Posted: Mar 25, 2020 10:09 am
felltoearth wrote:May be a stupid question but why the 24V power supply if you have to step it down so much?
No such thing, felltoearth.
The drive motors are used wheelchair units designed to operate at the 24 volt nominal battery voltage. Running them at 12 volts would be possible, but the amperage drain would double and that would shorten the life of the brushes and commutator that feeds the armature windings. The Arduino on-board regulator might be able to accept that high voltage, but it would be right at the upper limit of its tolerance. The easiest way to step down that 24 volts is by using the little solid state LM7805 (5V) regulator for the Arduino and the LD1117 (3.3 V) for the wifi module. Both of those large “transistor” looking devices are cheap and been used for years in these type situations.
The motor drivers (BTS7960B’s) and these regulators will generate some heat at maximum load, and I am seriously considering using a thermistor (a variable resistor that changes value when heated) attached to the large heat sinks incorporated into the motor drivers to switch on a computer cooling fan at some yet to be determined threshold. I have several analog inputs left on the Arduino Nano operating as the “Slave”, and it would be a simple addition to the code to output a signal once that threshold was reached. That Arduino signal would then in turn on an N channel mosFET to drive the fan. The Arduino itself is not capable of driving even a small fan, so this multi step system would have to be implemented. I already have all the parts on hand to do this, but will probably wait until I have tested the drive coding before spending much time on the issue. That fan would also cool the heat sink for the voltage regulators, but I really don’t expect much heat build up in those devices as their loads will be well below their upper limits.
RS
ETA:
Upon giving the power supply issue a little more thought it became clear that supplying the LM7805 with raw 24 volts from the batteries would be pushing its upper limit. In light of this I’ve added a third step down regulator. This third device is an LM7812 12 volt regulator capable of sinking a little over 1 amp, which should be well above the load demanded of the Arduino and wifi module. The LM7812 is more than capable of dropping the 24 volts down so the downstream regulators can operate safely. Considering the heat and rough conditions under which the entire “Slave” will experience I don’t want the electronics stressed by running at their ragged edges.