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I got the MakerShed Ultimate Microcontroller Pack and it came with a "Transistor NPN". I want to use it (or probably buy something else) as an electronically controlled switch so that on arduino I set a digital output to "HIGH" and it allows 12 volts though or set the pin to "LOW" so that it turns the 12 volts off. This is much harder to explain than it was in my head. Anyway, is that possible? Or do I need a bigger, better, transistor or some other special purpose circuit for this?

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After doing more searching it turns out I need a relay . . . which will take advantage of the transistor but I'll need more than just the transistor . . . but I thought maybe with only 12 volts I might not need to go that far. – tooshel Jan 14 '13 at 22:52
With no more information than that, I would have to guess that it's just a small-signal/general-purpose transistor that can handle maybe 40V and 0.5A (but not both at the same time!). – Dave Tweed Jan 14 '13 at 22:56
If this transistor can't handle your current or voltage requirements, there are plenty of bigger transistors that can. Probably no need for a relay, though if you tell us what the load is, we can say for sure. – Phil Frost Jan 14 '13 at 23:07
up vote 5 down vote accepted

Saying you have a "Transistor" is like saying you have a "vehicle", and you're asking us what your vehicle's towing capacity is.

Basically, there are many, many, many thousands of varieties of transistors.

Anyways, if you follow the "datasheets here" link, the NPN transistor included with your kit is a SS9013.

This particular transistor is rated for a maximum (collector-emitter) voltage of 20V, and a maximum current of 500 mA.

This means that you can switch a (small) 12V device with the transistors you have. However, if the item you're switching needs more then 500 mA, you're going to either need a bigger transistor, a relay, or some other switching device.

If you can give us more information on what device you're hoping to switch, we can probably help you determine if the NPN devices you have will work, or if not, what devices/relay/whatever you should use.

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Want to run two DC motors and I assume it's going to be way more than 500ma just by looking at other DC motors that Lego makes that are much smaller. philohome.com/motors/motorcomp.htm – tooshel Jan 15 '13 at 0:05
And, thanks for the "datasheet here" reminder. I clicked that before and remember reading that but it was awhile ago and this time when I was looking up what transistor it was I didn't notice it. Makershed should just put the part numbers right there! – tooshel Jan 15 '13 at 0:09
I thought transistors "leaked" so that some of the 12 volts that I apply will leak to the other side even if the base is held low. So even if it was less than 500mA a transistor alone wouldn't work. – tooshel Jan 15 '13 at 0:12
@tooshel - they do leak, but it's a tiny, tiny ammount that's completely irrelevant for a motor or other large device. – Connor Wolf Jan 15 '13 at 0:24
@tooshel - from that page on lego motors, it looks like about a third of them would work with the transistor you have. Which motor is it that you're working with, specifically? Alternatively, what is it's "stall current"? (The stall current is the maximum current the motor can draw). – Connor Wolf Jan 15 '13 at 0:26

As the (oh so fuzzy) datasheet on that site says, the (SS9013) transistor can withstand 20V across C-E and 500mA total. As long as you don't exceed those you should be fine.

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Getting close to both at the same time could cause thermal problems. – The Photon Jan 14 '13 at 23:26
@ThePhoton makes a very, very good point - you need to do more than look at the "maximum ratings" in the datasheet, you need to work out what will happen when you are using the thing. Motors are not a nice load, they are inductive, they can stall, they can be spun, reversed, over-sped, or over-run and generate back-emf and all sorts of nasty spikes. – John U Jan 15 '13 at 18:07

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