LM338K Voltage regulator, no current produced!

Thread Starter


Joined Oct 19, 2009
Hello all,
this is my first post and thread here :)

I made a simple voltage regulator with the common LM338K. Using the datasheet as a reference schematic, i sorted out the circuit (see attachment if does not show):

Behind this circuit there is a transformer with this ratings:
Primary: 0-220-400 V
Secondary: 12-0-12
VA: 100

That's good for a 4 amps at 24V, more than plenty for my uses.

That said, i made the circuit and worked good, at least without load i measured a good 1-30 Vdc range. But that was a prototype.

Then, in the final circuit i made the mistake of using old diodes (1N4007) that were bogus... they burned the set resistor (R2) a couple of times before i figured out that they were duds.

Bought new all the resistors and capacitors and replaced them, everything went fine (no burning resistors!), and i got the 1-30 Vdc without load.

BUT as i apply a load (dc motor, lamp...), Vout voltage drops to zero without having current to move anything, at any voltage setting.

Before the LM (so only rectifier and big capacitor) there is a meaty current to lit the lamp very bright, so it's not the rectifier fault.

I am thinking that with dud protection diodes, a backcurrent did damage to the LM. What do you think about?

Thank you for your replies!



Joined May 26, 2009
First of all, why do you have the fuse after the diode bridge circuit? It should be right where you connect the AC ternimals. An AC surge could damage the diodes. If you have the fuse where I suggested it will protect everything from surges.