Simple Motor Controller Circuit

[tehtehteh]

hello

I am trying to control a small gimbal using two 5v motors and this this switch:

http://uk.rs-online.com/web/p/joystick-switches/0516031/

which is basically 1 pin for each switch and a common pin, if this was a couple of DPDT switches it would be easy for me but it's not so I have come up with the attached circuit

is this a good solution? does anyone know of a better one?

also it will be powered off a small cable shared with a potentially sensitive 5v camera, so any advice on the capacitors would be appreciated too

 

[Arouse1973]

Wouldn't a DPDT switch work?
Thanks
Adam

 

[tehtehteh]

yes, but for size constraints and for the ergonomics of controlling the gimbal the switch I have chosen is more appropriate

 

[tehtehteh]

ok due to incorporating feedback into the circuit I will now be controlling it with a pic rather than directly from the switches, this has caused me some confusion

if I make the circuit in the attached image, either of the top mosfets will be seeing close to 5v at source right? making the difference between gate and source not enough to reliably switch?

 

[Kabelsalat]

I assume you're aware the risk of shorting by the mosfets in case both switches is closed.

 

[tehtehteh]

yes I'm aware, that's not the problem I'm having

 

[tehtehteh]

if I change the top mosfets to p channel wouldn't the voltage at source still interfere with their operation?

 

[Arouse1973]

if I change the top mosfets to p channel wouldn't the voltage at source still interfere with their operation?

You connect the source of the P-channel to the power rail of the supply.
Adam

 

[tehtehteh]

ahhh I see, the symbols I've been seeing have been confusing

as the mosfets have diodes built in can I leave out the flyback diodes? space will be a real issue on this board

 

[davenn]

can I leave out the flyback diodes? space will be a real issue on this board

only if you don't mind killing the MOSFETS

 

[tehtehteh]

please explain

 

[davenn]

the diodes are there to protect the MOSFETs from voltage spikes from the motor

google why flyback diodes are used on inductive loads

 

[tehtehteh]

I know what flyback diodes are for, I'm asking if I can ditch them because the mosfets already have them internally in the same configuration

 

[davenn]

I'm asking if I can ditch them because the mosfets already have them in the same configuration

what same config ?

not making sense

 

[tehtehteh]

these 2 diodes are the same no?

 

[davenn]

these 2 diodes are the same no?

yes the internal diode will do the same job
your earlier diagrams didn't show FETs with the diodes included .... hence the confusion and why I asked

 

[duke37]

The internal diode does NOT do the same job as a flywheel diode but some mosfets use a zener diode in this position to limit the collector voltage.
The normal internal diode conducts when the polarity is reversed.

 

[davenn]

The internal diode does NOT do the same job as a flywheel diode

I found a site that says that in this case with the MOSFET stated, that it would
hence why I responded as such earlier

edit .... found it again ......
from wiki

Body diode[edit]
It can be seen in figure 1 that the source metallization connects both the N+ and P implantations, although the operating principle of the MOSFET only requires the source to be connected to the N+ zone. However, if it were, this would result in a floating P zone between the N-doped source and drain, which is equivalent to a NPN transistor with a non-connected base. Under certain conditions (under high drain current, when the on-state drain to source voltage is in the order of some volts), this parasitic NPN transistor would be triggered, making the MOSFET uncontrollable. The connection of the P implantation to the source metallization shorts the base of the parasitic transistor to its emitter (the source of the MOSFET) and thus prevents spurious latching.

This solution, however, creates a diode between the drain (cathode) and the source (anode) of the MOSFET, making it able to block current in only one direction.

Body diodes may be utilized as freewheeling diodes for inductive loads in configurations such as H-bridge or half bridge. While these diodes usually have rather high forward voltage drop, they can handle large currents and are sufficient in many applications, reducing part count, and thus, device cost and board space.

 

[(*steve*)]

Body diodes tend to have very poor reverse recovery and are suitable only at low frequencies.

As long as you're not switching at high speed (i.e. for PWM control) it's not a huge problem.

 

[tehtehteh]

it will not be PWM switched, so I'm quite happy with that

I have about 30mm by 40mm space to work with, and I have to fit a 14 pin microcontroller on that, and some caps, so leaving out the diodes will be handy