1
\$\begingroup\$

I have two servos in an autonomous sail boat. One controls the rudder, the other the winch. I try to use both sparingly to minimize power consumption, and so I disconnect each unused servo through an n-MOSFET, and additionally, when neither are in use, I put the power regulator to sleep. Here's the circuit diagram:

enter image description here

I added the large caps based on recommendations from the interwebs to reduce "jitter," but given the fact that keeping a fixed position isn't really important here, are the caps doing anything besides effectively wasting energy? What I see when I scope the system is that when I power down the regulator, the caps discharge within a few seconds, meaning the power is wasted somewhere.

\$\endgroup\$
3
  • \$\begingroup\$ Any servo system needs integrators to account for inertia in the physical system it is controlling. \$\endgroup\$
    – user207421
    Sep 25, 2014 at 23:53
  • \$\begingroup\$ @EJP can you explain what you mean by "integrator" in this context? a bit of googling talks about circuits akin to what the name suggests - some sort of measuring mechanism. \$\endgroup\$
    – kolosy
    Sep 26, 2014 at 14:26
  • \$\begingroup\$ In an electronic circuit it will be an integrating capacitor such as the ones you're asking about. The basic idea is that the servo acts slower than the thing it is controlling. Otherwise the system may oscillate. On e.g. a governor for a steam engine it will be a little weight. \$\endgroup\$
    – user207421
    Oct 12, 2014 at 21:34

2 Answers 2

3
\$\begingroup\$

Typically, any feedback system requires capacitors, so that small fluctuations in the power supply due to movement of the motors do not create another unintentional feedback path. In this case, the charge lost in 1 discharge cycle of two 250uF capacitors is minuscule compared to your battery capacity: 250 uF * 5 V is about 1.25 mC, by comparison 1 mAh (where most batteries are on the order of thousands of mAh) is 3600 mC. Therefore, a typical 1000mAh battery can theoretically support many million charge and discharge cycles of your caps.

Not having capacitors may very well reduce your battery life, if small fluctuations in the power supply cause noise in your servo feedback readings and therefore cause the motors to move unnecessarily. Powering up the motor (even for a couple of milliseconds) is likely to waste magnitudes more energy than charging and discharging the caps.

\$\endgroup\$
2
\$\begingroup\$

Capacitors don't consume significant energy (just a tiny bit of leakage).

As batteries discharge their internal resistance increases so that circuits connected to them that do not draw steady current can malfunction while there is still plenty of energy left in the battery. The capacitors can counteract that if they're large enough (250uF isn't very large) in relation to the current drawn and the time scales involved.

There is really no downside to having them there, and they will likely have a very positive effect.

\$\endgroup\$
2
  • \$\begingroup\$ This is a bit of a tangent, but how would I go about sizing them correctly for the application? \$\endgroup\$
    – kolosy
    Sep 23, 2014 at 3:07
  • \$\begingroup\$ Sigh, that's a bit more difficult without knowing a lot about the load. Without involving the battery at all, a cap could be sized as C = I*t/delta-V, so if the load is 1A for 1msec and voltage can change 1V, then C= 1mF = 1000uF. \$\endgroup\$ Sep 23, 2014 at 3:38

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.