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Hello everyone I realised after burning through 6 L293D's that they were getting old. I hated them mostly due to the fact that they were fragile. I also burned 3 of them because of static electricity. So I decided to try and make a cheap H-Bridge from transistors.

This is just a test circuit, what I need to know is: is the circuit correct, can this type of H-Bridge handle more than 5 Amps depending on the transistors used, and do you guys have any suggested modifications if it is wrong.

EDIT 11/27/2013

Here is the newest versionenter image description here

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    \$\begingroup\$ Shouldn't you be limiting the base currents? \$\endgroup\$ – Joe Hass Nov 26 '13 at 22:16
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    \$\begingroup\$ Every time you edit the schematic, you are asking a very different question, and rendering all the existing answers wrong. \$\endgroup\$ – Phil Frost Dec 18 '13 at 17:47
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No, this circuit isn't even close to correct. Unless V+ is limited about 1.5 Volts or so, it will rapidly vanish into a puff of smoke.

On the left side, both trasistors will always be on if the the supply is more than about 1.5 V. These transistors are not voltage-operated. The base voltages will be around 700 mV from the base when on. Put another way, you have a very serious make before break problem.

On the right side it's hard to tell what is going on. You show the same part numbers, but the NPN/PNP polarity is backwards.

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In addition to the other points noted, the protection diodes D2,D4 ought to connect to GND not the PWM transistor collector, otherwise they simply pull the collector -ve instead of protecting the bridge transistors.

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  • \$\begingroup\$ At least that one's now been fixed! \$\endgroup\$ – Brian Drummond Nov 28 '13 at 22:21
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I noted two things, the first is that you don't control the base currents with resistors. The second is that placing the PWM transistor at the bottom means you have to use higher switching voltages which may or may not be a problem, if you are switching a couple of amps (likely) it probably would be. I would put the PWM transistor as a PNP at the top and then switch it using an NPN to keep the voltage off your switching pin. In a similar design I used an additional NPN on once side for an inverter and then had only a single pin for direction control.

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I suggest you study this other BJT H-Bridge.

It gives you control over the motor to move it forward, in reverse, braking and coasting. It also lets you modulate the signal to control speed to a certain frequency. The page says it can handle currents near what you are expecting (5A), with proper heatsinks. But, in my opinion, the nicest feature about that bridge is that it doesn't allow dangerous input combinations that short battery contacts.

See more details in this answer of mine.

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