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built variable power supply 1.2v-32volts can only get up to 9.4 volts? Answered



:First question, what are you putting IN to your power supply ? 


i have a 120 volt ac primary and 24 volts secondary with 6 amps with bridge rectifier to change to dc

That will JUST give 34 Volts. Have you got a nice fat reservoir capacitor on the output of the bridge - for something like 1A, 1000uF would be OK, 6A, more like 47000uF

What are you using for a circuit ? 



Circuit is pretty good. Can you see the 1.2V across Vout and ADJ ?

You've got something not where you think it is !


Ah, well what have you used for R2 ??? You pretty well have to use 220 or 240 Ohms.  It sounds to me like you used something like 100 Ohms.


ive tried 220 and 240 and even a 120 which is what the datasheet says and nothing different,  i did switch the lm338k with a lm317t and get the full 1.2 volts all the way up to 32 volts,  does that mean i had a bad lm338k regulator?

You will need to do a little investigating into what you have built.  First, you need to verify that EVERY wire and connection is wired per the schematic.  Often, when wiring something it is very easy to omit just ONE groundpoint, or just ONE little jumper that is small in the schematic. 

IF your circuit is NOT overheating then you can do some voltage measurements within the circuit to gain some information.... such as
How many AC volts is your transformer putting out??
How many AMPS is your transformer rated at???
How much DC LOAD are you placing on the power supply?
Is the dc LOAD exceeding your components capability?  for example, you cannot place 2 amperes of lightbulbs on a 1 ampere power supply!!! that will overload the supply and your voltage will "SAG" down as you describe. 
Is your BRIDGE rectifier wired properly??
How many VOLTS are present across the large filter capacitor?
Is your VARIABLE regulator IC wired correctly?  Is it the proper part number?
maybe you have a 9volt regulator, not a variable regulator???

If anything is OVERHEATING on your power supply, then you have a more serious problem of a miswired part or defective part .  You just need to keep HAMMERING away at the project like this untill you have success. If you get frustrated, that means it is time to stop and try again tomorrow.  Thats the basic method I use to fix things like you are having trouble with here.  Your question is not very specific, so i thought this answer giving you the basic
method to use would be of most help.  You might also verify your variable resistor is the PROPER resistance. maybe it is ten OHMS instead of ten K-ohms (or whatever value I dont know what you have).  and be sure the GROUNDS are connected to all spots shown on schematic. 
Good luck with it.