LM317 Regulator Problem

[Bryan]

Hey Guys,

For the life of me I can not figure out what is probably a SIMPLE issue.

I have a simlpe voltage regulator circuit for my heaters. It is based on the LM317 regulator. The differemces between my circuit and the one attached are that I have 50uF as the first filter cap, 100uF at the output, and a 220 ohm and 2K pot for the voltage divider, no led and no final 0.1uF at the tubes. There is also a small CM choke before the first filter cap after the bridge.

With no load I can set the output to 12.6, no problems. But when I apply load (ie attach the circuit to the heaters in parallel) the voltage drops to 10 volts with one tube, and 7 volts with two tubes. I can follow this voltage drop all the way to the bridge, which will read 13Vdc after the bridge. I should state the my tranny is 12V/2.5A, and with no load I have about 19Vdc after rectification.

I cant seem to figure out why I am dropping so much voltage?

Thoughts?

Bryan

[Shoog]

Is your transformer big enough, is the circuit sagging and dropping out of regualtion ??

Shoog

[Bryan]

Shouldn't be... The tranny is a 12.6VCT/2.5A. The center tap is not grounded.

The heaters should draw 0.3A each, so TOTAL is only 600mA current. The tranny shouldn't have any problems...

[aletheian]

What's the DCR of the choke? Maybe that combined with a high resistance in the tranny secondary is producing major sag.

[ilimzn]

Originally posted by Bryan
I have a simlpe voltage regulator circuit for my heaters... The differemces between my circuit and the one attached are that I have 50uF as the first filter cap, 100uF at the output... I cant seem to figure out why I am dropping so much voltage?

Sorry I have to say this but DUH! You substituted a 10000uF filter cap with 50uF, only 200 TIMES smaller - how can this be a proper substitute?! You are getting next to no filtering at the required currents!

[Bryan]

So you are saying if I boost up the capacitance before the regulator to 10mF I shouldn't have a problem?

[aletheian]

Now how did I miss THAT statement!? Yes, boost the capacitance and all will be well. That choke right after the rectifier might be forming a little choke input filter too, which will bring the voltage down to about 90%. And if it has a DCR of even only a few ohms, 3 ohms will drop almost 2 volts at 600mA.

[Gluca]

I made it this way

12VAC, bridge, 10000uF, 3mH, 10000uF, LM317, 10uF. No problem.

Ciao
Gianluca

[Bryan]

The choke has an max DCR of 0.39 ohm. I will try adding a large resivoir before the cm choke and see if things improve...

[Bryan]

Thanks guys,

4700uF in front of the cm choke and another 1000uF afther the choke cured all problems. Now I just need to heatsink the regulator and all should be fine.

Thanks,

BK