the voltage that it will deliver, and for the maximum current that it CAN deliver at that voltage. It is up to the load (the circuit that is powered) to draw the current it needs.

So, when you have a circuit that states that it needs 5V 1500mA, you need a power supply with a rating 5V, 1500mA OR MORE mA.

I doubt that a uC will use 200mA under normal circumstances, what you read was probably under the infamous ABSOLUTE MAXIMA section. But it will do as a safe maximum, so 1500 mA should be enough for your circuit. BUT I would feel a bit uneasy about powering a uC and two 0.5A servo's from the same 5V supply. I would go for separate 5V supplies, or if that is not practical, some other means to protect the uC from voltage drops. Maybe a Schottky diode + capacitor.

• Thank you. I think I will design two power supplies. My understanding is that each component will only draw what it needs.
– Jon
Dec 25 '12 at 19:55
• +1 - 200mA definitely sounds like the absolute maximum rating. This is a very common type of question - here is the question/answer(s) which attempts to cover most of the details on choosing a power supply Dec 25 '12 at 20:07
• Some bulk capacitors at the load points (servos and uC) with a Shottky will definitely help with voltage drops. I've dealt with that before, except it was 19 hobby servos off of a 4xAA pack. One for the uC after the Schottky is especially important, we first tried bulk caps at the power source but they drained way too fast to matter. Dec 26 '12 at 21:06