Talkin' PSUs

Sylvain wrote: Truly appreciate your guidance but between the mathematics and formulas and technical expressions ... i value the information but this spectator audience is a mere mortal.......and I chose to paint by numbers at least I have something at the end ....

Painting by numbers: primer 1



Edited by Graham Slee - 20 Jan 2021 at 1:06pm

Ripple voltage is peak voltage minus the gutter voltage of 26.5 V DC.

Vripple = Vpk - Vgtr

It's easier to test different ripple voltages, so let's restate that as gutter voltage plus ripple voltage equals peak voltage.

Vpk = Vgtr + Vripple

We find ripple voltage using CV = IT by rearranging it by dividing both sides by C, such that

V = IT/C

Now I = 1A and one times anything is whatever that anything was, so for I = 1A, we can forget I, so

V = T/C

For a full-wave rectifier (who'd use anything less?) T = χ-1 x 100 pulses per second, which is 0.01, so

V = 0.01/C

The original value of C (C1) is 4700uF, which is 0.0047F, so

V = 0.01/0.0047.

Do we need a calculator? Can we still do schoolkid maths? Canceling 0's we get

V = 10/4.7

10/5 is 2, so we can guess at 2.1 volts, and it's actually 2.13 volts.

Remembering what this is all about, obtaining Vpk, then

Vpk = Vgtr + Vripple = 26.5 + 2.13 = 28.63V

Therefore any input between 28.63 and 40 volts will work to produce 1 amp.

But now we need to know the transformer voltage.

√■ 2 = χ-¹ = 0.707

So 28.63 x 0.707 = 20.24V

But we now add the entry fee, the rectifier voltage drops, which are now hit by inflation!

The more current we draw, the larger the silicon is stretched (not exactly right, but I'm teaching Sylvain too), so it might now be 0.7V per diode or even 0.75V.

We have two diodes at any one time, so we must add 1.5V. Thus the AC voltage has to be 20.24 + 1.5 = 21.74V.

As Ashley noted, we could use twice the capacitance and reduce the ripple voltage by half. If we use 10,000uF, then instead of

V = 10/4.7 = 2.13volts, we have 10/10 = 1volt

Adding Vgtr, 26.5, we need 27.5V peak, and 27.5 x 0.707 = 19.44V

We add a 1.5V entry fee, and the transformer AC is 20.94V.

It looks like this is going to be awkward?

Great stuff! Really enjoying this unravel!