1
\$\begingroup\$

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.

enter image description here

\$\endgroup\$
  • \$\begingroup\$ 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? \$\endgroup\$ – loudnoises Oct 24 '17 at 10:50
  • \$\begingroup\$ You're not trying to power the bridge through the Arduino, are you? \$\endgroup\$ – Dampmaskin Oct 24 '17 at 10:51
  • \$\begingroup\$ Yes the voltage of pin 7 and 6 are complementary, I double check it with an oscilloscope. \$\endgroup\$ – Lucrus Oct 24 '17 at 10:56
  • \$\begingroup\$ And Vdd is power supply adjustable ( it can deliver 0-15 V and 3A), and no Dampmaskin ;) \$\endgroup\$ – Lucrus Oct 24 '17 at 10:58
  • \$\begingroup\$ 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. \$\endgroup\$ – Chris Stratton Oct 24 '17 at 11:59
2
\$\begingroup\$

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.

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

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.