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Problem with large currents flows that produce heat in H bridge

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Newbie level 4
Aug 8, 2009
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h-bridge problems

This is the circuit I used to drive my 6V DC motor and +V mentioned above is 6V.
I control this circuit using 16f84a PIC MCU .


The problem I found here is when applying 5V to A and 0V to B . Then it works .
But the problem is a large current flows through 1 and produces heat because A is 5V and +V is 6V hence 1 is turned on .

How can I get rid of this problem ?. I want to introduce a voltage drop there .
How can I get it done ?

6volt h bridge circuit

If the 6V source is regulated you can just add a diode in series with the base of the PNP transistors to achieve an additional drop of 0.7V (the arrows in the schematic are wrong). If not you´ll need a more complex circuit. Furthermore, you NEED to add limmiting resistors in series with ALL transistor´s bases (when you apply 0V to B you´re applying 6V to the junction of transistor 3).Can´t figure out how is it working. Also, a snubber circuit is welcome.


sry for the images

will it be sufficient adding single diode per base of a pnp ?

6v h bridge

No, need to limit the current at each base. Add a resistor in series with everyone.

h-bridge not working

can you please Mr.paul suggest me an improved circuit to get this thing done ?

Here I use PIC16f877a to switch the bases on and off

hbridges problems

You are applying 1V directly across the B-E junction of the transistor which is forcing it to sink high current. Try putting a resistor in line with each of the transistor base pins to limit the base current to a safe level. The transistors will still saturate so the motor current will not be reduced.


Re: H bridge problem related to driving a 6V DC motor

There are lots of ready ICs to do the job. They´re cheaper than your discrete circuit and more error proof. If you insist in doing it based on discretes I would like to suggest using the circuit shown here BUT you´ll have to invert all the voltage polarities and to invert all the transistors types and diodes to have controll using a 0-5V input signal.

H bridge problem

i think you need to tie the base of transistors 1 and 2 together and 3 and 4 together not like you are connecting, beside connect a current limit resistor on each transistor base.

Re: H bridge problem


In my circuitry will it be sufficient adding PNP power transistors and NPN drivers and changing the polarities like this .


but here I am gonna use +6V to drive my 6V DC motor

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