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I'm trying to learn a bit more about forward voltage and OHM's Law but I'm not sure how to wire this circuit.

I have: 1 x 4.5 DC motor (Operating Volts: 1.5- 4.5V, Nominal Volts: 4.5V, Current (no load): 0.25A) - http://www.jaycar.com.au/Electromechanical-Components/Mechatronics/Motors-%26-Gearboxes/Hobby-Motor---Medium-Torque/p/YM2707

3 x 5mm LED (IF Typical (mA)20, IF Max Continuous (mA)50, VF Typical (V)2.1) - http://www.jaycar.com.au/Active-Components/Optoelectronics/Standard-LEDs/LED-5mm-Red-Waterclear-10000mcd/p/ZD0156

1 x 3 - 12VDC Switchmode Plugpack (Output voltage: 3, 4.5, 5, 6, 7.5, 9, Output current: 1000mA (max))

Do I use the power at 4.5V and run the motor and the LEDs in parallel?

Do I need resistors on the LEDs?

Any help will be greatly appreciated!

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  • \$\begingroup\$ IF Max Continuous (mA)50, LED current should be limited to this value. You may consider ignoring DC Motor until you are able to appreciate ohms law using LED, supply and resistors. \$\endgroup\$ – User323693 Jul 13 '15 at 3:40
  • \$\begingroup\$ @Umar, but if I wire the motor and the LEDs in parallel, surely I don't need to cut the amperage down from 1A to 50mA? That was my question. \$\endgroup\$ – Daniel Gee Jul 13 '15 at 3:51
  • \$\begingroup\$ Yes and yes. One resistor for each LED : none for the motor. \$\endgroup\$ – Brian Drummond Jul 13 '15 at 12:56
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You can connect DC Motor and LED in parallel with the resistor in series with LED. LED can draw current about 24 mA (4.5 V - 2.1 V)/ (100 Ohms). If resistor is not placed in series with it, The LED will eventually get over heated and gets destroyed. use a fly back diode in parallel with motor if it is unidirectional. If Power is set to 4.5 V, then using a 100 ohm resistor with LED is recommended.

schematic

simulate this circuit – Schematic created using CircuitLab

LED do not have a datasheet, so assuming typical Vf for calculation.

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You should wire things like this:

schematic

simulate this circuit – Schematic created using CircuitLab

You require a current limiting resistor in series with each LED, to limit the current in that LED to 20 mA. The motor will draw whatever current it requires - no need for a current limiting resistor for it, as long as you don't feed it more than its rated voltage.

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