Here's my power supply (schematic except without the transformer since my source is already at 24 V AC and using a 100 uF electrolytic as my smoothing capacitor):
24 VAC rectifier -> 100 uF smoothing cap -> LM317 linear regulator (typical application on page 10 of LM317, running at 3.3V)
With the servo motor disconnected, I see the servo motor source voltage at 5V and the signal line is giving the pulses required to drive the servo motor. With the servo motor connected, there is a periodic voltage drop to ~1V, causing my MCU to brownout. I hear the servo motor twitching, so it seems like the circuit is trying.
Other stuff I've tried:
- Bypassing the 24 VAC and using a 3.3V DC power supply, the servo rotates fine, drawing about 200 mA from the power supply.
- Using a 24 VDC source, the servo rotates fine
- Using a larger smoothing capacitor (1mF) with 24 VAC, the servo still doesn't work. The voltage drop still occurs and the LM317 gets really hot.
I was thinking that the smoothing capacitance was not large enough so there wasn't enough current going into the servo but (3) disproves that. In addition, using smoothing capacitance calculations (C = (I * t) / dV), it seems like I should not need more than 100 uF. Any ideas what else could be wrong?
So, just to defend the LM317 choice a little,
- The servo motor is on no more than 10s per day and not more than a few hundred ms each time.
- With that, I was hoping that using a simple linear voltage regulator would be ok in terms of both cost and complexity.