Voltage regulator capacitors, what to use?
Hi All,
Happy New Year to you all. I have a question regarding the capacitors on either side of a voltage regulator. I have seen multiple schematics with different values. What are these values based on? How are the values calculated? Just wondering if the values are figured by trial and error AND an oscilloscope or is there a formula one uses to calculate them?
http://www.seetron.com/images/vschem2.gif
My circuit is for a vehicle application. I’ve seen this schematic for a vehicle and they use 100uF caps on either side. I have another schematic from Ford where they show them using 10 uF caps on each side and another schematic using 0.1 uF caps on either side along with a 10 uF cap on the input side and a 220 uF cap on the output side. Are they just connecting a basic voltage regulator to their intended circuit, using an oscilloscope and adding caps until they get the desired results?
The other thing I’ve seen is a Zener diode connected directly to ground as in the circuit above, but another circuit has a Zener going to a resistor and then to ground. Any reason why? I know you want to protect your circuit using the Zener, but is the resistor really necessary?
I’d like to make sure my circuit is properly protected, but don’t want overkill. What are your thoughts?
Thanks,
Tony
Capacitor & PCB trace length affect stability
Read the data sheets for the regulator you are using. Most specify a maximum distance between the regulator and the input and output capacitors to ensure no oscillations from the regulator. If in doubt go for two large capacitors (100 uF or more) adjacent to the regulator input and output and make the PCB traces as fat as possible for minimum impedance. Make sure you have a big safety margin on the input capacitor voltage rating. Automotive +12 volts should have a minimum 50 volt rating on the input capacitor, preferably 100V.
HTH
BrianT