Re: [CAD_CAM_EDM_DRO] cap for geckos

meneercen wrote:

> Hi,
>
> Alan's box shows 4 geckos, all without capacitors at terminals 1 and
> 2. Mariss white paper says to put a 470uF capacitor across these 2
> terminals if the cables from the power supply is more than ____
> inches. I put the capacitor across the terminals irrespective of the
> cable length. Which is a better practice?
>
> Also, is there anywhere on the web where one can learn the theory
> behind such things. As it is, one does not know what is the effect of
> not connecting the capacitos, or what if one uses capacitors less or
> more than 470uF, or what happens if one uses capacitors across the
> bridge rectifier (power supply) of value less or more than calculated
> from the formula in Mariss white paper?

To really understand the implications of this, you need a degree in
Electrical Engineering, or at least close to it. The Gecko drives (of all
flavors) are PWM power supplies of a sort, and draw pulses of current
from the power input. The capacitor acts as a local reservoir of charge,
preventing these current pulses from causing large voltage swings at
the input, due to the inductance of the power cables. Routing of the
power cables may be almost as important as length. Keeping the
supply and return wires close together reduces "loop area" thereby
reducing the inductance of the wires. You'd have to know the internal
details of the particular Gecko drive to evaluate the exact effect of
the capacitor. Things like the magnitude, duration and frequency of
the current pulses under worst-case conditions will affect the
performance of the capacitor. Other details of the circuit will determine
the susceptibility of the Gecko drive to whatever voltage fluctuations
are present. In serious cases, it could cause the drive to fault or
lose position, otherwise it might cause the motors to have less than
full power.

Jon