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# 220v to 12v transformer

Hi

I have two questions. In the drawing below, which diodes do i need to use and will this be compatible with a hand crank generator?

Thank you for any help!

63Views2Replies

Hi

I have two questions. In the drawing below, which diodes do i need to use and will this be compatible with a hand crank generator?

Thank you for any help!

The 1n400x series diodes are cheap, easy to find, and work well for rectifier circuits, like the full-wave bridge, in the picture you attached to this question.

https://en.wikipedia.org/wiki/1N400x_general-purpo...

By the way the numbers you have to worry about for diodes are:

peak reverse voltage (in volts)

peak forward current (in amperes)

If your circuit does not exceed those limits, the diode, or diodes, should be happy.

Another number is the forward voltage drop. For silicon diodes, like the 1n400x, this characteristic voltage drop is about Vd = 0.6 volts, at almost any amount of forward current. This number is handy for calculating the amount of power wasted by the diode when it conducts, which is Vd*I.

The only occasion when you have to worry about Vd, is when the rectifier circuit is rectifying AC with peak voltage

so lowit is about the same size as Vd. The reason why is the rectifier circuit is not power efficient when the voltage is this low.As an example consider a full wave bridge rectifying a AC voltage that peaks at 3 volts. There are two diode drops, and together those drop 0.6+0.6 = 1.2 V. So the voltage left for charging the capacitor is only 3.0 - 1.2 = 1.8 V, which is 60% of the peak input voltage, so the maximum efficiency is only 60%. In fact during large parts of the cycle, the AC voltage is less than 1.2 volts, not enough to forward bias the diodes.

I feel like I should also link to the Wikipedia pages for "Rectfier" and "Voltage doubler"

https://en.wikipedia.org/wiki/Rectifier#Rectifier_...

https://en.wikipedia.org/wiki/Voltage_doubler

because there are other rectifier circuits out there, besides the ubiquitous 4-diode, full-wave bridge.

For example the doubler named "Delon", or "Bridge", here,

https://en.wikipedia.org/wiki/Voltage_doubler#Brid...

is a good one if you have a transformer with just one output winding, or an alternator(generator) with just one winding. You get double the DC voltage, and power is only wasted across one diode drop, per half cycle.

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Thank you very much jack for not only answering the question, but giving me such a wealth of information on the subject.

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