Electronics 2000

Hi
I'm planning to use a Recom R-785.0-0.5 http://www.recom-international.com/pdf/ ... xx-1.0.pdf
switched regulator to convert a nominal 7.4V from a LiPo battery to a 5V output, to power the Rx & servos in a RC model glider

I have been told I need the following capacitors on the input & output sides
.. simply solder a 100uF on the input and 10uF output

Are these caps. necessary for an application such as this?
Do the values sound about right? - how does one calculate the values required? is there a rule of thumb / design criteria for this

Thanks, Geoff

Hi

Generally a decoupling capacitor on the input and output is required to smooth the supply and reduce the risk of the regulator circuit oscillating. The one on the input is most important, dependent on where the supply is coming from - with a battery supply it will be pretty smooth (since it isn't being rectified from an AC supply) so 100uF is a sensible value. The capacitor on the output is marked as optional in the datasheet. I'd suggest including it since the regulator will be driving servos what may introduce electrical noise onto the supply. Again 10uF sounds appropriate.

In the case of a supply originating from a mains AC supply there are formulae to calculate the required reservoir capacitor value based on the acceptable ripple voltage on the output, but in this case it is more of a 'rule of thumb'.

If you find you have any problems with sudden heavy loads (e.g. the servo moving) causing voltage dips and possibly interfering with the RX equipment then try increasing the input capacitor to, say, 1000uF.

Many thanks Simon - just needed a bit of reassurance