Hello,
Trying to light the 6.3v filament to a 6SL7 vacuum tube. Using the attached LM317 circuit to drop down 36v dc to 6.3v. Filament requires 300mA current. When I attach the output of the lm317 to the filament the output drops to zero. I read zero volts on the filament. Removing the lm317 from the filament I get 6.3v on the output again. Why does it drop to zero when I apply to the filament but goes back to 6.3 when I remove it?
Thank you for reading.
There seem to be a number of possible issues here.
Firstly the valve/tube heater (usually tungsten), will normally have a considerably lower resistance (just like a filament light bulb, which is also usually tungsten), when it is cold.
Maybe as much as a factor of 10x, or at least some big number. (Disclaimer: Some valve/tubes can have complications, which could change what I am saying here. But I'm ignoring them for now.).
Also unlike a light bulb which "instantly" turns on. Valves/tubes take considerably longer, like up to a minute, to heat up (because it is heating up a lot more, than just the filament...).
So let's say it was 1 Amp to 3 Amps (depending on the exact multiplication factor. Further confused because valves/tubes heaters run at a lower temperature, than bulbs).
So this huge initial current, combined with the huge voltage across the regulator (36V is relatively HUGE, a 30V+ differential), also I'm NOT clear about any heatsink arrangements. Sorry if I missed the details.
So I'd actually be much more surprised if it actually worked, SORRY!
In short, it has a wide range of protection features, built in. You are probably activating them ALL at the same time ? (partial joke, SORRY).
Any chance you could give it a more reasonable incoming supply voltage ? (If not there are other solutions).
Also there are higher current rated versions of the 317, which may be a better choice here, as well. E.g. LM317K.
Thank you for responding
I think I'm exceeding the max current of the lm317. the filament is 3.6ohms and if I apply 6.3 volts I'll draw atleast 1.75 amps right? The regulator may be shutting itself off when I apply the voltage.
Is that the COLD resistance reading with a meter, and did you subtract the meters probes short circuit resistance from the reading ?