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I am extremely new to circuits and using transistors, but after reading many other posts and articles I am still confused about terms and not sure if I am understanding this correctly.

I have been trying to use raspberry pi to use as a switch to turn on a motor. However I realized that I could not use PN2222 as the current limitation IIUC is less than a few hundred mA, much less than 1.5A. So I have been trying to get a new transistor but have been very lost on reading the datasheet. After much research, I think I have the basics, but wanted to double check from the experts here whether what I am doing is not going to blow anything up.

My main guide has been this post: Need help calculating resistance for transistor base

(Tried to simulate this but I don't seem to be getting any useful data - getting 0 V everywhere or N/A)

schematic

simulate this circuit – Schematic created using CircuitLab

Datasheet: sheet

Questions

  1. So the motor I have is 12V which takes in minimum 1.5A. Using I=VR, does this mean that the resistance is 8Ohm and so I should use 8 Ohm in my calculation?
  2. If I understand correctly, to calculate how much current is required from base, I need to use \$h_{FE}\$. At 1.5A collector current, gain is approximately 30 (from the graph, although I am not sure how that \$V_{CE}=4V\$ actually affects anything). So to support 1.5A, does that mean \$I_b\$ needs to be \$\frac{1.5A}{30}=50mA\$? If the max the source can output is 16mA, is the max current going through collector->emitter \$16mA * 30 = 480mA\$?
  3. So the base resistor required would be \$R=\frac{V_{BaseResistor}}{I_B}=\frac{3.3V - V_{BE}}{50mA} = \frac{3.3V - 1V}{50mA} = 44Ohm\$ But since I can't do 50mA, do I just calculate this for 16mA?
  4. From the calculation above, it seems like I can't actually use my current setup to enable the motor properly. Is this correct? In this case, is it better to get a new transistor (is there a way to find correct transistor more easily?) or is it possible to provide external power to increase amperage provided to transistor?

Sorry if my questions seem all over the place I'm still very confused whether whatever I am doing is correct.

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blah900
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    For this you would be much better off using a MOSFET instead of a BJT. If you must use BJT then you want a Darlington Pair to give you a greater gain. – Majenko Sep 04 '16 at 20:37
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    Don't forget a flyback diode across your motor to protect your switch. – efox29 Sep 04 '16 at 20:37
  • @Majenko wait, I'm confused, isn't MOSFET also a transistor? From what I read I thought that no matter what type I go with, they all have to go through this calculation? – blah900 Sep 04 '16 at 20:45
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    MOSFETs and BJTs are both forms of transistor, but they operate very very differently. And for just switching a motor on and off you will be in the saturation region, so there really isn't any calculating to do. As long as your threshold is well below your logic high, and the MOSFET can handle the current, and the on-resistance is low enough there's nothing else really to worry about. – Majenko Sep 04 '16 at 20:47
  • I'd suggest reading through the following tutorial to help you get started. Might help tie a bunch of things together. https://learn.sparkfun.com/tutorials/transistors/applications-i-switches – Fuzzy_Bunnys Sep 04 '16 at 20:48

1 Answers1

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Well thought out and written question.

You are quite correct in your calculations, the 4V Vce is important, it means you need a 16V supply or else your motor will only see 8V and run slow and under power. This lost power will heat up the transistor at 4V x 1.5A = 6W so require a heat sink. The current will likely be a bit lower as the motor is getting less than 12V but not neat 480mA

To get the full current even with the 16V supply the low hfe means that you need to find more gain and this is usually done with a logic buffer to give you the extra drive or an extra transistor to amplify the logic output.

As others have suggested a logic drive MOSFET is a strong contender as long as you are not trying to switch it at high speeds to control the motor speed, this will cause heating in the MOSFET if you do not make use of more elaborate gate drive control.

While starting out in an effort to minimise the risk of having motor voltage reach your controller (and for general galvanic isolation for a lot of reasons) I would recommend a relay as well. The relay you can usually drive with a single transistor and it would be selected to drive the motor with a safe margin.

Remember that your motor starting current may be much higher than the rated running current and if your transistor or relay contact are rated too low you may have regular failures. A 5A rating would be a happy margin.

As mentioned you want a fly-back or free-wheeling diode across the motor (or relay) coil to protect your transistor.

EDIT:
There are also darlington transistors available and these can be used but will have the same high Vce saturation voltage. Your load of 12V and 1.5A is often these days handled with MOSFET or relay when using microcontrollers.

Here is a picture search that may help find ideas. There are lots of alternatives that are worth considering to find what will suit you best.

https://www.google.com/search?q=motor%20+drive+transistor+arduino+schematic+12v+2a+-stepper&tbm=isch

KalleMP
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    I do not think that galvanic isolation is needed. A standard mos is enough, the only thing to watch for is the threshold voltage, for high current mosfets it can get quite high and 3V3 is probably not nearly enough. A great alternative is an optocoupler + mosfet, the opto should be open drain so it can give the mos a full 12V Vgs (which should be ok for fat FETS). Or the relay of course, depending if OP wants to PWM or not. – Vladimir Cravero Sep 04 '16 at 21:09
  • Yes, opto is nice compact alternative relay but may be more tricky to get right first time, 12V drive from OC opto is good idea to get around feeble 3.3V micro. But still relay is nice, it protects all the semiconductors from human error. – KalleMP Sep 04 '16 at 21:24
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    opto + mosfet is as tricky as a relay, costs a fraction, smaller, wider BW, looks cool, no sound, no moving parts, you get the isolation, you get to learn what an opto is... I mean, there is really no downside in using an opto imo. – Vladimir Cravero Sep 04 '16 at 21:33
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    Thanks everyone, I never knew there were other alternatives out there. I will check out MOSFETS, Optocouplers, and meanwhile see what I can accomplish with transistors at hand – blah900 Sep 04 '16 at 21:39
  • @VladimirCravero I like relays because you can see and hear them switch when debugging :-) In industry you often find both get used, the opto for primary isolation and the relay for cheap current handling. – KalleMP Sep 05 '16 at 10:23