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Daithi_D
modified 7 years ago

Motor transistor switch query

11
19
320
07:21:09
The positioning of the NPN transistor used as a switch to start a motor matters greatly but why? Doesn't the transistor just act as a switch to close the loop? Why does the motor (left) with switch placed after it work while the motor (right) with the switch placed before it does not work.
published 7 years ago
2ctiby
7 years ago
Look at the shown voltage going across each motor (immediately next to each motor + sign)...see how each motor is receiving a very different voltage.....The transistors are acting more like variable resistors rather than simple switches. The position matters, depending on requirements....That is why we normally see the load on the collector for this type of setup. Mosfets are more like simple switches, but they too need thought in their position.
Daithi_D
7 years ago
Thank you for your response. I see that the voltage is different and that the transistors don't act as a simple switch in this case but I'm finding it hard to see the difference as to why this is the case. Does a transistor behave differently with a positive voltage at the collector vs a negative voltage at the transmitter? I'm going to try the swapping the motors for lights and see if it simplifies the situation.
Daithi_D
7 years ago
Haha good man, I think that's as daft as my original question. I'm going to go study transistors and hopefully come back to this with a good description.
2ctiby
7 years ago
More help here... http://everycircuit.com/circuit/5341122480308224
Daithi_D
7 years ago
@2ctiby thanks very much for your help. I don't know why I was picturing the circuit as just either open or closed. It makes sense now.
2ctiby
7 years ago
It's a good starting point to then continue thinking about the transistor action....On the left circuit the base current can happily trickle through the transistor to earth and so increase the collector current for the motor. On the right circuit, the motor acts as a resistance which prevents any good current trickle through the base...look at the shown current through each of the 1k base resistors.
hurz
7 years ago
To drive a load with a voltage follower transistor "high side" or "top side" you need a higher control voltage then the load supply 12V. This is because the transistor needs current into base emitter. This. Curret can only flow if the voltage is higher then 12V. Here its about 15V to get across 1kOhm and base emitter 0.7V voltage drop enough base current to drive the motor. Hope this helps http://everycircuit.com/circuit/5733898145497088
2ctiby
7 years ago
Yes...agreed. Like I said...the motor acting as an emitter resistor prevents a suitable base-emitter current flow in the original schematic circuit on the right. To overcome that issue the Hurz description is the way forward. The original equal components quandary for both original left and right circuits is then seen to be an unsuitable option....A higher base voltage (centre divider volts between the 1k and motor) is needed to satisfy the requirements if an emitter load (motor) is to be used.
hurz
7 years ago
And the same is valid for Mosfets, while they do not need current but a voltage delta as well Vgs.
2ctiby
7 years ago
Yes, life is a lot easier when using mosfets if we keep the source directly to the top and bottom rail (pmos and nmos respectively) so that the Vgs can be identical to the Vg without further delta there to consider.
salehaljandaly
7 years ago
It is good
HR98
7 years ago
Good
hurz
7 years ago
No pmos here mr messy
2ctiby
7 years ago
and there is no such thing as your mentioned nmos number here... http://everycircuit.com/circuit/5767007612174336 ... which is just as well, because the settings are wrong in the one you have shown and there is over 2A passing to earth through it regularly.
hurz
7 years ago
You are such a baby.
2ctiby
7 years ago
We are all still waiting for you to admit making a mistake....any of them will do, there are loads to choose from...now we wait for the usual diversion and insult tactics. You don't go long without starting antagonism such as your little insult earlier here....you permanently look for someone to hate and argue with.....I will try not to be drawn in this year by such an idiot as you.
hurz
7 years ago
You will continue your hole life to turn correct things into wrong statements, by messing everything up. Already the way how you comment circuits from far away is messy and will help nobody. If you have questions then ask. If not, then better shutup, cuz you must realize how low your knowledge in electronic is and endups always in being rude instead of sit down and learn before open your big fat ugly mouth.
2ctiby
7 years ago
There we go...insults and diversion from a deluded 'would be dictator' who thinks that he is greater than everyone....A big fall is heading your way.
Daithi_D
7 years ago
http://www.instructables.com/id/12V-Motor-Control-With-5V-Arduino-and-NPN-Transist/

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