Maida regulator, these results expected?

[leadbelly]

I am building a SS HV regulator for my amp. I first started off building the Morgan Jones version of the Maida regulator, but I was suspicious of the results I achieved at first (he uses large value resistors for the divider circuit that do not provide the requisite 5mA minimum draw on their own), so I reworked it to be closer to the Maida circuit with lower value voltage divider resistors. This gives the same variability in the output voltage, and also much higher output voltage than expected. For the schematic attached, I get output voltages of: unloaded=425VDC, 300K load=419VDC, 77K load=400V. These voltages vary more than I expected, and also are much higher than the ballpark figure Vo=1.25(1+R2/R1) which would be 358 volts in this case.

So, any comments? Are these poor results, too poor for a properly functioning regulator? Is there something wrong in the circuit? Is the problem that the transistor I used has too low hfe?

 

[EC8010]

So, any comments? Are these poor results, too poor for a properly functioning regulator?

Yes, it should be much, much better than that. Don't forget that the Zener passes current into the load, so not only must the load draw enough current to make the 317 operate correctly, it must also sink the current from the Zener.

 

[Giaime]

I've built this, and it works very well.

 

[SY]

One other thing, I don't know what the dropout voltage is for the HV version, but you could be pushing that. Try increasing the Zener to a 10-12V unit.

 

[Zen Mod]

LM317 always need around 15mA through it ,for propper operation;
trick is just in output-adjust resistor;
I always use there max 91 Ohm (1,25V/91 ~13mA)

 

[leadbelly]

Thanks for all the suggestions. It appears that the 4.7K zener pass resistor was too small and was allowing enough current to pass through the zener to satisfy all my small test loads by itself. I think I will go with a 15K zener pass resistor, 12V zener, and an even heftier voltage divider to increase the draw on the LM317.