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# Electronics and Electrical Engineering Design Forum

## H Bridge with 4 NPN transistors (tip3055) using a 12v

Hi there,

I’m trying to build an H bridge with 4 NPN transistors (tip3055) with a 12 volt. I’m having trouble getting it to work. If you could help me, I’d really appreciate it.

Thanks!
-V

2 years ago

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Hi,

Does your schematic match this picture ? Make sure you put the flyback diodes, if not your transistor will burn from the voltage spike generated by the motor’s coil
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Right now it matches something like the picture below. I’m using 1k Ohm resistors to the base of each transistor. I do have the diodes in. I’m trying to control the transistors with 3.3v from an arduino board, and it seems that the emitter is only giving me the 3.3v, when I need 12v from an external battery.

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If you use a 12V motor, you need +V to be 12 V.

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Yes, that is where I have the battery connected. Im sending the arduino 3.3v to A and B

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3.3 V peak to peak, of PWM signal ? I think you need to lower the resistor values in the bases of the transistors and, if the motor drives a very large current, you may need Darlington transistors, because the output ports of the microcontroller can handle only 50 mA or so, and the bipolar need base current

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You can turn on the lower transistors with 3.3 volts through 1K resistors, if the motor doesn’t draw more than around 52ma! And, using NPN transistors for the upper transistors will require a much higher voltage on the bases. If you need to drive the motor with 12 volts, then the voltage at the base of each of the upper transistors will need to be at least 12V + 0.7V + whatever the saturation voltage of the lower transistor is (1.1Max at 4A).

So, there are two problems here:

1. Probably not enough drive on the base of the lower transistors. I’m assuming the motor will need more than 52ma to run, else why use 2N3055 which are intended for much higher currents (15Amps max!). The collector current is going to be the base current times the hFE (20-70 at 4Amps and 5 at 10Amps). So, if the motor requires 4Amps, then you need 4/20=200ma at the base. Since I doubt your arduino outputs can supply 200ma, you will need another stage of amplification — i.e. another transistor — and the base resistor on the 2N3055 will need to be (3.3V – 1.5V)/200ma=9ohms or less [why 1.5V and not 0.7V? Because, at 200ma into the base of the 2N3055, the Vbe is going to be more like 1.5V (whereas at 52ma it will be more like 0.7V)]. But, lets say your arduino can supply 20ma, then 20ma*20= 400ma. So, if your motor only needs around a half an amp, then you might get away with it (and the hFE might be higher at this lower current, so you might be able to get away with an even higher motor current, but then, you would be out of spec, so not a good idea if this is to go into production.) Or, you could use a darlington, as suggested by Mr. Valentin, in which case, 1K might be just right, but do the math.

2. The upper transistors need to be driven with a much higher voltage (look up emitter follower circuits). Again, more transistors—as you will need to apply a voltage of around 13.8 volts.

You would be much better off using PNP transistors for the upper transistors (like in the schematic supplied by Mr. Valentin).

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What is the exact problem? Also VCC is 6V, is your motor 6V?

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basically I’m trying to build an h bridge with 4 tip3055. Im using a 12v external battery, and a 12v motor. When I turn on the transistors they only send out about 3v through the emitter.

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The two BC337 are only providing about 5.7mA of sink (beta = 100). The TIP using a 470-Ohm base drive would deliver 24mA base drive which if the TIPs had a beta of 100 would give you 2.4A.

The BC337 are not saturated and in order to do that the base drive needs to be increased at least 5X, A 47K resistor won’t cut it, this could be a typo whereby perhaps it should have beed a 4.7K.

Here’s the deal, put your scope on the BC337 collector signal. It should go down to say 250-300mV when it’s on. If not you need to address that because they should saturate!

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Look again—it IS a 4.7K resistor. That’s what 4k7 means. Also, you’re looking at the wrong schematic. Mr. Valentin supplyed that as an example to Mr. Wong of a better way to do it.

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