another question about lesson 13 - DC motor

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Matt3036
 
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another question about lesson 13 - DC motor

Post by Matt3036 »

Hi all,

I already read the previous topic about transistors in lesson 13 in the forum. But there are two questions left I can't answer by myself.

First:
The breadboard layout (https://learn.adafruit.com/adafruit-ard ... ard-layout) shows the transistor with the flat side on the right. I had a look at the datasheet (http://www.mikrocontroller.net/part/2N2222) of the 2N2222 transistor I used (the same one that's used in the lesson). According to the datasheet the transistor, like it's posistioned in the layout, hast its collector south, the base in the middle and the emitter north. But the electric drawing in the lesson (https://learn.adafruit.com/adafruit-ard ... ransistors) says the opposite (emitter south). How is that possible? Isn't it the correct datasheet I looked at? When I imagine the conventional current (form + to -) flowing, the layout is only able to work if the emitter is south, because the only way current will be able to flow through the transistor is from north to south because of the wiring of vcc and gnd, right? So it has to be emitter south, but the datasheet says something else...

Second:
The lesson says, that a diode parallel to a motor blocks the drawback voltage the motor generates while braking. When I look at the sketch (https://learn.adafruit.com/adafruit-ard ... ransistors) I would assume that conventional current flows from vcc to gnd though the motor because of the transistor's emitter being south. So if the motor brakes it wants to generate a current flowing north (the opposite way) and the only way current can flow from south to north would be through the transistor no matter if there is a diode parallel to the motor or not. So how should the diode protect the transistor or the arduino like it's explained in the lesson?

Thank's for enlighting me ;)

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adafruit_support_bill
 
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Re: another question about lesson 13 - DC motor

Post by adafruit_support_bill »

Pinouts for otherwise very similar transistors may vary. This is the sheet for the part specified in the tutorial PN2222 :
http://www.onsemi.com/pub_link/Collateral/PN2222-D.PDF
Go by the data sheet for the actual part you are using:
So if the motor brakes it wants to generate a current flowing north (the opposite way) and the only way current can flow from south to north would be through the transistor no matter if there is a diode parallel to the motor or not
When the current flow through the motor coils reverses, the diode conducts and the current just flows back through the motor.

Matt3036
 
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Joined: Wed May 28, 2014 12:14 pm

Re: another question about lesson 13 - DC motor

Post by Matt3036 »

Hi,
thanks for the answer. The datasheet you posted makes it clear.
I still don't get how the diode works. What would be the exact path and direction current flows while the motor is braking refering to the tutorial sheme?

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adafruit_support_bill
 
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Re: another question about lesson 13 - DC motor

Post by adafruit_support_bill »

When you cut power to the motor it is still spinning due to inertia, so it starts to generate current. It flows out through the negative lead through the diode and back into the motor via the positive lead.

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