I have two A4988 drivers connected to one Arduino Nano. They drive motors that were purchased on eBay and similar to those shown in the picture; coil current rating unknown, coil resistance measured at 48 ohms. They are used on a scale model boat to rotate radar scanners, so essentially unloaded.
They are set up in pairs, so driver A (connected to motor through Orange plug) powers the small scanner, driver B (Blue plug) the larger one. Both motors worked fine on the breadboard setup. where the measured motor current into each driver was ~20mA. They worked initially when hard wired, but subsequently not so well.
Currently, both scanners work OK when each one is powered from driver A. Neither will work when powered from driver B. After first seeing this, I replaced driver B with a new A4988, but no change.
When powered by driver A one coil of each motor’s current was measured to peak at 8mA.
I have tried adjusting the current limit on driver B; the current is steady, and the minimum I can achieve on either coil is 20mA. I have tried adjusting it up to about 100mA, but the motor does not run and just gets hot. If I push gently on the scanner it will jump to the next position but stop there.
The testing has been done using alkaline batteries so a voltage of 7.5.
Attached are pictures showing the motors, hardware setup, the stripboard wiring layout, the wiring diagram, and the relevant parts of the Arduino sketch. The wiring diagram omits an Arduino used for the propulsion of the model, but it is shown on the other pictures.
Any advice on what might be preventing the one motor from running will be greatly appreciated.