LM317T not providing enough juice (?)

Ripthorn · February 10, 2009, 06:51:57 PM

Hi guys, I am building a power supply for multiple pedals and am having a little trouble. I run the mains into a transformer. Transformer goes into a diode bridge. Output from diode bridge gets filtered using 1000uF electrolytic. Filtered stuff goes to a 78L12 and an LM317T. I have the 317 set up to give me right about 9V which is exactly what I measure when there is no load. As soon as I hook it up to a pedal, the voltage tanks and won't power the pedal at all (on one, the LED won't even come on!). I am wondering what the deal is. The transformer is a 12.6Vac 2A, so there should be plenty of current to go around, but for some odd reason, it appears as though there isn't. Anyone have any idea why this might be?

esdiezy28 · February 10, 2009, 07:34:59 PM

Your problem is the 78L12. I think that the 78L12 is only rated for 100mA output. Swap that 78L12 with a 7812 (these can output 1A, I use this in my Valvecaster) that should solve the problem.

Ripthorn · February 10, 2009, 08:08:26 PM

The 78L12 works just fine, even when connected to a pedal. The problem is the 317 because whenever I hook up one of its outputs to a pedal, the voltage tanks.

esdiezy28 · February 10, 2009, 09:02:48 PM

Well I'm no genius in this stuff by any measure, but maybe your pedal draws less than 100mA? Then you wouldn't notice. My Valvecaster for example runs 300mA to just the heaters. On the other hand, you still might be entirely correct about your 317T. I've read alot about how many different kinds there are and that no two are alike and stuff like that, so try swapping that out if you have other 317's. Another option besides those entirely, is use diodes from the output of your 78L12 to drop the voltage to approximately what you want, did you say 9V, right? Say use 4 1n914/4148 diodes, these drop approximately 0.7V each which should get your voltage around 9.2V. Best of luck, and keep us posted

Jered · February 10, 2009, 09:43:52 PM

How much current does your transformer supply?

Ripthorn · February 10, 2009, 10:44:01 PM

Transformer supplies 2A at 12.6Vac. The 317T only handles 1.5A, but a single pedal only draws a few mA or so. I am just so baffled as to why it seems like it is not providing the power it should. Time to go poke around some more I guess...

R.G. · February 10, 2009, 10:57:51 PM

Supply us with the pin voltages on the LM317 both unloaded and loaded ("tanked").

Ripthorn · February 10, 2009, 11:46:41 PM

I will designate Pin1 as In, Pin2 is Adjust, Pin3 is Out.

When not loaded:

Pin1: 13.1V
Pin2: 10.6V
Pin3: 8.8V

When loaded:

Pin1: 12.5V
Pin2: 7.7V
Pin3: 4.5V

Another interesting thing that I noted is that if I were to disconnect the 78L12 (both the 78L12 and the 317T are fed by the + side of the diode bridge after filtering) then the voltage on Pin3 goes from 8.8V to like 15V. But even still, when I load it, the voltage drops to 4.5V like in the case above.

Vaum · February 11, 2009, 01:26:42 AM

Backwards.

Paul Perry (Frostwave) · February 11, 2009, 08:53:47 AM

Is it hot?

Ripthorn · February 11, 2009, 09:23:12 AM

Turns out it was backwards...

But then again, I now also know why, the data sheet I looked at had the pinout one way but the pinout for my brand of chip had it listed as the other way. Well, now I just need to change my resistor ratio, since it is now giving me 15.5V out (even though I am only putting in 13 or 14V), but the voltage doesn't drop when I hook up a pedal. I should have some time this evening to make sure that such is the case before saying "problem solved". Thanks for all the help guys.

~arph · February 11, 2009, 09:32:46 AM

Note to self.. see if I was using them backwards all the time.

EDIT: A quick glance at the ds seems to indicate I have indeed been making the same error. Oh my...

Ripthorn · February 11, 2009, 10:31:31 AM

At least I'm not alone anymore.

~arph · February 11, 2009, 10:41:20 AM

Yes, so when anyone sees my replies about pentode filament powering trouble, keep this in mind

Ripthorn · February 11, 2009, 11:11:53 AM

Are you using the 5672's like frequencycentral? My tubes are 6.3V filaments.

~arph · February 11, 2009, 11:15:08 AM

Yup..

Ripthorn · February 11, 2009, 11:25:33 AM

I am thinking of doing a tube amp with switchable power. I have some 6aq5 tubes coming, should give me about 10 watts, but I am debating as to whether I should make the other power section from 2 5672's or 2 5902's. The 5672's would give like 120mW or so, and the two 5902 would give roughly 2W. Not sure which would be best for bedroom levels.

~arph · February 11, 2009, 01:06:06 PM

Quote from: ~arph on February 11, 2009, 09:32:46 AM
Note to self.. see if I was using them backwards all the time.

..affirmative

frequencycentral · February 11, 2009, 01:20:44 PM

Quote from: ~arph on February 11, 2009, 01:06:06 PM
Quote from: ~arph on February 11, 2009, 09:32:46 AM
Note to self.. see if I was using them backwards all the time.

..affirmative

Quote from: Ripthorn on February 11, 2009, 11:25:33 AM
I am thinking of doing a tube amp with switchable power. I have some 6aq5 tubes coming, should give me about 10 watts, but I am debating as to whether I should make the other power section from 2 5672's or 2 5902's. The 5672's would give like 120mW or so, and the two 5902 would give roughly 2W. Not sure which would be best for bedroom levels.

@ Ripthorn - where are you getting your 5902 from?

Ripthorn · February 11, 2009, 01:23:27 PM

I have been looking at ebay, but I also found some obscure radio place in NY, though I am not sure they are in stock there. I also know that vacuumtubedepot (or something like that) has them for about 7 USD a piece (about the best price out there). I am just wondering if the difference between 10W and 2W will be enough or if I should just go with the 5672's.

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LM317T not providing enough juice (?)