Problem with H bridge and Arduino

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.

• Did you make sure that the square wave put out of pin 7 and pin 6 are complementary i.e. opposite polarity? What voltage is Vdd? – loudnoises Oct 24 '17 at 10:50
• You're not trying to power the bridge through the Arduino, are you? – Dampmaskin Oct 24 '17 at 10:51
• Yes the voltage of pin 7 and 6 are complementary, I double check it with an oscilloscope. – Lucrus Oct 24 '17 at 10:56
• And Vdd is power supply adjustable ( it can deliver 0-15 V and 3A), and no Dampmaskin ;) – Lucrus Oct 24 '17 at 10:58
• You don't want a bipolar bridge. Use FETs chosen for low on ressitance at a low gate drive - but bear in mind your high side will be complicated. If planning less than an amp, just use a bridge IC. – Chris Stratton Oct 24 '17 at 11:59

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.

• Whoever downvoted this, what exactly do you think is wrong? – Olin Lathrop Oct 24 '17 at 11:24