Hi

I am designing a circuit for driving the stepper motor, the link attached (https://www.alibaba.com/product-det...m=a2700.7724838.2017115.17.149a279bsYy1CA&s=p) using 2xAA (2.4V) NiMH 800mAh batteries. Also, I am using a boost converter U1V10F5 (0.5V-5.5V)to step up from 2.4V to 5V to power up the motor as well as the driver (DRV8834). I've found that the motor that runs at 2.7V consumes approximately 70mA. The question is if I am using normal 2xAA(3V) battery or bench power supply (set it to 2.7V) the circuit is working fine and the motor is running. Whenever I set the voltage to 2.6V or 2.4V on the bench power supply or connect the 2XAA rechargeable battery my motor stops working.

Is that the rechargeable battery not having enough capacity? or the step up booster is not able to supply enough current to drive the motor at 2.4V?
Please let me know why the motor is not running at 2.4V and runs at 2.7V. I have attached the rough schematic. I am using two PSU for the microcontroller(5V@3A) and the motor(2xAA(2.4V)) indicated in dotted lines in the schematic. Your guide is much appreciated. Thanks in advance.

 

NiMh batteries terminal voltage is 1.2V. Two in series would give 2.4V. You have established that your circuit doesn't work at 2.4V so it's not going to work on two NiMh batteries.

What is the output voltage of the booster with a 2.4V input?

 

NiMh batteries terminal voltage is 1.2V. Two in series would give 2.4V. You have established that your circuit doesn't work at 2.4V so it's not going to work on two NiMh batteries.

What is the output voltage of the booster with a 2.4V input?

5v out

Do you think booster part is not having enough current ?

 

Verify voltage at the motors. Or use resistors as a load instead of the motor. Measure voltage across the resistors.

 

Do you think booster part is not having enough current ?

If the booster is providing 5V out then it doesn't seem to due to the booster. Equally if the stepper driver has a 5V supply then it ought to be working but isn't. On the face of it it is the stepper driver that has the problem but it does work when the booster input is higher.

Confused.

What is the booster output when its input is 2.7V?

 

Hello,

The step-up converter can handle a max input current of 1.2 Amps.
At 2.4 Volts input voltage I would expect a max output current of about of 0.5 Amps, if the batteries allow the 1.2 Amps input current.
I have the idea that the internal resistance of the batteries is the cause of the failure.

https://www.pololu.com/product/2564/specs

Bertus

 

I have the idea that the internal resistance of the batteries is the cause of the failure.

But it doesn't work on the bench supply set to 2.4V either

 

Hello,

It looks like you can not draw more than 300 mA from the regulator @ 2.4 Volts.



Bertus

 

5v out

Do you think booster part is not having enough current ?

Ni-Mh are greatly inferior to Ni-Cd when it comes to instantaneous current draw.

When the Ni-Cd AA cell in my shaver gave up, I tried a Ni-Mh replacement - the run time of the 2400mAh Ni-Mh cell was about the same as the old knackered 600mAh Ni-Cd cell. The cell got warm, which was a clue to where the missing energy went.

A rechargeable lithium would make life easier, but they're dangerous if not charged correctly - you can get a USB/lithium charger PCB from eBay as little as 0.99p and no postage.

 

If the booster is providing 5V out then it doesn't seem to due to the booster. Equally if the stepper driver has a 5V supply then it ought to be working but isn't. On the face of it it is the stepper driver that has the problem but it does work when the booster input is higher.

Confused.

What is the booster output when its input is 2.7V?

5V is the booster output when its input is 2.7V

 

Still confused. The supply voltage to the stepper driver is the same whether the supply voltage is 2.4V or 2.7V, yet with a 2.4V supply it doesn't work. How does the stepper driver know what the supply voltage to the booster is?

 

Still confused. The supply voltage to the stepper driver is the same whether the supply voltage is 2.4V or 2.7V, yet with a 2.4V supply it doesn't work. How does the stepper driver know what the supply voltage to the booster is?

That is the problem now. I have found that when you drop the voltage from 2.7V to 2,4V slowly using bench power supply, the current draw suddenly goes down to zero when it is at 2.4V and the voltage across the output of the booster collapses as well. Finally, the driver stops. When it is at 2.6V the driver struggles to drive the motor and the current consumption will vary towards zero. Any idea?

 

That looks like the booster cannot provide enough output current with only 2.4V input. Either you need to draw less current or a more capable booster.

 

That looks like the booster cannot provide enough output current with only 2.4V input. Either you need to draw less current or a more capable booster.

Yes you are right. That makes sense. I've ordered another booster. Thanks for your help.