Fault condition on MD01B


I have a MD01B and I’m trying to drive a 7.2V drill motor. I have it hooked up to a battery (7.2V) and I read about 3A no-load. A lot of times when I turn on the PWM line the motor driver (MD01B with VNH2SP30) goes into fault on at least one of the h-bridges. The motor just makes a little chirp. I assume this is some level of overload, but I don’t know what I should do about? Is the current at start-up just so high, or is it an LC-problem? Should I try to find a bigger motor driver that can handle larger currents? If I hook it up to my 3A current-limiting power supply then it doesn’t fault and I’m able to control it with different speed and direction. Even with the current-limiting power supply it still errors out, and 3A should be way below the max rating for the MD01B, right?


Doing some more research, this seems to be a similar issue as this:

Since I’m also using a drill motor. I measured 0.5 ohms over the motor (actual with 10’ of wires, so it’s probably even less). Sounds like a ton of current and that’s what trips the fault condition. The question is what should I do? The motor fits mechanically and torque-wise very well. Should I try to find a different speed controller?

Thanks for any input,

Hello, Hakan.

I agree that it is probably the high-current at the start that is putting the driver into a fault condition. To reduce the starting current, you can lower the motor voltage to decrease the stall current, or you can slowly ramp up the PWM duty cycle.

- Ryan

I’m already starting it with a low PWM duty cycle, but it’s erroring out even then (I assume the inrush current applies even at a low duty cycle since the output is fully on, even if it’s for a short period of time).

You could try lowering the voltage to see if that helps. If you have an oscilloscope, you can use it to look at the power and motor voltages. It could be that the drill is drawing enough current to drop the battery voltage below the operating voltage of the driver. If you notice that happening, you can try to mitigate it by adding some capacitance (as much as you can partically but at least a couple hundred microFarads) between motor power and ground close to the board.

- Ryan

I don’t have a lot of options to lower the voltage, I’ve tried 7.2V (battery) and it causes the issue. My next step is 5V power supply (PC), but it’s under the 5.5 v min, so that didn’t work. I’ve been looking at adding a capacitor, but since I’m doing both directions, what kind of capacitor with 100s of uF should I get that isn’t polarized?


What PWM frequency are you using and over what time period are you ramping up the speed? Also, the capacitor Ryan was suggesting goes across your power supply leads (as close to the driver as possible), so it can be polarized.

- Ben

I’ve tried a couple, typically 4-6khz, tried 10khz as well, about the same result. Ahhh, didn’t realize I can put it on the power supply side, very good, I’ll try that. Is there a way I can measure the inrush current with my oscilloscope?

I think you would need a special current probe for that. Can you just try looking at VIN near the driver to see what’s happening to the input voltage when you try to drive the motor?

- Ben

Thanks, I’ll definitely try that. It would be worth a lot to know if it’s an over-current fault or under-voltage. The range for this chip isn’t that big, especially when I need to keep the voltage low to limit current. Are there any other tricks to limit current on the motor, like adding a resistor? I think the motor has plenty of torque available (it’s just moving a skeleton, hehe).

A resistor would work, but keep in mind that you would be dumping quite a bit of power through that resistor, which is kind of a waste. You would also need to make sure you select a resistor that can handle that kind of power. A better solution would be to limit the voltage to the motor by using lower duty cycles. Even though the motor driver outputs are alternating between full-on and full-off, the inductance of the motor should effectively average the current out, assuming you have a high enough PWM frequency. Is there a duty cycle you can reliably use that doesn’t trigger a fault?

It sounds like you are looking for a solution that works with parts you already have, but you might just consider getting a more appropriately sized motor, or getting a more powerful driver (e.g. if you get the dual version of that board, you can use the two channels in parallel to get twice power).

- Ben

Yea, since this is a one-off for a Halloween display I’m more looking for a bandaid solution :slight_smile:
It would solve me a lot of effort to use the motor that I have since it has enough torque and gearing to move the skeleton at a good speed, finding a new motor within time may be tricky. So I’m trying all other options. Another driver may be an option, but they are kinda pricey so I was hoping to avoid that. I’d be ok with wasting power, but I realize I’d dump quite a bit of power on it.

I’ll try to increase the frequency and make sure it doesn’t fault on a low duty cycle, I kinda gave up when it just errored out on me and I didn’t have any tool to tell me why. But this discussion has been very helpful to me, I’ll make sure to report my progress.