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I'm pretty new to 'breadboard-ing' and I'm beginning to work with and need help understanding some basic concepts that I can't grasp.

Basically I've made a basic circuit with a push button for an LED light. There circuit is a basic circuit, powered by two double A batteries and there is a 220\$\Omega\$ resistor on the cathode side of the LED (from what I understand it doesn't matter which side it's on but that's the way I have it set):

enter image description here

The LED lights up when I press the push button. Victory.

The next set up is with a 3VDC Motor (350mA) and when I plug that into the batteries directly it spins. When I plug it into the circuit where the LED is (with the resistor) and press the push button, it doesn't spin:

enter image description here

BUT, when I remove the resistor and put a straight wire into the circuit it works great when I press the push button:

enter image description here

So obviously the resistor causes enough resistance to not spin the motor in the circuit BUT 220\$\Omega\$ seems like a very small resistor and would seem as though that should resist the circuit so much that the motor doesn't work at all.

I don't have a resistor smaller than 220\$\Omega\$ to try out but obviously anything more than that won't work if this doesn't work. I'm just trying to understand the concept of why this wouldn't work because I'm trying to chain some LEDs and motors together and the resistance obviously causes an issue.

I tried a capacitor after the resistor too but that didn't work (but maybe my concept is off so far; as I said I'm pretty new to this and am having trouble even knowing what to search for to get answers).

Thanks for your help!

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  • \$\begingroup\$ Measure the voltage across the motor with the resistor in circuit. \$\endgroup\$ – Leon Heller Nov 8 '15 at 19:37
  • \$\begingroup\$ You might just be able to feel the difference in torque when you turn the motor by hand. Turning it clockwise will feel different from turning counter clockwise. Obviously from your previous results the torque in itself is too tiny to spin the rotor. \$\endgroup\$ – jippie Nov 8 '15 at 19:44
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You say that the motor draws 350 mA. If you pass 350 mA through your 220 ohm resistor, Ohm's Law says there will be a 77 volt drop across the resistor - since you only have a 3 volt battery, this obviously won't work!

Ohm's Law indicates that 3 volts across 220 ohms will only result in a current of 13.6 mA.

Common LEDs typically have a maximum current of 20 - 30 mA, so they cannot be connected in series with your 350 mA motor - the LED would vanish in a puff of smoke!

If the motor is designed to operate on 3 volts, it should be connected directly to the 3 volt battery, with no series resistor.

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  • \$\begingroup\$ This is great, thank you! Very well explained. One quick question, when you say the LED cannot be connected in series with the motor, you mean on a direct current, correct? I could resist the current after flowing through the motor or am I misunderstanding that concept? I'm still reading on a lot of this and I really appreciate the quick help here. Thanks! \$\endgroup\$ – MillerMedia Nov 8 '15 at 20:21
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    \$\begingroup\$ There is no "before" or "after" for current: it's the same at all points in each loop of a circuit. (Kirchoff) \$\endgroup\$ – pjc50 Nov 8 '15 at 20:30
  • \$\begingroup\$ As a side-note, to sink 350mA at 3V, the motors resistance would have to be ~8-9Ω. Note that it's more complicated (back EMF from the motor contributes, the static resistance of the motor windings is probably considerably smaller, but the point stands). \$\endgroup\$ – Connor Wolf Nov 9 '15 at 2:53
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220 ohms is enough for the motor not to spin. A lot of motors need a specific minimum input current(you can always go higher). You should look at your motor datasheet for this. There is an important thing that you should know about motors:

When connecting a motor to a circuit, you should always add a diode to either the ground or the positive voltage input of the motor. This prevents the inductive kickback of the motor from potentially damaging other parts of the circuit.

According to what I think you want to do(which is connecting motors and led's to the same circuit) you should connect the motors and led's in parallel. Your question has basically been answered here:

How to wire circuit DC Motor with LED

If you connect the led and motor in series, then there is a resistance issue, but if you connect it in parallel, you should be fine.

Go to this link for more info about series and parallel circuits:

https://www.swtc.edu/ag_power/electrical/lecture/parallel_circuits.htm

A parallel has multiple paths which current can flow through and because of this connecting a motor and led to the same circuit in a parallel circuit won't cause any resistance problems. On the other hand, a series circuit only has one path for current to flow through, which causes the resistance problems that you talked about.

Just a note, when asking a question like this always mention how you are powering the circuit and at what voltage and current. Also, try making a circuit diagram with the schematic maker.

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    \$\begingroup\$ Actually I think a lot of people (not just some) frown upon the quality of diagrams that Fritzing generates. The built in circuit editor is a much better advice. \$\endgroup\$ – jippie Nov 8 '15 at 20:08
  • \$\begingroup\$ Ya i mentioned that. But for a lot of people the schematic diagram maker is more complex. If you know how to use the bulit in schematic tool then it is better, but for people who aren't as comfortable with drawing schematics, fritzing might be more appealing because it has an easy breadboard interface and it has more parts. \$\endgroup\$ – zack1544 Nov 8 '15 at 20:12
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This here is a simple diagram of what your circuit should look like:

schematic

simulate this circuit – Schematic created using CircuitLab

][1]][1]

Basically, make sure the resistor, LED, and motor aren't on the same wire, but are in parallel. I didn't put this in the diagram, but put a regular diode before the motor to prevent hazardous backflow

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