Electronic – arduino – Problem with H bridge and Arduino

arduinoh-bridge

I have built an H bridge to control a coil but I've got issues …

I built my H bridge with 4 darlington transistor : Tip122 (datasheet : https://www.onsemi.com/pub/Collateral/TIP120-D.PDF) and I control it with an arduino MEGA.

I made a program that sends a square wave signal of 5V through the pin 7 & 6, and it works when I observe it on an oscilloscope. However I can't have more than 0.3V at the level of the coil. I need roughly 3-4V and 3A for my coil(the resistance of my coil is 1.4Ω).

I put 1K resistor and the TIP122 have a gain of 1000 so it should be satured.

I have try everything but it still not work… I read on internet that with this type of circuit I will not have more 4 V at the level of the coil but I only have 0.3V, wich is clearly not suffisant.

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Best Answer

There are several problems here:

  1. Darlingtons are a poor choice since they have relatively high voltage drop when on. At only 15 V supply, the 1 V or so across each darlington is significant.

  2. Not only will the voltage drop of the darlingtons rob voltage from the load, but it also causes the transistors to dissipate power. At 1 V across and 3 A thru a transistor, it will dissipate 3 W. That will require heat sinking.

    At 3 A of load current, the transistors will waste about 6 W total.

  3. Emitter follower makes no sense for the top transistors. You want this H bridge to work with up to 15 V supply and to control it with 5 V logic signals.

    The emitter followers as you show them provide current gain but no voltage gain. Even worse, they will drop about 1.4 V from input to output. With 5 V in, you won't get more than about 3.6 V out at the emitter.

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