I have a simple circuit, with 5x 3v/20ma LED's in parallel (each with a 330 resister in front of them) and a dc motor (rated 6v to 24v) also in parallel, all running off a 9v battery.

The problem is the motor is slowing down thanks to the LED's. I would have thought that with having everything being in parallel the motor would not be slowing down, but it is.

What is happening here? Why is it slowing down? Is it to do with the voltage, or the current (amps)?

Can I speed the motor up using a transistor, or is there something else I can use to speed it up?

Thank you!

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    \$\begingroup\$ Put a voltmeter on the 9 V battery terminals. Observe the voltage without the motor attached, but with the LEDs attached. Then, while still observing, add the motor and see what voltage you get. \$\endgroup\$
    – jonk
    Commented Dec 21, 2016 at 22:23
  • \$\begingroup\$ First, measure the voltage of the battery with the motor not running, then with it running. Does the voltage drop significantly? EDIT: jonk, you beat me to it. \$\endgroup\$ Commented Dec 21, 2016 at 22:23
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    \$\begingroup\$ A cheap fix is using the same type of battery in parallel with the current battery to double the amount of amps you can pull. There's some drawbacks, but you might just be interested to see what it does for yourself. \$\endgroup\$
    – Mast
    Commented Dec 21, 2016 at 22:43
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    \$\begingroup\$ Don't put them in series, put them in parallel. You really want to take a beginner course in electronics to answer the rest of your questions. There's plenty of things you can do with a transistor, but none of them make sense in the context you provided. More parallel batteries or less parallel LEDs. Once you can handle a large load all at once, increasing the voltage by using series batteries could work. \$\endgroup\$
    – Mast
    Commented Dec 21, 2016 at 22:51
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    \$\begingroup\$ No, that will cripple both your LEDs and motor. Don't do that. \$\endgroup\$
    – Mast
    Commented Dec 21, 2016 at 23:04

1 Answer 1


The battery has an internal resistance. This means that if it has to supply more current (by adding leds in parallel) it will supply less voltage so the motor slows down. Use a bigger battery or several in parallel.

  • \$\begingroup\$ Thank you. But surely I can increase the voltage by using a transistor? \$\endgroup\$ Commented Dec 21, 2016 at 23:06
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    \$\begingroup\$ No a transistor will only let you control (reduce) voltage you already have. \$\endgroup\$
    – RoyC
    Commented Dec 21, 2016 at 23:21
  • \$\begingroup\$ Ah, ok confusing, I thought transistors amplified current and voltage. Hmmm, transistors are complex :) \$\endgroup\$ Commented Dec 22, 2016 at 4:07
  • \$\begingroup\$ Think of the transistor like a switch. You wouldn't expect more voltage or current to flow if you put a switch in series with your motor. The same goes for the transistor. A transistor uses a small current to control a larger current - but your battery must supply that larger current. \$\endgroup\$
    – JRE
    Commented Dec 22, 2016 at 5:51
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    \$\begingroup\$ " I thought transistors amplified current and voltage." They do, but only for voltages less than the power supply voltage. If they did what you think, they could be used to make perpetual motion machines. \$\endgroup\$ Commented Dec 22, 2016 at 14:34

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