Figure 1. Motor control schematic.
I am trying to replicate this circuit: Parastaltic pump driver.
The problem I have is after I lay out the circuit exactly as described, with a terminal block for the 12v power supply, the positive terminal of which goes up junctions to the Anode of the diode, then into the + of another terminal block, which runs to the pump, then comes from the - of the terminal block, junctions to the Cathode of the diode and then into the collector ([Pin 2]).
I have a 2.2kO resistor connected between the third and final terminal block + to the base pin, then I have the collector from the TIP120 and the - from the third terminal block going to the - from the 12v supply terminal block.
My understanding is that both power supplies need to have a common ground because obviously otherwise the + from the 3.3 PWM wont have anywhere to go and wont trigger the base of the TIP120.
Now, where it gets weird, for me...is that when I hook up the 12v power supply and I measure the voltage between the + and - on the 12v terminal block I get...12v...like the circuit is complete. The motor does nothing though. And no current is going through the base pin. If I take my pprogrammable power supply that I'm using in place of the 3.3 from the Arduino, plug it into the wall, do NOT turn it on, and touch the + lead to the + of the terminal block leading to the resistor connected to the base pin...the motor starts to slowly turn. If I touch the - from the power supply to the - terminal of the same block, it stop turning and something starts smelling funky (shut it off before I could investigate, the TIP120 is cold to the touch however).
Now, I did this in a project board and you know them, you have to do some nasty solders, so I thought, ok, I don't see any shorts or measure any shorts but lets do it in a breadboard just to be safe. Recreated the whole thing, same way, EXACT same result.
How the hell is touching the positive terminal of the powered off (if I unplug the DC PSU from the wall nothing happens when I touch the + lead to the resistor on the base) PSU causing the motor to spin? It also acts like its got plenty of current but low voltage as the motor spins up right away just very VERY slowly.
I've measured the DC motor and it takes about 375mA to get going and once its humming along with no load it draws around 80mA anything less than 375mA and it slowly ramps up, but obviously if I drop the voltage and leave the output current at 375mA it slows the RPM without impacting its ability to start.
I freely admit I'm new to this and learning but my understanding what, no base current, no circuit. But it acts like its free to carry all the voltage it wants, just no current. As to why hooking up the positive to a non powered on PSU allows it to start turning...and worse still why then also touching the negative from the powered off CPU appears to allow it to draw a LOT of current...I'm at a loss.