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# need help with power supply

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#### Blaster

##### New Member
I gotta make a 0-20V 1A DC power supply for school. so i looked for a scheme and i found this.

i tested it and it works
now my question, is this a good one or can i improve it by changing some values of components and how can i add a current limit that limits to 1A (using a transistor)
just discoverd this forum, hope you guys can help me

The easiest current limiter is a fuse.

yeah, i know a fuse is easy, but i dont want to change the fuse everytime i overload the thing

i know you can limit it with only one transistor more but i have no idea how. (i have seen an example wich limits to 0.7A but i have no idea how to make it work that it limits to 1A)

and for the supply of mechie, i need to explain how my supply works. so i prefer a simple one

At what current is the 24 volt input rated?

what do you mean? :?:

its a 40VA transfo
so it can deliver 1.666A

So with a s/c current of 1 amp the BD13x gets very hot.....of course it will, its dissipating lots of energy.......if you maintain the s/c for too long the transistor will simply burn out. Its disipation isnt high enough even mounted on a modest heatsink Change it for something with a higher rating. A BD711 is more suitable.

As for the s/c calculation, application of Ohms law will tell you how to arrive at the value for the resistor......

You need 0.6v across the emitter-base junction of the transistor you added.

So from V=I*R, 0.6=1* Xohms

so your resistor needs to be 0.6 ohms, nearest preferref value is going to be either 0.56 or 0.68 ohms

Now you know how the resistor is calculated perhaps this will help you in working out how the current limiting action works.......

ok tx for your help dude
ill buy that better transistor and test it

srr i deleted previous post, was a little confused :shock:

Well, what I mean is that when current is a consideration, I like to begin a design at the begining - the supply current. Knowing what current is available can help select componebts that can stand up if the input is taxed to the max. Thanks for the info.

Hmm just a thought......the 1N400x didoes are only good for 1 amp......current rating is a bit on the lo side...1N5400x would be better..

and for the bd711 , isnt that a bit overkill?
what about the bd241, it can dissipate 40W

BD139 SN 80V 1.5A 12.5W
BD711 SN 100V 12A 75W
BD241C SN 100V 3A 40W

A few basic calcs can yield a lot of info when designing.......

24v ac supply from tranformer, offload dc = 24*1.414=33.96v however there are losses across the diodes...assume worst case 1v per diode that's gonna be around 4 volts..so across the cap you got around 30v assuming the transformer puts out 24v ac.......it could be higher !

So 30vdc across cap, your series pass transistor is seeing 30v at its collector with the wiper of the pot at max, your output is going to be 20v less the b-e volt drop of both transistors around 1.2v
Assuming the s/c current control circuitry works,max dissipation of series transistor is going to be max volts across c-e* s/c current.....(30-20)*1 watts, around 10w so yup BD241 looks good if thats the case
Otherwise, its total meltdown........use a fuse! lol

I said a BD711 coz I got 1, thats all..... 8) equally a TIP41A would do too.

HTH Chip

even when i get the limiter to work, iam going to use a fuse of 1.2A or something. i take no chance
and some nice volt and ampere meters and a on/off switch offcourse

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