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power supply design

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ant9985

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hi, for one of my assignments i have to design a linear power supply to meet the following specification:-

selectable input voltage 110/230v ac
Adjustable output voltage 24-26V
maximum output current 1.5A
load regulation +/- 1.0%
line regulation +/- 0.5%
ripple noise <50mV pk-pk
your design should:- identify the devices and passive component values used also circuit connections.

iv looked on the net for powersupply design, and in my textbook but im left feeling confused after it because they dont cover all of the variables here.Im quite confused as to how to make it selectable input voltage and if anyone here can help me a bit with it, like how to calculate some of the values for parts needed it would be very much appreciated.
thnx
 
I'd like to think your course has prepared you for some of this assignment but I'll help get you started. They do make some transformers with selectable primaries - the transformer is wound so that by selecting the right taps you can work with various input voltages. You'd need to select a transformer with the primary voltages you have in mind and with a secondary that is of adequate capacity.

Keep in mind that the transformer might not be the first component you select and that this might be an iterative process, especially if the requirement is to specify real world components. An example - you might first determine that a 28 vac secondary is sufficient but find the only transformer made is 32 vac. That might put a greater demand on the regulator, pass transistor, heat sink, etc.

Note also that there are quite a few books out there on power supply design. I know that a number of my ARRL amateur radio handbooks provide lots of good info though it's fragmented requiring that you think thru the whole process.
 
A 3rd yr EE student from MIT visited me (know his parents) last year and told me his professors insisted he have several books as references. One of them was Paul Scherz' "Practical Electronics for Inventors". I find it's quite the handy reference and might just help with this sort of thing - not all the answers but maybe enough to guide you. I know that in there is a very clear explanation/calculation regarding the ripple reduction characteristics of IC regulators.
 
im just trying to calculate the ripple suppresion capacitor. the textbook says if the mains frequency is at 50hz then the dips fall to zero at 10ms intervals because the mains frequency has a periodic time of 20ms.

now the forumla is: I x t = C x V
where I is load current, t is ripple period(seconds), C is capacitance and V is ripple voltage.
so i need I as 1.5A, V as 50mV (0.05V),

But if my power supply is to be selectable 110/230v ac meaning the mains frequency can be either 50Hz or 60Hz what do i put in the forumla for
t (ripple period)?
 
ant9985 said:
But if my power supply is to be selectable 110/230v ac meaning the mains frequency can be either 50Hz or 60Hz what do i put in the forumla for
t (ripple period)?

Use 50Hz, because 60Hz is a higher frequency, so will always have less ripple than 50Hz.
 
Keep in mind that some (maybe all) IC regulators have some capacity to reduce ripple if in the correct configuration. That might ease the burden of the filter capacitor somewhat.
 
In calculating the reservoir capacitor, bear in mind that the ripple voltage will not be 50mV, but the difference between the peak voltage delivered by the transformer/rectifier and the minimum input voltage of the regulator. This will usually be several volts. A mistake here can result in a HUGE capacitor, and a fatter rectifier .
 
do u mean the ripple suppresion capacitor?
wont it be (1.5 x 0.01) / 0.05 = 0.3F?

also are the parts i need:-
ripple suppresion capacitor
zener diode
resistor
transformer

or do i need some other stuff aswell
 
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