Hi,
I have a simple LM317 voltage regulator to drive the heaters of a couple of pentodes (about 800 ma for both). I am using 560ohms and 2.2K to get 6.2V. But when I connect the 2 heaters the voltage drops to 5.7V?
Any suggestions will be greatly appreaciated?

Thanks

 

Yes, you're drawing 1.6amps, are you using a large heatsink, or better still use a LM338.

 

Yes, you're drawing 1.6amps, are you using a large heatsink, or better still use a LM338.

I am drawing 0.8A for both (0.4A each), is that the cause?
Yes I am using a decent heatsink.

 

What is the voltage to the input of the LM317 when the output is loaded with the two heaters ?

Les.

 

Hello,

As @LesJones asked, what is the input voltage?
It should be at least 2 volts higher as in output voltage:



Also I see that you are using a voltage divider of 560/2200 on the Adj pin.
According to the datasheet it is better to use 120/470.

Bertus

 

What is the voltage to the input of the LM317 when the output is loaded with the two heaters ?

Les.

Ok, I will have to confirm that when I get home, it should be 12 volts. I use am using a 12V 5A wall wart, to power the filaments and the tubes to 35V via a dc-dc boost converter, it works well enough, but the filament voltage is going down to 5.7V?

 

If you indeed have 12Vdc on the LM317 input and it is on an adequate heatsink (how hot does it get when operating?) then the output should stay at 6.2V as shown in the LTspice simulation below.

Where is the top of R2 physically connected with respect to the LM317 OUT pin?
It should be connected directly to the pin to avoid any degradation of the output voltage due to the wire output resistance.




Note that you only have to reduce the resistor values as Bertus suggested, if you need the output to stay in regulation with no load.

 

Ok, I will have to confirm that when I get home, it should be 12 volts. I use am using a 12V 5A wall wart, to power the filaments and the tubes to 35V via a dc-dc boost converter, it works well enough, but the filament voltage is going down to 5.7V?

The heaters have a PTC characteristic - is the cold current forcing the internal current limiting and stopping it getting up to speed?

 

Switched to Lm350, works now. Maybe Lm317 was faulty ?
The heat sink does get pretty hot - just around the chip it is about 150F.

Thanks for your inputs.

 

Switched to Lm350, works now. Maybe Lm317 was faulty ?
The heat sink does get pretty hot - just around the chip it is about 150F.

Thanks for your inputs.

Various appnotes for 3-terminal regulators describe how to wire an external bypass transistor for boosted current.

 

The heat sink does get pretty hot - just around the chip it is about 150F.

The LM317 could be current limiting due to excess temperature.
Is the voltage from the LM317 correct right after you turn it on?