hi all, how do i add a current limit to an already built power supply?
this power supply has current of up too 2A and 30V. i want the current limit to be able to reach 50mA to 2A.... thank's

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It's likely to be far easier to add it internally, rather than externally. Have you built the PSU?, or did you buy it?, and do you have the circuit diagram?.

Any external circuit will require a device to pass the full current, and large heatsinking as well, plus it will drop some voltage and reduce the regulation of the output voltage.

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it's not yet built...
basicly it's a double PSU that uses the LM317T regulator, the schematic below shows one of the LM317's.... the Pot. is 5.5K

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Download datasheet for LM317T. There is an example of regulated power supply with current limiting (uses additional 317 for current limiting).

 

i looked there, but isn't there something simpler? plus i can't find those FET'S anywhere.... and inserting a negative voltage is not an option too... any ideas?

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I don't think there's going to be a simple option, the limitations imposed by using a 3 pin regulator IC prevents anything simple.

Assuming you're wanting to build a nice versatile lab PSU, I would suggest dropping the 317 and building something more complicated - perhaps using a 723 IC and external transistors, there are loads of designs about.

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after snooping around a bit i think i'll take your suggestion, but i think i'll build this one. is it a good PSU?

http://www.electronics-lab.com/proje...011/index.html

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Strange! - the URL wasn't visible at all, but it is when you click "Quote" - anyway, that circuit is rather crude, there's not even a voltage reference in it. But it should work OK, and probably be fine for many purposes, it's certainly simple enough!.

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but would you recomend this one? or is there something better, yet simple enough?
the problem is that there are going to be high-school kids working on this (one's that don't understand much... :-) ) so i need to make it fail-safe... any suggestions how should i improve this circuit (other than adding a fuse..hhh)?

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Presumably then it's for a school project?, as such it would be a useful tool, and a good subject for research - they could perform tests on it, and plot graphs of it's performance, then try and figure out how to improve it.

A fuse would be a good idea, and probably having the mains transformer in a separate box (where the kids can't touch it) would be a good safety idea as well.

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yes, as always you were right, this will be for the use of the school... my school asked me to supply them with a few of cheap and good supplys so i'm building it and selling at the price of the materials...
i need to build a double power supply, so i assume that i just share the output of the transformer with two of the circuits?

another thing is the cost of the transformer... do you know a web site that sells these cheap? and ship internationally?

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You really need separate transformers, or at least separate secondary windings - this will keep the two supplies totally separate, allowing you to connect them in series for a higher voltage.

The UK isn't terribly good on cheap parts, plus transformers are heavy and expensive to ship - but they are simple to manufacture, you should be able to source them locally at reasonable prices.

Also, don't forget, you will need substantial heatsinking on the output transistor!.

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thank's, but i think i'll stick with one transformer due to the price and the fact that i can't get an 220/2x24v transformer...
electronics is a funny buisness... i tried to make the cheapest power supply and the price of the material's for each one climbs up to 33$ not including the PCB and box....

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Very often you can buy complete items for less than you can buy the parts, basically due to the huge volume discounts for manufacturers buying their parts directly from the manufacturers. By the time you come to buy a part, it's already had profit added to it by two or three different companies.

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