design of circuit for 0-24v 5amp regulated power supply

[VickySalunkhe]

Hi neo
I starter making the power supply
I tried it total 3  times at 3 different places
First two time it was working fine

But at the 3rd location the something happend in the pot and so now i cannoy regulate the power supply
Was willing to know what could have happend?
Can i use 47k pot instead of 5k pot and what will happen if i do this?

Thanks 

[Ian.M]

No.
Pots are rated fror dissipation by the power the whole track can handle.  Convert that to a current by I=sqrt(P/R), and that gives you the macxmum current for any part of the track.

A 47K pot would need to be rated for 5W dissipation, and only a small fraction of its rotation would be usable.  You probably burnt out the pot, as a 5K pot needs a minimum 1/2W rating to handle the 10mA minimum load current of the LM317 regulator (via the 120R resistor), and most small pots are rated for less than that.

[VickySalunkhe]

Thanks Ian.M

So what do you suggest for which should i go for

I need to regulated around 38V 5Amp

Thanks 

[Ian.M]

If you want a linear PSU, use a LM723.

I posted a LTspice sim of the regulator section of a Philips PE 1535-00  0V - 40V, 0A - 0.5A. single output PSU, [here].  Given a big enough heatsink, it would be fairly easy to extend to 5A: 

* To beef up the pass transistor to handle 5A, add three 2N3055 transistors B and C paralleled with 0.1R individual emitter resistors for current sharing, to V26, the BDY20 to make a Darlington pair, with an extra  47R resistor from the 2N3055 bases to the lower end of the emitter resistors.   

*Replace V111, V112 with fast >100V, >7A diodes.

*Decrease R137 to 0.22R to increase the current limit to 5A.

Caution: if the peak input voltage is >45V, increase the Zener voltage of V109.  It needs to be Vin_peak-16V, or a few volts more.   If you use a 50Hz or 60Hz transformer and a bridge rectifier to feed it, you will probably need a preregulator as the PSU cant tolerate more than about a 12V swing between the peak unloaded input voltage and the troughs of the ripple at full load.   Another pair of paralleled 2N3055 transistors should do the job in a Darlington capacitance multiplier circuit with a >1A driver transistor, with the voltage across the capacitor clamped to 45V by a chain of 15V zeners.

The heatsink needs to be *MASSIVE* and probably fan cooled, as worst case, with the output shorted it will be dissipating 225W + whatever the preregulator dissipation is.  e.g. with a 44V 8.33A secondary feeding it, at 10% high mains voltage that could be another 115W.   

A better option would be to find a used 48V, 300W or greater telecoms SMPSU, and either hack it to drop the output voltage to a fixed 38V, or trim it down to 45V and use it to feed the modded Philips ciruit I have just described if you need a fully variable output.