H bridge issue

Hi everybody !

I have build and H bridge to control a coil but I've got issues ...

So I build 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.

So I made a program that send a square wave signal of 5V through the pin 7 & 6 (I put a screenshot of my program), and it's work when I observe it on an oscilloscope however when I connect it to the transistor my signal is only 1V so the H bridge doesn't work (I rougly have 0.3 V at the level of the coil). What should I do ?

I have try everything but it still not work...

Thanks so much :wink:

Lucas

H-bridge-tipe.png

First, it's about attention to detail. Have a good look online of various darlington pair h-bridge circuits, and notice that the left-side of the circuit will have an npn darlington pair and a pnp darlington pair. That is...two different types of transistors on the left-hand side. The same case will apply to the right-hand-side of the circuit.

Now take a look at the circuit that you showed...... and look at the left-hand side. Does that look like two different types of transistors? Or does it look like the same type?

I search on internet and they are H bridge composed with 4 npn transistors, I don't understand why it can't works ?

However you're right I'm wrong with my schematics, I reverse the polarity of the transistor on the right side but I don't do it on my real electric circuit I check.

H-bridge-tipe (1).png

If you do use NPN for the high-side drivers then they don't need base resistors, that will marginally increase the
output voltage. The low side drivers definitely require their base-resistors though.

Trying to use darlingtons for a 5V bridge is basically a fail, you lose maybe 1.5V at each end, leaving
very little left for the load.

What is the load exactly?

Ok thanks, I will remove the two resistors at the top, and I need roughly 4 V on my coil.

I will not damaged my two upper transistor ?

And thanks for the answer :wink: !

You won't get 4V with darlingtons. If the current isn't too high simple BJT's will work (you'll also
need 4 freewheel diodes for them). BJTs in saturation will get to 0.2V perhaps, the high side one's
cannot saturate if NPN though, will need to be PNP (and need base resistors and will be inverted
drive).

You haven't told us what the load is yet.

I have roughly 3A in my coil, it's why I choose darlington.

Then you need a MOSFET H-bridge that will run at 5V, and add a 0.33 ohm resistor in series with the coil
so it gets 4V.

Simple BJTs don't have the (saturation) current gain to drive 3A from an Arduino pin.

Alternatively you could rework the darlington bridge to work from about 7.5 to 8V, with level shifting to the high
side drivers.

With this circuit how much can I get ? Because actually think they are a problem, in my coil I have 0.5 V exactly like the tension of the arduino on the base pin of each transistor but I doesn't have load or less than 50mA.... It not works with little diodes...

Ahd how can I do level shifting ?

Thanks so much for answering me :wink:

Google "level shifting high-side switch Arduino" perhaps?

With this circuit how much can I get ? Because actually think they are a problem, in my coil I have 0.5 V exactly like the tension of the arduino on the base pin of each transistor but I doesn't have load or less than 50mA.... It not works with little diodes...

I really can't understand most of this paragraph I'm afraid. I've told you what darlingtons from a 5V supply
can do.

lucrus91:
With this circuit how much can I get ? Because actually think they are a problem, in my coil I have 0.5 V exactly like the tension of the arduino on the base pin of each transistor but I doesn't have load or less than 50mA.... It not works with little diodes...

By saying that, I would like to say that it is strange thats actually I have less than 0.5 V at the pin of the coil, no ? On internet some have around 3 V with this circuit.

Its not in the least bit strange as I explained above. Using darligtons as emitter followers for the high-side
switches loses you volts, using darlingtons at all loses you volts, everything about the circuit loses volts and
the load is very low resistance in the first place.

Ok thank you a lot for all your awnser MarkT !