Voltage regulator capacitors, what to use?

[malc-c]

Tony,

Mostly just browsing the web and seeing what others used, or generally guess work.. most of the stuff I needed just a simple 5v supply - If I found a problem with ripple I just substituted the capacitors for higher value ones.

[Plcguy]

The filter capacitor cleans up the ripple to give you a flat DC supply. There is an equation for it (ripple and load based), but since the supply is DC, not AC the ripple is minimal. A 780 amp hour battery is an amazing cap, so a 10uf input cap for an auto system is fine.
As Alain alluded though, higher frequency stuff sits on the DC that is VERY bad. Those intermittent wipers keep firing a DC relay and kicking pulses of CEMF on the line. so, add a .001 and a .1 cap in parallel with the 10. Also, I'd put a .1 ohm resister in series with the regulator and hang a 25v zener on the regulator input. The output needs a 1uf to prevent oscillation FB. A typical 3 terminal regulator can handle up to 35vdc, though only for a short time without heat sinks.

[BrianT]

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

[ERMEGM]

Hi guys,

Thanks for the inputs, but the original question is not getting answered. How are you coming up with these values? Other than what other people have used in the past, are the values just trial and error using an oscilloscope and changing out the different values, or is there a formula?

Does this have to be done AFTER I build my circuit, or if I follow this setup, I can use it on all of my different setups? Here's an example from another company for their product:



Can I use this, or is it specific to their circuit? This is what they're using to power their circuit which is an amplifier. Mine is just to supply power to my pics and transistors controlling relays.

Thanks,
Tony

[Plcguy]

C*ΔV = q = I*t, where ΔV is the voltage sag (ripple), C is the capacitance in Farads, q is the charge lost in Coulombs, I is Amps, t is time in seconds.


for a true DC supply t = 0 -> C = 0
for a small I, the ripple is small.
If you have a 3 phase alternator, you may have 180hz freq, but prob no ripple due to the battery acting like a massive capacitor.

generally, this is for ac rectifiers at 60 hz.
the ripple freq is 120hz for 60hz sine wave
Example:
Given full wave rectification, a load current of 3.5A, and a capacitance of 6800µF, calculate the ripple voltage.
From equation (3)
Δv = 3.5A 120Hz x 6800µF = 4.29V