which transformer to use for this regulated supply?

godslayer · 2013-05-25 4:55 pm

Hi guys,

while stumbling over the internet looking for +-15v good regulated supply using 317/337 i stumbled upon this link which i chose for the construction of my supply as this is the easiest no frills +-15v schematic i can ever get

http://audio.gotroot.ca/minipow/

i havent been able to find the transformer rating. can someone help me with that? will [email protected] >25VA work fine with this?

does someone have a BETTER and simpler circuit for a clean +-15v than this?

Rundmaus · 2013-05-25 5:12 pm

I don't see the point of so much filter capacitance before the regulator. Otherwise, it seems pretty standard as shown in the 317 application note.

Rundmaus

Rundmaus · 2013-05-25 5:31 pm

... just noticed that I did not really answer your question.

12VAC will give approx. 18VDC rectified, minus the voltage drop of two diodes (1.4V). This does not leave enough headroom for the regulator, especially with some mains variations, IMHO.

LM337 is capable of 1.5A, I seem to remember, better check it in the datasheet. So the maximum sensible current draw is 1.5A per rail, meaning 30V*1.5A = 45W. I would use at least 50-70VA transformer, unless you definitely know you will not be drawing that much current.

Rundmaus

godslayer · 2013-05-25 5:41 pm

i wont be drawing much current, i just need the supply for p88 (esp) and one bass filter cutoff ie all in all driving 4 opamps... ill go for higher one, maybe 15-0-15 @ 2a(60va) thanks for the help

as for the complications you mentioned, will this circuit work? given that i use only 15v network? is it suitable enough for Audio application(i see varied register networks than previous circuit)?

AndrewT · 2013-05-25 7:04 pm

15+15Vac is perfect for a regulated 15+15Vdc

godslayer · 2013-05-25 7:31 pm

AndrewT said:
15+15Vac is perfect for a regulated 15+15Vdc

can i also have your critique on the above circuit?

AndrewT · 2013-05-26 8:37 am

LED5 is being run very hard
R38 might be a bit high.
The switching is going to send ~23V pulses to the outputs.
C1, 2, 9 & 10 are shown in the wrong locations.
The continuous Zero Volts wire from chassis to GND is wrong.

TonyTecson · 2013-05-26 10:29 am

AndrewT said:
15+15Vac is perfect for a regulated 15+15Vdc

yes, but down to 3 volts out your regulator is wasting most of the power...

godslayer · 2013-05-26 6:48 pm

Tony said:
yes, but down to 3 volts out your regulator is wasting most of the power...

i have a spare 13-0-13v @ 75va one lying in my drawer. i guess that will be perfect.

26v-> 36vdc minus the diode drop
2a-> 1.27A dc

36v dc 3v for each regulator to regulate. i guess things are better like this.

i will go with minipow supply then.

sofaspud · 2013-05-26 8:14 pm

AndrewT said:
LED5 is being run very hard
R38 might be a bit high.

Typo?
Looks like the LED receives ~15mA; R38 could probably be increased in value.

Tony said:
yes, but down to 3 volts out your regulator is wasting most of the power...

Maybe not "most of the power," but more than necessary.
For a handful of op amps, either the 26VCT or 30VCT transformers should be more than big enough.

Art M · 2013-05-26 9:15 pm

So I was wandering around the Texas Instruments LM317 data sheet per the regulator in discussion. I need a bit of guidance as to how the shown AC regulator circuit functions with AC applied to both Vi terminals ? The circuit might have some function as a raw Pre regulator before a Bridge ?

sofaspud · 2013-05-27 2:48 am

It would probably work as an AC "preregulator" to some extent, but it just as probably isn't a very effective or efficient method. The bridge and filter caps don't need this regulation.
The AC clamp circuit functions quite normally - the 120Ω + 480Ω resistors set the regulated voltage at 6V. The output waveform is more of a chopped triangle wave due to the input sine wave reaching the regulator operating point. Then the voltage ramps to the regulated 6V, remains there for a portion of the cycle, and ramps back down.

Art M · 2013-05-27 4:54 am

The regulators will only pass in the Positive direction, how can the Negative return path be completed by the opposite regulator ?

sofaspud · 2013-05-27 5:24 am

The regulators will only pass in the Positive direction

Not true; that's why many of the LM317 circuits include the reversed-biased diode from input to output - to provide an alternate current path for input shorts. The AC clamp is both low-voltage (6Vp-p) and low-power (2W). That is probably a circuit requirement. You'll note that at low voltages the datasheet states that the diodes (in/out & adj bypass) are not necessary.
In my analysis I was WAGging it a lot, but it should be a reasonably close description.

TonyTecson · 2013-05-27 5:36 am

Art M said:
The regulators will only pass in the Positive direction, how can the Negative return path be completed by the opposite regulator ?

correct for normal operation, in case of shutdowns or input shorts where a possibility of current flowing in the reverse direction and damage the series pass trannie, a diode or a zener can protect the series pass trannie from damage...

godslayer · 2013-05-27 6:16 am

sofaspud said:
It would probably work as an AC "preregulator" to some extent, but it just as probably isn't a very effective or efficient method. The bridge and filter caps don't need this regulation.
The AC clamp circuit functions quite normally - the 120Ω + 480Ω resistors set the regulated voltage at 6V. The output waveform is more of a chopped triangle wave due to the input sine wave reaching the regulator operating point. Then the voltage ramps to the regulated 6V, remains there for a portion of the cycle, and ramps back down.

Indeed, regulating AC doesnt sound like a genius method to me right before being rectified? it might decrease the smoothing capacitors required but i guess, you will still need a regulator circuit after smoothing :\

godslayer · 2013-05-27 6:31 am

sofaspud said:
Typo?
Looks like the LED receives ~15mA; R38 could probably be increased in value.

Maybe not "most of the power," but more than necessary.
For a handful of op amps, either the 26VCT or 30VCT transformers should be more than big enough.

I'd still come as close as i can to regulation voltage assuming the variation in power supply i guess most of the people use 24-30vct rather than one that is near 15v.

AndrewT · 2013-05-27 9:14 am

a 24Vac centre tapped transformer is the same as a 12+12Vac transformer except that the two secondary winding are joined at the centre tap.

A 12Vac transformer is too low for a 15Vdc regulated supply.
You need very approximately a 14Vac to 18Vac transformer, 15Vac having been proved by many builders to be adequate.

godslayer · 2013-05-27 2:48 pm

i have 13x2 @ 2A one lying in my drawer,i havent measured how much it actually shows unladen. after constructing the circuit, i will let you know if that was sufficient.

godslayer · 2013-06-27 6:19 pm

AndrewT said:
a 24Vac centre tapped transformer is the same as a 12+12Vac transformer except that the two secondary winding are joined at the centre tap.

A 12Vac transformer is too low for a 15Vdc regulated supply.
You need very approximately a 14Vac to 18Vac transformer, 15Vac having been proved by many builders to be adequate.

Okay guys i built this supply using a 13-0-13 @ 2 amp transformer the rectified and filter voltage is 17.2v on each rail but unfortunately THE REGULATED VOLTAGE IS 14.52 volts. is it okay to drive ne5532 with this voltage or it needs sharp 15v for it?

which transformer to use for this regulated supply?

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