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I Have connected 8 servo motors to 5V DC power supply as shown in the figure.

  1. When ever I switch on the power supply there is not enough power supply to servo motors. Voltage across each servo motor is 1.5 Volts only. Should the circuit must be changed?

  2. Is there any better way of circuit design?

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    \$\begingroup\$ Try just connecting one servo and check the voltage. If it´s ok, then add another etc. When the voltage drops more that 1V or 2V, you´ll know you´ve reached the limit and will need a bigger power supply. Otherwise, you have some other problem. \$\endgroup\$ – F. Bloggs Apr 19 '16 at 12:33
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The design is ok, the reason is that your power source is not able to provide enough current (as others have mentioned already). You need a 5V 16A power supply (it is a serious stuff, like a PC PSU, not a wall-wart) to provide full power for these servos in all circumstances.

It is a good idea to add 2200uF/16V electrolityte capacitor to the power, or a 470uF/16V at each motor power in if there is a serious wiring distance.

It is also important to consider that you do need thick wires on the PSU rails.

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Could be possible that your source cannot drive a lot of current which the servos usually need.I used 4 servos and they needed about 2 amps current.So check if you are really using a source which can supply the needed current too. If you are using a voltage source from the laboratory then don't limit the current to a less values.

I used this interface to genrate the PWMs https://www.adafruit.com/product/815 for my robotic arm.It works really well and is also easy to generate the needed PWMs with less direct connection between the controller and the motors using the I2c interface.

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