Linear power supply design

[teklucifer]

Dear all,

I have been designing a bench power supply circuit that supplies 24vdc at 5A. The design took me 3 weeks to tinker with and still couldn't get it correct. So, the power supply should be able to adjust the voltage as well as limit the current. I have used the LM338 [TO-3] regulator since the capacitors that i might use may increase the voltage input to the regulator. Moreover, I put a current sensing circuit that i was told it can limit the current (correct me if I am wrong). I still cannot find precise voltage and current limiting circuits. I am stuck with this for 3 weeks reading online articles to  schematics. Please help out as I am struggling alot with design. If there is something wrong with circuit please do correct me!

EDIT: The voltage regulator is LM338 not 1084T



best regards,

teklucifer

[tautech]

I have been designing a bench power supply circuit that supplies 24vdc at 5A. The design took me 3 weeks to tinker with and still couldn't get it correct. So, the power supply should be able to adjust the voltage as well as limit the current. I have used the LM338 [TO-3]

R3 is in the wrong place.

[retrolefty]

Isn't D1 wired backwards?

[Paul Moir]

Isn't D1 wired backwards?

I'm thinking it should be a zener?  Either way without something to limit the current going from the base to emitter when you put the pot all the way up the transistor and diode (and pot) will fry.

[MarkF]

See this description of Series Voltage Regulators that details your circuit.  It appears to me that you're missing R₄ in Fig. 2.2.5 which sets your reference voltage with zener D_Z.  R₃ and TR₂ adjust the output voltage.  R₅ and TR₃ perform the current limiting.  Adjust the value of R₅ to set the current limit.

[teklucifer]

Thank you for replying to my topic, and i apologize for my late reply. I am still unable to understand how will i control the current output. for example adjusting the voltage to 10v at 2.4 A. I have read Ti's datasheet and still wasn't able to find the circuit that i want. There is a current regulator and there is a current limiter. I have posted on other forums and some told me to use an operational amplifier to adjust the current and voltage.

Thanks!

Edit: Is it possible to use a variable current source circuit for controlling the current

[Kleinstein]

At this power level, and if an adjustable current limit is needed,it's not a good idea to use a regulator IC that includes the output stage. The more logical solution would be a circuit based on the LM723 or similar. That is a regulator chip and separate power transistor(s). At 24 V and 5 A chances are you need more than 1 transistor. Alternatively separate reference and OPs could be used instead of the LM723.
As for a power supply I would suggest to start with a smaller version (e.g 0.5 or 1 A) first. It's less magic smoke and less expensive parts - so it's better for learning.

For the current limiting, there are three options:
1) the supply turns off, once the current gets to high. So a kind of electronic fuse.
2) the current is limited to the set value and the supply chances over from a constant voltage mode to a constant current mode. This is how most lab ppower supplies work. So you can choose maximum values for voltage and current, usually only one these values active at any time.
3) use so called fold back current limiting: the current limited to a miximum level like before, but the current limit goes down as the voltage goes down. This method is used in may of the integrated regulators like 780x or lm317. Foldback reduces the possible power dissipation and thus allows more current with given power transistors / heat sink.

[danadak]

The output Vadj circuit should look like this, attached.


Regards, Dana.