1N4007 or MBR340 for Flyback ?

I have a Mega 2560 with Ramps 1.4 and 4 steppers ( similar setup to, but not a 3D printer ) and a 4x20 LCD screen connected to the Ramps board.

The PSU ( 12V 18A ) also directly drives a 12V DC motor from a remote control offroad truck ( 5000 rpm low speed, high torque, brushed motor ).

Am sometimes getting a scrambled screen on the LCD, and I suspect it could be caused by the motor, so wondering if the 12V motor should have a flyback diode ? The motor in this setup is the only thing different to my 3D printer that uses all the other same parts.

I have a 1N4007 diode, and a much larger MBR340. Would either of these be suitable ?

Try the 1N4007.
BTW, always have a fuse in your circuit where large currents are flowing.

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larryd:
Try the 1N4007.
BTW, always have a fuse in your circuit where large currents are flowing.

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Thanks Larryd

Had not thought about a fuse - I have some 10A glass fuses with holders so will add that to the PCB with the diode.

DaveO:
Am sometimes getting a scrambled screen on the LCD, and I suspect it could be caused by the motor, so wondering if the 12V motor should have a flyback diode ?

Of course, you have a highly inductive load and presumably don't want 100V across your RAMPs board
even for an instant!!

The diode's pulse rating should comfortably higher than the maximum load current the motor takes.
(you didn't say what the motor was).

MarkT:
Of course, you have a highly inductive load and presumably don't want 100V across your RAMPs board
even for an instant!!

The diode's pulse rating should comfortably higher than the maximum load current the motor takes.
(you didn't say what the motor was).

The motor is an AX24007 Axial 55T Electric motor ( Axial Racing - 55T Electric Motor ) but I can't find much in the way of specs, and no response to enquiries sent to Axial.

edit : the highest rated glass fuse I have lying around is 3A ( not the 10A that I thought it was )

You didn't mention if the motor is being PWM'd or on continuously.

  1. If PWM'd I would use the MBR340 + a 0.01µf ceramic capacitor.

Reasoning: Assuming your switching times are fast (i.e. the time it takes to transition from On to Off and vice versa. The 1N4007 is not good because it will conduct in the reverse direction for a short period of time when switched quickly (Google reverse recovery time and /of look here Reverse Recovery Time of a diode)

The 0.01µF capacitor is to reduce voltage noise spikes from the brushes. You would really like more but a large capacitor will interfere with the switching. 0.01µf seems to be a good compromise.

  1. If no PWM I would put a 1.0µf or at least a 0.1µf ceramic capacitor across the motor.

Reasoning:
Same as above but with no PWM the capacitor can be much larger. You don't want to go much larger because the larger capacitors and non ceramic capacitors are not as capable at high frequencies (of the brush noise).

Grumpy_Mike had an excellent tutorial on capacitors at high frequencies.

In both the above cases, the capacitor should be as close to the motor terminals as practical.

Good luck

JohnRob:
You didn't mention if the motor is being PWM'd or on continuously.

.... .... ....

  1. If no PWM I would put a 1.0µf or at least a 0.1µf ceramic capacitor across the motor.

Hi JohnRob

The motor is on contimuously with the 12V from the PSU to the motor controlled on / off with a manual switch.

I have some 10nF ( which I think = 0.01uF ) ceramic caps from here ( supplier link

So I should use the capacitor across the motor terminals, and the 1N4007 as the flyback diode.

Any suggestion on how to determine the fuse rating that would be required ?

20% less than the power supply capacity.

larryd:
20% less than the power supply capacity.

so if the PSU is 12V 18A then the fuse should be 14A ?

I think 15 is a value you can purchase.

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larryd:
I think 15 is a value you can purchase.

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Thanks. I see 10A and 15A listed at our supplier.

But would it not be better to use a lower rating that is closer to the current draw of the motor ?

I mean that if the motor is rated at, let's say 4A ( althought I can't find any specs for this motor ), then go with a fuse at the rated current + 20% ?

DaveO:
But would it not be better to use a lower rating that is closer to the current draw of the motor ?

Depends on how hard it is going to be to get to that fuse in the finished product.

Delta_G:
Depends on how hard it is going to be to get to that fuse in the finished product.

I have all the electrics and electronics for the machine in a large aluminium control box with a hinged door, so access is very easy.

You are protecting against an accidental short circuit, 15 amps would do that.

Go smaller, sure, some fuses blow at 2X their rating.

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@DaveO,

With a capacitor the diode is not as important.

Info on motors in general:

When you turn off a DC motor, it will create a short voltage spike due to the inductance of a motor. Inductors don't like changes of current, they will generate a voltage to try to keep the current from changing. This will be very short (10's of µsec).

After that, the motor which is now un-powered but still spinning, will act as a generator and generate a voltage. In your case it will start at 12V then decay to zero as the motor slows down. This should not be an issue but you should know about it.

Also the motor will take longer to slow down if you add the 1N4007 than if you stayed with just a 1µF cap.

As for a fuse, as suggested a 10A will probably be OK, however if you start the motor under load you may find the initial current draw to get the motor moving is too much for the 10A. In that case you look for a 10A slo-blow fuse which was created for such a load.

Does your power supply have any built in protection?

Good Luck

JohnRob:
Does your power supply have any built in protection?

The spec on the psu is : Protection: Shortage Protection, Overload Protection, Over Voltage Protection