linear power supply design help plz

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ant9985

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hi, i got an assignment asking me 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 and noise <50mV pk-pk

iv worked out the transformer is going to be rated at 39VA
and the ripple suppresion capacitor is 0.3F.
But now i need to work out what zener to use and the resistor's resistance that goes in series with it and i dont know how to work out the bias current it needs. please can anyone help. also if anyone could tell me anything else i need for this, like what load and line regulation are or how i make my power supply to fit them. My lecturer just gave me this assignment without any teaching for the subject, and i do not remember doing this kind of stuff on my previous course. thnx :shock:
 
You seem to be designing the most horrendous inefficient power supply imaginable.
A 300,000uF capacitor in a 1.5 amp PSU is just plain silly.
Your specification imposes limits on ripple, line and load regulation and you are trying to achieve this with an enormous capacitor and a zener diode.
You also have the output voltage as being adjustable from 24 to 26 volts, how are you going to do that with a zener?
I suggest that you think again.
In your other post on the subject of this power supply, you say that you cant use ICs. Try using a few discrete components to make the error amplifier and a pass transistor.

JimB
 
iv got no idea, iv never designed a power supply before, iv been following the text book they recomended and thats what it came to, i thought it was an overly large capacitor, and i asked my teacher about it, and he said yes thats what it will be. but thnx for telling me where i went wrong. ill just tell him i cant do it.
 
you can do this for about £40 buy a transformer that outputs 30v there sould be 3 wires on one sid and 2 on the other. this means you have 1 coil on the mains side and 2 coils on the output side if you put the ouput coils in parralell you put 240vac in the mains. if in series you put 110vac i would say use 5 caps 22000uf, 100uf, 1uf, 1pf and 1nf put them all in parralel from the output of the transformer. use a tip32c transister to vary the voltage. here is a curcuits diagram i through together.

just being nosy, how old are you.
 

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That's pretty horrible as well :lol:

And why all the capacitors in parallel?, all you need is one reasonably sized one.

The idea of using a transistor to stabilise and vary the output voltage is the correct one, but NOT just fed from a potentiometer!. It needs to be part of a proper stabilised PSU, of which there are a great many varients.
 
btw if i sounded a bit rude in my first response i didnt mean to, i am greatful for all the help i can get with this.

thnx daniel.. but how do u did u come to those capacitor values and do i choose a transistor rated at 39VA just with 3 input wires and 2 output? i cant just put in a circuit without them knowing how i came to it. but thnx for the help.... why do you want to know my age? im 20
 
i used the equation p = vrms x irms to get 39VA
and I x t = C x V to get 0.3F capacitor
did i use the wrong equations?
 
ant9985 said:
i used the equation p = vrms x irms to get 39VA
and I x t = C x V to get 0.3F capacitor
did i use the wrong equations?
Click to expand...

Your whole premise is wrong!.

The regulator circuit takes a higher DC voltage, that may have a reasonable amount of ripple on it (so you don't require a massive capacitor), the regulator then outputs a lower stabilised voltage with a far lower degree of ripple.

For a simple example:

Imagine the peak output voltage on the capacitor is 40V, and under full load this drops to 30V in the troughs - so that's 10V ripple on the supply. As output you're only wanting 26V maximum, this gives 4V margin (30V-26V) from the output voltage to the lowest part of the ripple. A discrete voltage regulator circuit would be quite happy with that amount of leeway, and you would get a smooth, ripple free, output (presuming the regulator is designed OK).

In practice it wouldn't be a good idea to have 10V ripple on the incoming supply to the regulator, I used that value as an exaggerated example.
 

the caps just get smaller, as long as they get smaller it should work. not transistor, transformer. i have built this circuit and it works ok for me. it does vary from 30v to 0 and has no short circuit protection. the bigger the transistor and transformer the more wattage you will get. i used a power transistor but a medium transistor will do for what you want, i wanted 20amps at 30v. hope that helps.

ant9985 said:
why do you want to know my age? im 20
Click to expand...

just curious. i was thinking gcse level.
 
danielsmusic said:
smaller caps will reject smaller ripples
Click to expand...
Nope.
Sometimes it is necessary to do a similar thing on a PC board with high speed logic or RF, because different-valued caps become inductive at different frequencies. This isn't necessary on a power supply preregulator filter.
 
You have no regulation with this circuit. All it provides is a way to vary the output voltage.
 
The pot feeding a power emitter follower isn't a variable voltage regulator. It is a current limiter with a set max voltage without a load. As the load current increases, the transistor's base current also increases which causes the pot's slider voltage to drop and the output voltage too.
 
Ant9985

I am sending a PM to you with some pages scanned from a book:

"Solid State Design for the Radio Amateur"

The pages take you through a design process for simple power supplies, this will get you a lot nearer to your goal than your current path.

If your "teacher" thinks that your current approach is fine for a PSU with the specification you have shown here, he is VERY MISTAKEN.

JimB

Bummer!!
There is no obvious way to add an attachment!
Can you send me a PM with your e-mail address?
 
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