Let's Make Robots!

driving motor with bjt

i want to thank every body here for this usefull forum and the great answers i have found.

I want to run applications with PIC MCU but PIC can not sink them(like DC motor).so i need transistor.

but as i look to for example C945 datasheet,the betaof transistor is not specified!!!!!!
only a range is specified for beta.

so how can i have precise design to let my transistor work with the exact CE current I want????? 



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am I right?
in the schematic
27K resistor pull down the base while the MCU pin goes to tri-state condition.
if we do not use it,is it the leakage current of the transistor that might turn the motor on?

i guess this resistor has another effect too!
it cause the motor to stop faster.right?cause it turn the transistor off faster.am i right?
The 27K resistor does ensure that the transistor is off when the pin goes tri-state which can happen during power up or reset. BJT transistors are not really a problem buth this circuit can use a FET instead of the BJT in which case the 27K resistor is more important.

by the way

really appreciate your answers.

they worth a world.

thanx every body 

what i understand about noise was that as frequency increase then reactance decrease(1/(jwc))
so for high frequencies if the capacitance be high lots of current flow through capacitor which damage the capacitor and decrease motor power.
am I right?
but EMF?
take a look at the picture you have posted for me,(a relay coil with a doide across it.)
normally current flow from Vcc to ground(up to down in the picture)
as you said, while EMF happens the current flow reversly in the coil(from bottom to up)
if so,the diode can not pass the current,
That is the only time the diode across the relay coil in the schematic can conduct. look at the arrow on the diode.
thank you oddbot and thank you Al1970.
about EMF:
motor is an inductor so when we cut the voltage of base of transistor the inductor wants to flow the current in the same direction it was flowing.
but why does this current cause a damage?
it is not a high current or it is not in the reverse direction.

and about PWM and noise:
I applies 10nF cap for reducing noise of my motors in my robot.
u mentioned for higher frequencies use less capacity capacitors(less than 1nF)
I thought about but I have no idea why?

When the magnetic field around an inductor collapses it creates a current in the reverse direction.

As for capacitors across the motor, capacitave reactance decreases with frequecy. So if you are using high frquency pulse width modulation then you need less capacitance otherwise the capacitor acts like a short across the motor, reducing power to the motor and potentially damaging the capacitor. You want just enough capacitance to short out the spikes which are typically high frequency.

thank you both.I studied all the links,
for example we want to design the common emitter to work in saturation area,to let the maximum current flow through collector.
maximum current is:[ Vcc-VCE(saturation)]/Rc
then we say base current is: Ic/beta
here is my problem.
the beta is not specified in datasheet.only a range is specified.
so how is it possible to calculate base current to finally calculate base resistor to let the transistor work in maximum current?

beside,I have a general question which you would help me alot if you answer:
dude as a beginner who is so keen to learn electronics and such practical information,what do U recommend?
where can I start to learn such practical things (like you said to put a diode across transistor for EMF protection?)
at university the only thing that I learnt was some formulas which almost non of them have been useful for practical works.what's your opinion?

For the Beta assume the worst case senario. For switching a motor on or off it does not matter if you oversaturate the transistor so assume your transistor has the worst gain of the range given.

Oversaturation is only a problem when switching at very high speeds as it can take longer than normal for the transistor to come out of saturation.

My knowledge is mostly self taught. Practical knowledge come from practical experience which very few uni proffesors seem to have.


If you're trying to adjust the load current by varying the base current it becomes quite complicated, and I'd reccomend against it unless you really really have to. If you just want to calculate your basic resistor values for a switching transistor (full power or no power only), try http://www.daycounter.com/Calculators/Transistor-Switch-Saturation-Calculator.phtml

There are a few other transistor calculators there as well which might be useful.