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How do I use a DC motor with a circuit's power?
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ghuppi



Joined: Feb 19, 2017
Posts: 2
Location: el dorado hills

PostPosted: Sun Feb 19, 2017 5:43 pm    Post subject: How do I use a DC motor with a circuit's power? Reply with quote  Mark this post and the followings unread

Hi there! New here. Basically, I have a 9v DC motor and I want to mount it inside a device such as a keyboard, that runs on 6v... How would I do this safely? Is the only way to use a separate power supply for the motor??? I was thinking of using a 9v power supply and connect the dc motor, and run the keyboard through a 6v regulator? would this be a bad idea? Would I get interference from the motor? Thank you!

Shorter version: Can I use the power of another circuit to power a DC motor without the motor taking away too much power? Maybe using a transistor? Thanks!
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gdavis



Joined: Feb 27, 2013
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PostPosted: Mon Feb 20, 2017 2:14 am    Post subject: Reply with quote  Mark this post and the followings unread

To determine if the motor can be driven with the existing supply, you need to now how much margin you have. In other words, the amount of current the supply can provide must be greater than the amount of current the keyboard and motor will draw together.

If it's using a wall wart supply and you can find the current ratings of the supply, keyboard and motor, you can determine if there's enough. If there isn't, you can probably find another supply with a higher current rating.

If the supply is built into the keyboard, it will be difficult to determine the margin.

Adding a 9V supply would probably work, though I think a DC-DC converter will be more efficient that a voltage regulator for getting 6V from the 9V. Again, you need to make sure the supply is capable of providing the total current needed.

DC motors are inherently noisy, though it's impossible to say if you will get interference as there are many variables to consider. Some things you can do are to put 0.1uF ceramic caps across the motor terminals and make sure the wires are short and routed away from other circuitry that may be sensitive to interference.

A transistor isn't necessary unless you want to switch the motor on and off from some other circuit.

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JovianPyx



Joined: Nov 20, 2007
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Location: West Red Spot, Jupiter
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PostPosted: Mon Feb 20, 2017 8:15 am    Post subject: Reply with quote  Mark this post and the followings unread

To what gdavis said I will add that when devices like relays or motors are used in a DC circuit it is a good idea to also put a diode (power type with reasonably high voltage capacity) across the motor terminals. DC motors will internally have a commutator which is a kind of switch. The commutator switch will cause inductive spikes that can blow a transistor if one is used to control the motor's speed or on/off. The diode is wired to be reversed biased under steady state conditions. When the spike happens, it will be reverse polarity and the diode will conduct the current and protect the transistor and other parts of the circuit. The voltage of the spike will be many times the voltage applied to the motor's coils.
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ghuppi



Joined: Feb 19, 2017
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Location: el dorado hills

PostPosted: Tue Feb 21, 2017 9:48 pm    Post subject: Reply with quote  Mark this post and the followings unread

Thank you both so much! Makes sense now! So much appreciated!!!
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