LM317/337 dual supply

[jcx]

if you get thru Rod's info there's more: http://www.diyaudio.com/forums/head...capacitors-opamp-power-rails.html#post2872703

 

[Bibio]

bumping this up for another check as i'm bound to have got something wrong

its not for audio so not needed to be super dooper.

Schematic


Board


BOM
C1-2 1000uf, Aluminium Electrolytic
C3-4 1UF Tantalum
R1-2 220r Metal Film
VR1-2 2k Variable Resistor
D1-4 1A-60V Schottky Diodes SB160
BR1-2 2A-50V Bridge Rectifiers
LM317 Positive Regulator
LM337 Negative Regulator

 

[scottjoplin]

Hello again.

That looks fine

I discovered recently that it's necessary to use fast diodes or the rapid charging of the output capacitors can cause the regulators to lock up. You have used fast diodes so there should be no problem. Worth baring in mind though

Also reread what AndrewT posted

 

[Bibio]

thanks peeps, superb

should have the other boards right so its time for sending of to the fabricators.

on a side note, bloody brexit, lost a good 30p in exchange rate over the past few months :-(

 

[scottjoplin]

You're welcome (code: break seat)

 

[Bibio]

another question. given the above schematic and an input of 12vac what sort of max dc voltage can i expect on the output?

guess i should ask what size of secondary's do i need for 12vdc output with a little headroom?

 

[scottjoplin]

Hello again.

What does 12V dc output with a little headroom mean? It's a regulator

 

[Bibio]

ok, if i'm using 12vac secondary's what would the maximum vdc be at the output? LM317/337 adjustable regs.

 

[scottjoplin]

Approximately 12 * 1.4 - whatever the minimum drop across the LM317 is (about 2V?) I can't remember, it's all in the data sheet which I would have thought you'd know off by heart by now

 

[Bibio]

i could not remember the formula for the transformer/bridge, whether it was * or -. so i'm good for 12vdc and a little headroom.

i know i could have used fixed regs but i might need to turn it down to 9vdc

thanks again Scott

boards are fabbed on their way

 

[scottjoplin]

Gotcha. If in doubt about things like that the ESP site is the clearest most accurate place to go. It's surprising how much false info there is on the web, and some is downright dangerous.
Yes, you'll be fine for something around 15V from the regulator outputs.

 

[Shhh its me]

Worthy to note that the mounting face of the LM317 is connected to output terminal, so insulation needed if that heatsink is @ 0V via ground plane.

 

[AndrewT]

another question. given the above schematic and an input of 12vac what sort of max dc voltage can i expect on the output?

guess i should ask what size of secondary's do i need for 12vdc output with a little headroom?

12Vdc from a regulated supply usually works with a 12Vac transformer. This rule Vac ~ = reg Vdc holds for voltages from ~10v to ~30v
As you go higher the AC voltage can be a bit less than the required DC voltage.
As one goes lower, one needs to use a slightly higher Vac as the supply, say ~7Vac for that reg 5Vdc.

The exception is a Shunt style regulator where you need to take account of the extra voltage drop across the current limiter that must precede the Shunt regulator. Most 12Vdc shunt regulators will work with a 15Vac transformer.

 

[Bibio]

a momentous occasion for me. my first very own designed and built circuit board

ok so all the specs are straight off data sheets and help from forum members, but hey..



just need to see if it blows up

 

[scottjoplin]

Very neat. I've only ever built them on vero....

 

[Bibio]

Very neat. I've only ever built them on vero....

thanks Scott.

well i wired it all up switched it on and no bang

that went well so i then tested for voltage by probing 0V + and got 6.9v. i then probed 0V - and got -7.2v. i haven't twiddled the pots yet but things look promising

i dont have a scope so cant tell you what sort of voltage is coming out. all i know is its DC... lol

am i chuffed.. you bet

 

[Bibio]

+12vdc no problem but -11.5vdc is the max it will go and the trim pot still has a good bit to go before the stop. this to me sounds like either the transformer is not rated correctly (not tested) or i need to up the 240r resistor to 330r?

 

[scottjoplin]

How high will the positive rail go? Measure the voltages at the inputs to the regulators

 

[AndrewT]

2000 for the VR and 240 for the Out to Adj gives (2000/240)+1 * 1.25V +-5% gives 11.66V +-5%

You need to make the 240r just a bit lower, you can try 220r or 200r
But some 317/337 require a minimum current of 10mA and this is ensured by choosing 120r.

 

[Bibio]

ok just measured the trafo and its 14vac supposed to be 12... lol so its not that

so i go down not up on the resistor.. silly me

as for how high the positive will go. i have no idea as not gone past the 12vdc.

i'll dig around in the resistor bin and see what i have thats around 120-200r

thanks again

TBH i could just use 11.5vdc but i would rather get it right. in a way i'm glad its not worked perfect as then i dont learn anything.