I created a circuit that uses a transistor to run a small (1.5 - 3v) motor. The transistor I used was a 2n3904 NPN. I calculated Beta to be about 11 when it's hooked up to an Arduino the base resistor I used was 47 Ohms and it worked well.

Given that I moved the circuit from the Arduino to a HiTechnic prototyping board, It's digital pins only put out 3.3v and from what I can tell (they don't say this online or in any documentation) the current is really tiny, about 8mA. Because of that it seems like I need use a different transistor - at least that is my guess, because even when I put a base resistor with 0 Ohms of resistance the motor barley turns and the current into the collector is barley 170mA.

To get a better idea of what I am doing. I have essentially taken this video tutorial and migrated it to this setup and I am using the Arduino to turn it on and off using the wire library and bitWrite.

The motor I am using is a Radio Shack Model: 273-258. It says it is .18amps to .25amps at no load, it will start spinning at just over .21amps.

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    \$\begingroup\$ You left out the obvious all-important information on how much current the motor draws. Once you have that finding a transistor or some other setup is easy. You can't start designing something without specs. \$\endgroup\$ – Olin Lathrop Feb 12 '13 at 20:53
  • \$\begingroup\$ Sorry I didn't make myself clear. It's this motor right here. radioshack.com/product/… it says it is .18amps to .25amps at no load, it will start spinning at just over .21amps \$\endgroup\$ – Kenn Feb 12 '13 at 21:16
  • \$\begingroup\$ @Kenn - On stack exchange, it is better to edit your question in response to a request for more information in a comment, that way the comments can be tidied up (deleted)later so that they don't distract from the question. \$\endgroup\$ – Mark Booth Feb 13 '13 at 1:41

You say you want to control a motor that requires from 180 to 250 mA at no load from a 3.3 V digital output of questionable current sourcing capability.

First, the no load current is pretty useless. The important parameter is what the maximum load will ever be. That is what you need to design to. If the circuit can handle the maximum current the motor will draw, it will certainly be able to handle a lower amount. The maximum current, also called the "stall current" is probably 500 mA at least. Let's figure 1 A max.

Here is a circuit that should work nicely:

This FET has a guaranteed maximum on resistance of 80 mΩ at 2.5 V gate drive. At 1 A it will only drop 80 mV and dissipate 80 mW, which is fine for a SOT-23 package. You will notice it getting a little warm, but it will be well within its limits.

A FET is a better choice here than a bipolar like your 2N3904 for several reasons. First, it will drop less voltage and therefore take less from the motor. At such a low drive voltage, even a few 100 mV can become significant. Second, it will drop less voltage and therefore dissipate less power and not heat up as much. Figure a bipolar would drop at least 200 mV although probably more at 1 A. Even just 200 mV at 1 A is 200 mW, which would make a SOT-23 pretty toasty. Third, it won't load the digital output, which you say has low current sourcing capability. The FET gate will only look like a capacitor to the digital output. That will slow down its edges, but they will still be instantaneous on the scale of anything the motor will react to.

A 2N3904 is a pretty crappy transistor for more than just signals anyway. I ususally use 2N4401/4403 for jellybean bipolars. They have more current capability and are generally more robust, but still have good gain and are also cheap and widely available. But I wouldn't use even the 2N4001 in this case. The FET is a better choice.

  • \$\begingroup\$ Thank you, that's a great answer. Just one more quick question. If you go to mouser mouser.com/_/?Keyword=irlml2502&FS=True there are two, they look identical except one has a slightly higher power dissipation. Which one would I want to use? \$\endgroup\$ – Kenn Feb 13 '13 at 13:59
  • \$\begingroup\$ @Kenn: You need to think for yourself a bit. I've already shown you how to calculate the power dissipation. Get a transistor that supports the current you need, the open circuit voltage you have, and that can be driven well enough with 3.3V on the gate. Compare the specs to what you need. If the transistor meets or exceeds them, then you can use it. \$\endgroup\$ – Olin Lathrop Feb 13 '13 at 14:28
  • \$\begingroup\$ good grief, OK - let me rephrase the question. While it appears either would work please remember I am just a hobbyist and given that I have never worked with this type of electrical component before would my assumption be correct or am I missing something? \$\endgroup\$ – Kenn Feb 13 '13 at 21:27
  • \$\begingroup\$ @Kenn: You can see from the two datasheets that both have sufficient specs. \$\endgroup\$ – Olin Lathrop Feb 13 '13 at 22:00

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