Hi,

I am trying to understand bridge rectifier circuits on an AC voltage source a friend made for me. I have managed to rectify the AC signal into a DC signal using a full bridge rectifier. The issue arrises when I use a smoothing capacitor. If I use a small 10uF capacitor I get a slowly increasing smooth DC voltage around 50V, if i use anything bigger than 100uF i don't get any kind of output (0V). If I use a smaller pF capacitor I get a flat (near instantaneous) stable voltage output (again around 50V) but when I add a load (usually a 10,000 ohm resistor) the capacitor does not charge, and I get no voltage across the load.

The Vrms of the rectified signal is 100V with the Vpeak being around 500V. The frequency is around 240Hz.

Why does the capacitor not charge in the instances with a resistor as a load and why if I increase the capacitance it then doesn't charge.

Thanks.

 

Sounds like there is some parasitic resistance that does not allow caps to charge fast, or there could be some impedance issues.

 

You need to give a lot more detail than you have done as the facts you have given do not make sense. You need to give the following information.
1 The supply voltage and frequency to the primary of the transformer.
2 The transformer primary & secondary voltage rating. The frequency range rating of the transformer. Some idea about the size of the transformer so we have some idea what we are dealing with. (Power rating, output current rating or if you dont have these the pysical size of the transformer.)
3 A schemaitic of how you have wired it up and the voltage rating of the capacitors you have used.

Les.

 

As noted, the results don't make good sense. What should happen is that, with no capacitor, your meter will read an average voltage somewhere between zero and the peak voltage of the rectified pulses. The exact reading depends on your meter and a few other things and is not completely predictable. It will usually be higher than the quoted AC output of the transformer since that quote is usually specified under load. With no load, adding a capacitor of almost any size will filter the output and act like a peak detector. Your meter should read the peak voltage of the rectified sine wave.

As you add a load, there is a complex but predictable relationship between the voltage ripple and the size of the load compared to the size of the capacitor. A big load and a small capacitor will act as if the capacitor isn't there. A big capacitor with a small load will show only a small ripple.