windozeuser said:
Thanks for all your replies
One Last Question How Do U Figure out what size cap u need for any applacation is there a math formulae?
I'm going to make a few reasonable assumptions here:
1) You are in fact using 120V 60Hz voltage as your input.
2) You are indeed using a bridge rectifier as shown above.
3) Maximum load current is going to be 1A (this is the max you'd get from a 7812 regulator)
Looking at the data sheet for the 7815 (
https://www.onsemi.com/site/products/summary/0,4450,MC7815A,00.html) we see that it has a dropout voltage of 2.0V typical. We're going to add a margin of safety here, and make sure that we're always going to be above that threshold by having a minimum input voltage of 16V.
Since 120V is an RMS voltage, we need to convert it to a peak-to-peak voltage by dividing by multiplying by sqrt(2). If you really want to understand the math behind that, use google, otherwise, assume it to be true. This gives us approximately 170Vpp coming in from the wall.
If we get a 9:1 transformer, this will give us 18.8Vpp coming out of the secondary. We then have to take into account the voltage drops in the dioes, which for a bridge rectifier is 2 * Diode Drop. For silicon diodes, Vdrop=0.7V roughly. This means that we're getting around 17.4V
If we have 17.4Vpp coming from it, and we want a minimum of 16V coming out of our rectifier, then our Vripple = 1.4V. This gives us all the variables we need.
Note: Since the bridge rectifier is a "full-wave" rectifier, it has a ripple frequency of 120Hz, since it is working on both the positive peaks and negative peaks.
The formula for ripple voltage is:
Vr = I / fC
or, in words:
Vripple = Max Current / ( ripple frequency * Capacitance )
Re-arranging this, we get:
C = I / (f * Vr)
and plugging values in, we get:
C = 1 / (120Hz * 1.4V)
C = 0.005952 F = 5952uF = rougly 6000uF (a somewhat commonly available value)
The 6000uF capacitor will give you roughly what you're looking for, but if you want the voltage to be a little more stable, then go for a larger value. Jameco has a 4700uF 25V capacitor for a little over a dollar, and two of these in parallel would be ideal.