motor control problem

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bananasiong

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hi all, i'm now doing my final project, and i found some problem. i'm using 68HC11 to control the motor. i need 12V to supply to the motor, since the 68HC11 drive only at 5V. i have constructed the circuit as shown in A. the 2K resistor is assumed as a motor. when the 68HC11 give a '1' to the PB0, which is 5v, the motor should run, in more than 12V, but it is unexpected. is it necessary for me to connect the '-ve' of port b to the negative terminal of the battery so that they have common ground?

when i use ewb to simulate, as shown in B, assume that the 1K resistor is the 68HC11, when the 5V is injected to the base across the 1K resistor, the voltage across the 2Kresistor is 11.xx volts.

can anyone help me with this?? i'm still new in electronics.. thanks for helping.
 

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what is the voltage developed across the 2K in fig A , also u need a current limiting resistor b/w micro and the transistor (~1k)
 
akg said:
what is the voltage developed across the 2K in fig A , also u need a current limiting resistor b/w micro and the transistor (~1k)
Click to expand...

what u mean by need a current limiting resistor b/w micro and the transistor?? u mean need a resistor between the microcontroller and the base??
 

yes. otherwise the micro will see only low resistence(Fwd Bias res of BE Jn), and the voltage at pb0 will be pulled low (Vbe)
 
oic!! thanks.. but, is it necessary for me to connect the -ve of the port b to the -ve terminal of the battery?? so that they can have common ground.
 
bananasiong said:
oic!! thanks.. but, is it necessary for me to connect the -ve of the port b to the -ve terminal of the battery?? so that they can have common ground.
Click to expand...
i don't know abt that micro. and i don't understand this " is it necessary for me to connect the -ve of the port b to the -ve terminal of the battery"
why a port has a -ve terminal ?.

AFAIK ur 1st dia is correct when a res: is placed as mentioned earlier.
 

i mean.. the port B of the 68HC11 microcontroller has 10 pins, 8 of them are for output and the rest 2 are +ve and -ve.
 
bananasiong said:
i mean.. the port B of the 68HC11 microcontroller has 10 pins, 8 of them are for output and the rest 2 are +ve and -ve.
Click to expand...
that is the supply for the IC .
u need to have a common point for IC and the driver ckt.
 

okay!! thanks for answering!! hope my project is done well..
 
it doesn't work!! even though i've already connect a resistor to the base (between the microcontroller and the base), the voltage across the 2k resistor is only 2.++ volts (12 volts is supplied)..

i make the question simpler.. is there any circuit that can use 5 volts to switch 12 volts?
 
what is the value Vce .?
 

i get around 9 volts for Vce.. how to increase the Vc?
 
bananasiong said:
i get around 9 volts for Vce.. how to increase the Vc?
Click to expand...
the transistor is not turning ON , it seems to be in the active region..what about Vpb0 and Vbe
 

the microcontroller is supplied 5V, the Vpb0 is around 4.xx volts. and the Vbe is less than 1V.
 
that voltages seems to be good , so normaly the trans should be ON, is the transistor good/correctly connected.?
 

yes! the transistor is good! i've tried the circuit with a few transistor (same value).. but the Vce still high.. what can i do??
 
Assuming you're doing what we think you're doing?, then the only possible explanation is faulty transistors or a wiring error.
 
The 2N3904 is an American transistor with its pins EBC. European transistors like the BC547 have their pins the opposite: CBE. If the E and C pins are swapped in the circuit then the transistor has very low gain and won't saturate.
 
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