Simple Motor Controller Circuit

[BobK]

If you are using an H-bridge configuration, the body diodes are indeed exactly in the positions called for (you cannot use a diode across the motor with an H-bridge since it will see both polarities). But, you must ensure that they can carry the full motor current. Often, they are not rated as high as the MOSFET for current.

Bob

 

[(*steve*)]

Often, they are not rated as high as the MOSFET for current.

And it's often a dissipation issue as well. The voltage across the body diode is generally higher than the voltage across the mosfet when it's switched on.

However, if you're looking at very short transient currents, heating is not likely to be an issue.

 

[tehtehteh]

the motor is 40ma free spinning, it won't get anywhere near stalling and is geared down a lot, so it should run relatively close to 40ma at all times even when driving the gimbal, that said I've no idea where to find what the diode is rated at, if it's in the data sheet it's not obvious

 

[Arouse1973]

I have seen body diodes used for this purpose but there tends not to be much info about them. Its up to you but if it was me I would rather use a diode that I had a data sheet for and knew its limitations. With the body diode you do not have this info so I would not use it.
Adam

 

[Alec_t]

the motor is 40ma free spinning, it won't get anywhere near stalling

Don't forget it is in the stalled state immediately it is started up.

 

[tehtehteh]

ok good point, but I think it'll be alright as it gets up to speed seemingly instantaneously so by the time flyback diodes are needed it should be stable

went with this circuit in the end, an advantage was also that I can't easily see it being in short circuit, not that it matters now that it's microchip controlled, I will put in appropriate delays to make sure it stays safe

 

[Alec_t]

it gets up to speed seemingly instantaneously so by the time flyback diodes are needed it should be stable

The flyback diodes could be working hardest when the motor is first switched on, if there is intermittent brush contact with the commutator.
Your specified PFETs are marginal for their power handling. I'd choose something with a lower Rds(on) rating.

 

[tehtehteh]

ok thanks for the advice, I will change the mosfets

in addition, I've got most of the pcb layout complete and it seems I can get this all to fit on a 23mm * 46mm board with a little room to spare, so it may be that I could fit some diodes or extra capacitors (or both)

capacitors across the motor terminals? from terminals to ground? both? any advice would be appreciated

 

[Alec_t]

capacitors across the motor terminals? from terminals to ground? both?

For best noise suppression you may need both.

 

[tehtehteh]

can't find any p channel mosfets the same size that can take higher current, this one can take 3 amps for 300μs, I don't know how long the current surge would last for when spinning up the motor, I'll just have to try it and see

I was reading about capacitors across motor terminals yesterday and there was one response that suggested they may make the noise issue worse, I don't know enough about it I'm afraid

 

[tehtehteh]

here's the PCB layout, I put 100nF caps across the motor terminals and there may still be room for diodes, but as there's 8 of them they would need to be very small

any thoughts? someone with experience of making PCBs might have some tips for me (this will be my first!)

 

[Alec_t]

they would need to be very small

SMD types are very small.

 

[tehtehteh]

not convinced I'll be able to solder surface mount

 

[tehtehteh]

found a through hole diode that was small enough if I mounted it vertically

the PCB layout is a bit of a mess now but it's all free of design rule errors so I can't complain

this will be what I get printed unless anyone has any further suggestions or spots errors that I've missed

 

[BobK]

The diodes can be mounted directly on the motor terminals instead of on the PCB.

Bob

 

[tehtehteh]

do you mean the capacitors? I'm not sure I'd be able to anyway, the motors are tiny and I have even less space to work with than on the PCB

 

[BobK]

No, I meant the diodes, but if you don't have space, you don't have space.

Bob

 

[tehtehteh]

I don't understand how I could translate the diodes as they are in the circuit diagram to being on the motor terminals, at least not without running extra 5v and ground leads to them

 

[BobK]

Never mind. I did not realize you were using an H bridge.

Bob