Hello everyone ,
I am trying to build a H bridge for a DC motor of a toy car that I have found from my childhood ,
motor is 6 V I think and I have used BC547 NPN bipolar transistors.
http://html.alldatasheet.com/html-pdf/11551/ONSEMI/BC547/179/1/BC547.html
I connected transistors in darlington so I can get current amplified to get the motor running and used two resistors ,
R1 = 120 ohms and R2 = 8k ohms because without them transistors were getting really hot.
For controlling transistors I use MSP430 microcontroller from texas instruments with ~3.3V DC output on the each digital pin
jpg images
So I am encountering really strange problem for example when I connect battery as on the picture above I get no motors moving one the one darlington pair of the transistors and on the other ones I get motor going but it's just really slow ,
but If I short circuit collector and base electrodes of the any transistor I get motor running at full speed, but since C-E gets forward biased it destroys the transistors eventually.
This is what I have measured with the voltmeter , where transistor 1 is getting input from the microcontroller:
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Vce2 = 5.6V , Vbe2 = 0.71V
Vce1 = 6.2 V , Vbe1 = 0.7V
and VRB = 2.36 V ,
where VRB is voltage drop across the base resistor
from the base emitter voltage it seems like both transistors are forward biased,
and base current of the first transistor is Ib1 = VRB/RB = 2.36 / 820 = 2.88 mA which is pretty high current and it should get amplified twice with both transistors.
What is more confusing is when I take the battery and connect it to the opposite sides the motor gets going with very low speed, and I measure voltages:
Vce2 = 0-82 V, Vbe2 = -0.19 V
Vce1 = -1.39 V , Vbe1 = -0.87V
I can even disconnect microcontroller pin to feed the base electrode of the first transistor and the motor is still running since the BE is reversed biased.
Would really appreciate your help.
I am trying to build a H bridge for a DC motor of a toy car that I have found from my childhood ,
motor is 6 V I think and I have used BC547 NPN bipolar transistors.
http://html.alldatasheet.com/html-pdf/11551/ONSEMI/BC547/179/1/BC547.html
I connected transistors in darlington so I can get current amplified to get the motor running and used two resistors ,
R1 = 120 ohms and R2 = 8k ohms because without them transistors were getting really hot.
For controlling transistors I use MSP430 microcontroller from texas instruments with ~3.3V DC output on the each digital pin
jpg images
So I am encountering really strange problem for example when I connect battery as on the picture above I get no motors moving one the one darlington pair of the transistors and on the other ones I get motor going but it's just really slow ,
but If I short circuit collector and base electrodes of the any transistor I get motor running at full speed, but since C-E gets forward biased it destroys the transistors eventually.
This is what I have measured with the voltmeter , where transistor 1 is getting input from the microcontroller:
img host
image sharing sites
Vce2 = 5.6V , Vbe2 = 0.71V
Vce1 = 6.2 V , Vbe1 = 0.7V
and VRB = 2.36 V ,
where VRB is voltage drop across the base resistor
from the base emitter voltage it seems like both transistors are forward biased,
and base current of the first transistor is Ib1 = VRB/RB = 2.36 / 820 = 2.88 mA which is pretty high current and it should get amplified twice with both transistors.
What is more confusing is when I take the battery and connect it to the opposite sides the motor gets going with very low speed, and I measure voltages:
Vce2 = 0-82 V, Vbe2 = -0.19 V
Vce1 = -1.39 V , Vbe1 = -0.87V
I can even disconnect microcontroller pin to feed the base electrode of the first transistor and the motor is still running since the BE is reversed biased.
Would really appreciate your help.