Voltage sag on heater PSU
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 9th January 2013, 08:43 PM #11 grommeteer   diyAudio Member   Join Date: Apr 2012 Location: Kiel, Germany To get a clearer picture can you please: Give all component values, Measure DC and AC voltages on all regulator pins, Measure the current from reg-out to heater wiring when volts are low, Test the circuit with a dummy load 22 Ohms 5W or a dial lamp 7V 0.3A. The 6922 is rated at 0.3A. Test the heater wiring with a bench supply, a 6V battery, 5V wall wart, or a simple heater transformer. Use a fuse! Just a thought: 1.24V is the typical Vreg of the LM338.
DF96
diyAudio Member

Join Date: May 2007
Quote:
 Originally Posted by Preamp Where are those numbers from? Sounds interesting...
I can't remember, but you should be able to find them in books on power supplies. The basic idea is that the charging pulses for an electrolytic reservoir cap have a much higher RMS/average ratio than the resistive load assumed by transformer VA ratings. You have to derate by some factor around 2 or 3 (I can't remember the actual figure).

If the DC is powering a Class B/AB power amp then you can get most of that factor back again because of the duty cycle of real music. Thus a 50VA transformer can (very roughly!) just about power a 50W amp (or 25W stereo). Heaters draw continuous current so you lose this advantage.

bg3009
diyAudio Member

Join Date: Jan 2013
Quote:
 Originally Posted by grommeteer To get a clearer picture can you please: Just a thought: 1.24V is the typical Vreg of the LM338.
Hi, I am using 1/4 watt resistors to set the reference voltage (2 x 180R). Would there be a concern with the size of these resistors? I can't seem to find anywhere in the data sheet where it specifies the rating for these resistors.

Cheers!

 10th January 2013, 01:19 PM #14 DF96   diyAudio Member   Join Date: May 2007 250mW power, maybe 200V voltage? Should not be a problem for a 6.3V supply.
bg3009
diyAudio Member

Join Date: Jan 2013
Hi,
With load, the waveform to the bridge rectifier is about 9.5 volts p-p.

The voltage at the input of the power regulator is about 10.4 volts DC. The waveform pretty flat with some noise at the 1mV setting. This measurements were taken with just the 6922 plugged in. With a good 10volts supply entering the Lm338K, I am beginning to suspect the power regulator is not set up correctly. The circuit was built as per the following schematic.

The bottom section is of course the heater section that's causing grief.

Cheers!
Attached Images
 image.jpg (396.7 KB, 47 views)

bg3009
diyAudio Member

Join Date: Jan 2013
Hi,

Sorry I had split my reply up. This is the waveform of the AC input at the bridge rectifier with load. I am getting a good 10.4 volts input at the power regulator.

Cheers!
Attached Images
 image.jpg (370.9 KB, 44 views)

 11th January 2013, 04:42 PM #17 grommeteer   diyAudio Member   Join Date: Apr 2012 Location: Kiel, Germany Case of LM338K is out, but In and adj can easily be interchanged.
Robert Kesh
diyAudio Member

Join Date: Oct 2012
Quote:
 Originally Posted by bg3009 Hi, I am using 1/4 watt resistors to set the reference voltage (2 x 180R). Would there be a concern with the size of these resistors? I can't seem to find anywhere in the data sheet where it specifies the rating for these resistors. Cheers!
1.25^2/180 = a lot less than 250mW

The other resistor can sometimes need a half watt or more, but not at 6.3V

I'm wondering if your heat sink/case of regulator, is shorting with something

metalsculptor
diyAudio Member

Join Date: Sep 2010
Location: Melbourne
Quote:
 Originally Posted by bg3009 Because I power up through a variac, would the ramping up cause a voltage violation issue with the LM 338K? Cheers
It would not help, the drop out current is less than 1 amp with low voltage differential. I do not know the fold back characteristics of this device but it may have gone into some sort of foldback.
Try applying full power to the circuit then switching on a heater load. The transient load from a cold heater can be determined by measuring the cold resistance, the valve datasheet may mention it as well.
There is a simple soft start circuit on the TI lm138 datasheet, this might be another option if the variac is causing trouble

Just check you have connected the 338K the right way

 30th January 2013, 09:01 AM #20 bg3009 diyAudio Member   Join Date: Jan 2013 Gents, After a good summer holiday, a fresh re-look into the circuit revealed what was wrong. Thee feedback resistors were built into a small PCB that was mounted onto the voltage setting terminal of the LM338K. A wire was to provide voltage from the output of the LM338K back to this circuit. However, due to a fault in connection, no voltage was flowing back from the output to the resistor/trimpot network. Re-did the connection and the circuit came up beautifully. Thanks every one for the input!

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