Help!! LM317T Filament Heater Problem

[ron944]

Hi, I did a complete rebuild on a Counterpoint SA-3.1. When I fired it up I am getting no voltage to the heater filaments. The high voltage is fine. I put in a INXYS full wave bridge which then goes to a 1000uf cap and then to the in of the LM317T. Output from the LM317 goes to a 110 ohm and then a 402 ohm resistor. The ADJ pin goes to a 10uf cap and then to a point between the two resistors. I have 15v coming out from the bridge rectifier (my ac is a little high 123v)and 15v showing at Vin on the LM317T but 0 volts on the Vout and ADJ pins. Can something on the load side be causing this or can the delay circuit be causing the problem (the led goes green after 30 seconds and I have high voltage readings). I tried three different LM317T's the back is isolated from the chassis with thin rubber. I am going to unwire the Vout and ADJ pins and see if there is an output voltage. Any Ideas will be helpfull Thanks, Ron

 

[boyofwax]

Check the datasheet for connections on the LM317. (Tab to back and pins down it should be adj-out-in for TO-220. This differs from the 78XX series.) With adj to GND it should give 1.23V out. Keep in mind that filaments have a high inrush current.

 

[ron944]

It is a TO-220 package with the pins as you described. I will check adj to ground and see what Vout reads.

 

[Strychnine]

...hi !

Here i have the same problem... LM317 linked to a 12ax7 pin 4+5 and pin 9 , normal routing for 6,3v and nothing
When i unplug the tube i can read 6,3v. and the led of the regulator PCB shine... i plug the 12ax7 , the led switch Off and no volts...

I use a LM317 with a dc power supply rated at 15V 800mA.



My PCB is :
1000uF 50v+ 1nF 250v on the input
150R + 5k Trim pot at V Adj
1000uF 50v+ 1nF 250v + LED + 1k5

Power supply 15vDC 800mA

The board works fine ...but not with a single tube with 300mA hungry ^^

Why ???

 

[DUG]

Measure the cold resistance of the filament load.

This is what you are initially applying to the 317.

Try a 10R (10W) between supply cap and regulator as a "soft start" current limiter.

Install tube and then apply power.

Then review the results.

 

[rayma]

shutdown

Measure the cold resistance of the filament load. This is what you are initially applying to the 317.
Try a 10R (10W) between supply cap and regulator as a "soft start" current limiter.

Yes, you will need over 4 Ohms in series with the 6.3V regulator output to avoid tripping the 1.5A current limiting and sticking in a stable state.
The best solution is Kevinkr's, a shunt power resistor across the input/output of the regulator. An old trick, but a good one.

 

[Strychnine]

Will try the serial resistor ...thank you...but i'll trick it also with a woolly LM350 just to see if the 1,5 A is a cause ...i'll tell you if i survived the electrocution soon

 

[formantjim]

I too have been having similar problems with both LM317 and the LM1084 and I have found that the capacitor of 10uF they recommend between the adjust pin and ground too small and should be increased to 47uF. I scoped it and found it would go into oscillation lower than this I too have placed a 1000uF on the output and it works beautifully very little noise and about 5mV regulation.

 

[Strychnine]

so...still alive ...problem fixed, it was a bad silicon....or maybe i fryed it when i swaped the static from the protection ...
I replaced it with a new one and it rocks.
So far so good ...Under 1W with 14.8vdc input to 12.6 heater, no heat, no noise , perfect regulation ^^