Bridge Rectifier Question

[Kempachi343]

I am building a basic power supply "Transformer - Rectifier - Smoothing - Regulator" and I have one easy question I cant solve.

I have a 240vac > 12vac transformer capable of putting out 8amps.
I need a 12vDC output, capable of at least 6amps.

For my rectifier, I cant find what I see to be a suitable bridge rectifier on jaycars website, the only bridge rectifiers capable of passing 8+ amps, are all rated at 50v-1000v.

Am I wrong, and do I just choose a bridge rectifier based only on it's maximum current, regardless of it's voltage range, Or am I missing something altogether?

Cheers,

Daniel.

 

[dknguyen]

The voltage rating for the diode, like the current rating, is a maximum so it is okay.

Just keep in mind that when a component has lots of one good thing, it probably had to sacrifice something else in order to get it. For example, a 1000V diode might have a higher forward voltage drop than an equivelant 50V diode but they will both probably still work in a 30V circuit(unless the circuit is really sensitive to some difference between the two diodes), but the 50V will probably work better than the 1000V one . So don't go too overboard.

 

[Kempachi343]

Thanks dknguyen, Just what I wanted to hear!

Cheers.

 

[Kempachi343]

dknguyen, I've ran into another puzzling thought.

I've been reading through this article related to the smoothing process of my dc waveform,

Smoothing - Electronics in Meccano

I can only work out that to achieve the "The ripple voltage should not be more than 10% of Vs" I would need at least a 0.05F Capacitor.

The maths I'm using is that of the website,

Vr = I/CF

Where I = 6A
C = .05F
F = 100hz

which = 1.2v (10% of Vs)

The largest capacitors I see on Jaycar are 4700UF which is not nearly big enough according to my maths, Do these capacitors exist, is it what I need, or am I approaching this in the wrong way?

Cheers.

 

[Ubergeek63]

you are calculating based on RMS instead of peak.

the peak is 17V at full load and 20V at no load.

There is absolutely no way to maintain 12V +/- 1V with out a regulator.

Dan