Baby steps. I'm working on a project where I want to control a DC motor, bi-directional with just 3 control lines. Before I get to the big part of the project, I'm trying to just get/understand some of the smaller sections working first.
My problem is getting the H-bridge working. I'm currently using IRLZ44N mosfets. If I just wire 5v to the mosfet to the motor and turn it on, everything works as expected. (My final project will have 12v to the motor) I get a 4.9v drop across the motor and .001v across the trasistor, which I would expect due to the low mosfet Rds. But, when I wire the 2nd mosfet in the circuit, the motor runs very slow and only has a 2.8v drop across the motor and now a 2.1v drop across the first transistor. Shouldn't both transistors have extremely low Rds and all the V drop across the motor?
Please post a pic of a hand-drawn schematic (wiring diagram).
It really is not worth the effort to make your own H-bridge, as there are too many pitfalls involved in coming up with a reliable design. Pololu has a great selection of well engineered motor drivers, backed by product support.
Thank you for your quick reply. I hope this is what you are looking for. I first tried this with the full H-bridge and got the same effect, so I just concentrated on half of the bridge with the same results. I'm fairly new the the mosfet transistors. All mosfets are IRLZ44N.
The problem with that design is that it is impossible to turn on the upper transistor(s) fully.
The key specification is the gate to source voltage, Vgs, not the gate to circuit ground voltage.
In the future, please post clear, hand drawn diagrams, not low contrast, illegible screen shots.
Thank you again for a quick reply. It makes sense that the results are due to the transistor not being fully turned on. Now the question is how do I get the 1st transistor to turn on fully.
Eventually I would like just 3 control lines. 1. Motor on/off 2. Direction 3. Speed (controlled by pwm). There will be TTL AND gates and an Inverter for some of the control logic. The And gates will help prevent both sides being turned on at the same time and the PWM will control the speed connected to just the lower transistors. This is why I chose to build my own and not a premade H-bridge.
Now the question is how do I get the 1st transistor to turn on fully.
Set Vgs properly, using appropriate circuitry.
If you insist on building your own H-bridge, do the research. There are countless technical articles on line discussing H-bridge design, and, of course, some articles written by people who have no idea what they are doing.
To turn on the top transistor you need a supply voltage Vgs threshold above the drain. In ic motor drivers using n channel MOSFETs they have a charge pump to generate this.
Hi, @cmndr-brain
Welcome to the forum.
This is a very good attitude, small steps. 
1/2 H-Bridge is a good start.
In your analysis you also have to measure the Source to Gate voltage of each MOSFET.
This will tell you which MOSFET is being turned ON or OFF and why.
for your choice of MOSFET, logic level will save a lot of problems trying to get any circuit working with 5V logic.
Tom.. 
Hi,
If you Google;
arduino discrete h-bridge circuit
You will find many tutorials about building a H-Bridge and its control components.
There are different circuits for instances where the motor supply is the same voltage as your logic level and when motor supply is higher than your logic level.
That is Vmotorsupply = 5v or Vmotorsupply > 5v.
What model Arduino are you going to use the H-Bridge on?
Thanks.. Tom..
Tom,
Thank you for the information and kind words. Currently I have a couple of Arduino Nano's to play with. This is just a hobby and was trying to learn something new. I'm kind of new to Arduino as most of my other projects were Pic Microchip. Software... I can hold my own, but lack a lot of the hardware background. I've worked with TTL logic mostly. Unfortunately, there is a lot of bad/wrong information out there about these circuits. At least you gave me a starting point to the right track. Thank you again.
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