So, after our first blog post, which you can read here, Watterott kindly pointed out to us that our post didn’t make sense, so we looked at TL-Smoothers for 3D printers again. Turns out, our explanation of TL-Smoothers doesn’t apply to TL-Smoothers, since these small boards don’t use freewheeling diodes. Instead – and to our surprise – TL-Smoothers simply put diodes in series with the motors.
Now, what’s important to understand is that this increases power dissipation significantly because the full motor coil current drops in these diodes. With a 0.8A motor, and a voltage drop of roughly 1V (maybe less or more, depending on the number of diodes) you get 0.8W additional power dissipation per coil, i.e. 1.6W per motor.
What will the series diodes cause?
By dropping voltage they generally eliminate energy. Eliminating energy causes heat and reduces efficiency, but it also may cause dampening and thus reduce resonance. To understand in more detail, we need to look at the different chopper phases and what the diodes will cause in each phase:
- On-Phase: Less voltage applied to the motor coil, so basically we lose dynamics, like with a lower supply voltage.
- Fast-Decay phase: Fast decay may become a bit faster, like with a higher supply voltage. Assuming that the fast decay phase was optimum before, now it will be too long.
- Slow-Decay phase: The slow decay phase is the phase with lowest power dissipation, as the motor coil current just re-circulates. In this phase, we get probably the most sensible effect: While eliminating desired motor current, at the same time the diodes will also eliminate current fed back from the motor’s back EMF due to resonances, thus offering a kind of electrical dampening. However, this is paid with a lot of energy wasted.
What does fast decay mean? Normally, stepper motor drivers not only put current actively into the coil, but they also feed back coil energy into the supply in order to reduce coil current during the falling slope. This is necessary to allow the motor current to follow the desired sine waveform. Most drivers do not control this so-called fast-decay phase, but they use a pre-determined time for fast decay. This time might be too long, which basically will just waste some energy by causing higher current ripple, but it can also be too low, which will cause a non-optimum sine wave. This might be exactly the point, where the diode array also helps. The control of this time is not easy, because the required fast decay period depends on many factors, like supply voltage, motor type, velocity and mechanical load.
So, to get back to the question if you should use TL-Smoothers on your 3D printer, you shouldn’t. At least not when using Trinamic motor drivers. Our drivers actively measure motor current, also during the fast decay time when operated in SpreadCycle, so it automatically becomes optimized.