How do you select the appropriate power supply for an application?

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Offline RyanGuyardo

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You are given an assignment to design and build the regulated power supply (with input at 240V ac) and the Optical Detecting
Module in the electronic system that will be used in a sporting goods manufacturing plant for controlling and counting the
number of baseballs going into various sizes of boxes for shipment
The specification given for power supply are
Input voltage : 240V at 50Hz
Regulated Output voltage : 5.1V +/- 10%
Max ripple factor : 3%
Max load current : 100 mA
« Last Edit: 02/10/2009 08:20:42 by chris »


Offline graham.d

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And your question is??

You wouldn't be asking someone to do your homework assignment would you??

Here's a start:
Full wave rectifier
Zener diode
Medium Power transistor
Various resistors and capacitors.

Nowadays you can get an integrated regulator to do this. All that you need is a transformer, full-wave rectifier, a capacitor or two and maybe a few resistors.


Offline Bored chemist

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If you want it really cheap and are prepred to make frankly rather dangerous sacrifices of safety, you can do without the transformer.
What are you trying to optimise for?
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Offline that mad man

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For a simple and stable circuit just use a bridge rectifier with a large ripple capacitor and add a 3 pin +5v voltage regulator such as the LM805 range.

You don't state what the output of the transformer is but the LM805 can operate up to 30-35v input with minimum being 7.5v. 


Offline techmind

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I think "that mad man" means LM7805 series of voltage regulators.
The 78L05 is rated for 100mA, but I'd probably choose something a bit heavier if I really wanted 100mA all the time (maybe the 78M05 or 78S05 - not sure of my memory).

These days though, and to avoid regulatory (legal) hurdles it'd be well worth considering just buying an off the shelf switched-mode power-supply module (from a Chinese manufacturer) which will also be cheaper, and lighter-weight than anything you could design yourself. Well, ok, maybe for 100mA a homebrew is good enough. If you wanted a few amps then a commerical switcher is your best bet!  (Probably not what the examiner was looking for.)
"It has been said that the primary function of schools is to impart enough facts to make children stop asking questions. Some, with whom the schools do not succeed, become scientists." - Schmidt-Nielsen "Memoirs of a curious scientist"