H Bridge for switching the polarity of a DC motor

Hello the forum

I don't have an electrical or electronics engineering but i know some basics about electronic

I'm trying to build a thermostat please help

I want to use this H Bridge for switching the polarity of a DC motor (to allow DC motors to run forwards or backwards) with is used to turn the eggs in the incubator

I know that inductive loads (motors) can generate large voltage spikes that can damage the transistor, the diodes as the 1N4002 may help absorb the reverse but what I don’t know why they use the two resistances shown on the picture

Please help me understand why this two resistances

Thank you

The resistors won't make any difference to the operation of the motors. If there are no other components in the circuit the resistors won't make any difference at all.

However, if the transistors and the diodes are turned off, the voltage on the motor terminals could be anywhere between ground and the supply voltage, so oscilloscope measurements or other voltage measurements could lead to inconsistent readings.

For instance, if the motor is stopped and the transistors are off, both sided of the motor will be at the same voltage. However, if you measure the voltage between a motor terminal and ground without the resistors, you could get just about any reading, depending on the impedance of the voltmeter, leakage current in the transistors, other leakage or a variety of other causes.

If the resistors are there, and the motor is stopped, the voltages on both sides of the motor will be the supply voltage and that can be read with just about any voltmeter.

Somewhat indirectly, your circuit diagram isn't clear. I am assuming that the wire at the top is the +ve supply but it isn't stated. Also where the wires between the diodes cross the wire between the motor terminal and the transistors, I assume that there is a connection. On the diagram you should move the top or bottom diodes slightly to the left or right so that it is clear that there are two T connections, not one cross. A cross might be a connection, or it might be a cross-over with no connection, while a T can only be a connection.

Your circuit will never turn off the PNP transistors. The Base-Emitter voltage of the PNP transistors needs to be less than 0.6 V when they are off. With the circuit that you have, the base current in the PNP transistors will be about half what it is in the NPN transistor when the driver transistor is off. In effect, all the transistors will be on, and they will get hot or short the supply or both.

Finally, beware of labeling a motor with +ve when it is a reversing motor. It's better to label it as "forwards" or "clockwise" or "raise" etc, because that terminal will be -ve when the motor is doing the opposite function.

As noted, that circuit will never turn off the top transistors.
To correct that, remove the base resistor connections for the bottom transistors and connect to the opposite control signal
Thus the left bottom transistor (it would help if you labeled them as Q1, Q2, etc.) would be controlled by the right signal, and vice versa.

You can also get the LMD18200 complete with mounted components cheap on Ebay.
Max.

Not a direct answer to your question, however have you thought about using a cheap L298 module, everything you need for the bridge is on the module.
Obviously the module needs to handle the current the motor requires.

AMIRAAM said:

Please help me understand why this two resistances

Thank you

Click to expand...

This circuit will never work because all the power transistors are always on shorting the supply.
Put the NPN transistors on top and the PNP at the bottom with the emitters of each pair linked, the bases of each pair are linked too. This way it is complimentary emitter follower and there is never current flowing from one transistor in the pair to the other one. You save 6 resistors. The diodes are in the right place to protect the transistors.

Hello

Thank you very much

Diver300 said:

The resistors won't make any difference to the operation of the motors. If there are no other components in the circuit the resistors won't make any difference at all……
………………………………………………………………………..
………………………………………………………………………..

For instance, if the motor is stopped and the transistors are off, both sided of the motor will be at the same voltage………………………………………………………….

…………..If the resistors are there, and the motor is stopped, the voltages on both sides of the motor will be the supply voltage and that can be read with just about any voltmeter. .

Click to expand...

It seems that the two resistors are for stopping the motor, I can ignore these resistors!?

Diver300 said:

Your circuit will never turn off the PNP transistors. The Base-Emitter voltage of the PNP transistors needs to be less than 0.6 V when they are off. With the circuit that you have, the base current in the PNP transistors will be about half what it is in the NPN transistor when the driver transistor is off. In effect, all the transistors will be on, and they will get hot or short the supply or both.

Click to expand...

crutschow said:

As noted, that circuit will never turn off the top transistors.
To correct that, remove the base resistor connections for the bottom transistors and connect to the opposite control signal
Thus the left bottom transistor (it would help if you labeled them as Q1, Q2, etc.) would be controlled by the right signal, and vice versa.

Click to expand...

So I modified the diagram as you suggest please look at it and tell me the correct values of the pul-up resistors

Diver300 said:

Finally, beware of labeling a motor with +ve when it is a reversing motor. It's better to label it as "forwards" or "clockwise" or "raise" etc, because that terminal will be -ve when the motor is doing the opposite function.

Click to expand...

Did you mine like this?

MaxHeadRoom78 said:

You can also get the LMD18200 complete with mounted components cheap on Ebay.Max.

Click to expand...

dr pepper said:

Not a direct answer to your question, however have you thought about using a cheap L298 module, everything you need for the bridge is on the module.
Obviously the module needs to handle the current the motor requires.

Click to expand...

Thank you, that because i want to use a simple parts !?

I m sorry zahwi

zahwi said:

This circuit will never work because all the power transistors are always on shorting the supply.
Put the NPN transistors on top and the PNP at the bottom with the emitters of each pair linked, the bases of each pair are linked too. This way it is complimentary emitter follower and there is never current flowing from one transistor in the pair to the other one. You save 6 resistors. The diodes are in the right place to protect the transistors.

Click to expand...

i didn't see your reply, please i will see that tomorrow thanks

AMIRAAM said:

Did you mine like this?

View attachment 122945

Click to expand...

I don't know if there is a standard symbol for motor direction. My point was that "+" isn't useful in this case.