>On Sat, 25 Oct 1997 17:09:13 +0100 Alec Myers writes: >>I'm having a bit of difficulty with some cheap RC servo units (being >>driven >>from a PIC). >> >>Every now and then, the servo unit goes into a funny oscillating mode >>(frequency the same as the driving pulses). It draws lots of current >>(>500mA), > >Drawing 500mA is normal. It takes power to turn the motor. When the >servo reaches the set position (without having to hold against external >torque), the motor will cut off and the power consumption drops to just a >few mA. The motor is driven with PWM at the drive-pulse frequency, so >even at less than maximum effort, the servo power rail will have to >supply relatively large pulses of current. Budget about 1A peak for each >servo. If the servo power supply voltage is dropping, the servos may >malfunction like you describe. Mike, I'm afraid it's not that simple. Most of the time, the unit draws about 500mA while moving, then reaches the set position and stops. Then the current drops to about 10mA. Only sometimes, it gets to about the right place, then oscillates, and the current doesn't fall off. It _feels_ like the pulses are random lengths - the motor is turning full torque one way then the other - but on the 'scope, the pulse lengths are steady as a rock. If I push against the servo, or stall it with excess friction, then I'd expect heavy current draw. But not at the low load it's under. Actually, running the servos from a current limited source (say max 200mA) helps, because the oscillations aren't as severe. But it doesn't seem like a good solution to me. I have tried different signal voltages (including Andy's suggestion of a 270 ohm resistor from the PIC output) but it doesn't make much diference. I will report more when I get a bit more time to play with the system. Alec