I am using a Bridge Rectifier LOW VF 1A 40V Schottky with 7.4 VAC input. The DC output starts at 6.5 V for one second then drops to 3V for one second.
Why?

 

Does it then go back up to 6.5? You need a capacitor in parallel with the input to keep the voltage up, otherwise you are basically seeing the peak and valley of the voltage minus the diode drop. Turn your meter on ac and it will give you an average.

 

Yes, it cycles every second. What size cap?
The load is 100ma.

 

Truth is, the meter can't keep up with the actual output pulses, so it throws different measurements out. The DC pulses off a full wave bridge with a 60hz in, are 120hz.

Your unloaded output voltage should be 7.4*1.41= 10.43, - .44v bridge drop... 10VDC.


The size of the cap is dependant on how 'smooth' you want your output.

The basic formula is I (current of circuit) / f (frequency of ripple) * V (desired peak to peak ripple voltage)

So, if a .1v p.p ripple is what you want, then .1 / 120*.1 = .00833 Farads or 8330 uF

the more ripple you can allow, the lower capacitance you need.

Now, if you have a cap in hand and am curious what it will do to your output, this is the equation...

I / f * C = V p.p ripple i.e. you have a 1000uf cap, at .1A / 120* .001 = .83V Your output will fluctuate by nearly a volt.

It all depends on what you are powering. But hopefully this gives you an idea.