Thread Starter

Jean1320

Joined Oct 11, 2019
13
Hello, I need to design a linear regulated supply to power an dac output stage based on op amp. It must be a negative + ground + positive one.
The problem I encounter is that I don't know wich topology to select between the first and second arrangement.
I suppose that the first one induce a greater voltage drop and losted power, besides costing a little more and taking more space. I also think that the output power may be higher (provided that the diodes used in the two differents circuits are of equal total power) and that it can be more stable? Compared to the second, what is/are its benefit(s) or drawbacks? I think that the second emits less switching noise because 2 less diodes are in conducting state at each AC cycle and that it can work with center-tapped transformer while the first cannot?
As the only thing that matters to me is the performance so the added cost is not a concern, I will add positive and negative linear regulators at the end of this circuit.
Finally, what's the purpose of the (optional) input capacitors paralleled with transformer's secondary (c46 and c45 for the first and c43 and c44 for the second)? I added them because I learned that they can be beneficial for less noise and more stability (forming a LC low pass filter with transformer's coil) but can they be detrimental and what will be the recommended general value?
In the third example, the only difference with the second schematic is that there is only one paralleled capacitor (c63) placed within secondary's outputs, and it's not raccorded to ground.

Thanks.
 

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dl324

Joined Mar 30, 2015
12,256
I need to design a linear regulated supply to power an dac output stage based on op amp.
  1. What is the current requirement?
  2. What is the secondary voltage and current?
  3. What regulated voltages do you require?
  4. Do you already have this transformer? Dual secondary transformers are probably more expensive than a single secondary with center tap. If you don't need a lot of current, you can use a single secondary transformer to make a bipolar supply.
I think that the second emits less switching noise because 2 less diodes are in conducting state at each AC cycle and that it can work with center-tapped transformer while the first cannot?
The ripple will be 120Hz which is easily filtered.
Finally, what's the purpose of the (optional) input capacitors paralleled with transformer's secondary (c46 and c45 for the first and c43 and c44 for the second)? I added them because I learned that they can be beneficial for less noise
As you said, they're for noise suppression and aren't always used. Most voltage regulators won't be affected if they have sufficient headroom.
 

Thread Starter

Jean1320

Joined Oct 11, 2019
13
  1. What is the current requirement? About 0.3-0.4 A
  2. What is the secondary voltage and current? It's 12v rated at 0.5A
  3. What regulated voltages do you require? -15v 0 +15v
  4. Do you already have this transformer? Dual secondary transformers are probably more expensive than a single secondary with center tap. If you don't need a lot of current, you can use a single secondary transformer to make a bipolar supply
  5. I have multiples 12v transformers of wich some are center-tapped and others have dual secondary.
The ripple will be 120Hz which is easily filtered. Ok
As you said, they're for noise suppression and aren't always used. Most voltage regulators won't be affected if they have sufficient headroom. Ok

Thanks for your reply.
 

Thread Starter

Jean1320

Joined Oct 11, 2019
13
My budget is quite large, I only have to make the best choice possible.

Thanks a lot, I will eventually consider this but I would prefer to make it by myself, with the transformers that I own. I think they are appropriate?
Based on your recomendations, I think that I will choose the single bridge rectifier configuration and it will be fine.
 

SamR

Joined Mar 19, 2019
2,912

Thread Starter

Jean1320

Joined Oct 11, 2019
13
There is an adjustable power supply kit sold on AliExpress for less than $7 postage included. Excellent design and quality parts. 5-40VDC depending on which transformer you use. Mine came with Nichicon Capacitors that measured 0 ESR. I am feeding it with a 115/230/28/28 transformer and it works very well. Getting +/- ~25VDC out of it.
Lusya DIY Kits LM317t LM337t Linear Regulated DC Power Supply Adjustable Filtering Board 5 40V DC F7 007|Amplifier| - AliExpress
View attachment 226678
Thanks, I will study this design and why not buy the board.
 

Thread Starter

Jean1320

Joined Oct 11, 2019
13
Some specifications if you need extra low noise
https://www.osaelectronics.com/product/ulps1225a/
I saw this psu and it seems very high end! A thing is not a supposition though: the price! I saw the even higher end one, with better PSSR and less noise, in wich there is low value decoupling caps, generous input reservoir capacitors, some analog devices linear regulators paralleled and the double bridge rectifier (one for each polarity). The interest of the double bridge rectifier is that it can allow the supply to be used for each voltage side independently, as desired, that's right? Also, I learned somehow that output capacitors after linear regulator side must be rated below a certain capacitance, in order to restrict linear regulator's oscillations. Is that right? I ask those questions to confirm my knowledge/thoughts and to eventually complete/confirm those of other minds that can fall on this thread!
 

dl324

Joined Mar 30, 2015
12,256
About 0.3-0.4 A
It's 12v rated at 0.5A
You can get 300mA, but not 400.

This is from Hammond Manufacturing:
clipimage.jpg

A 12V secondary will put you close to the edge at 300mA. The power line noise you're worried about could be an issue because you don't have much voltage headroom.

EDIT: You'll have more headroom if you use low dropout regulators.
 
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Thread Starter

Jean1320

Joined Oct 11, 2019
13
You can get 300mA, but not 400.

This is from Hammond Manufacturing:
View attachment 226743

A 12V secondary will put you close to the edge at 300mA. The power line noise you're worried about could be an issue because you don't have much voltage headroom.

EDIT: You'll have more headroom if you use low dropout regulators.
Yes I will use low dropout regulators and, ok, I will power the op amps with less voltage, 14v will be fine. I didn't knew that there was an average rectified DC voltage and the formula belonging to it! I only knew about the "1.41 X secondary V A.C"
Thanks a lot
 

dl324

Joined Mar 30, 2015
12,256
I didn't knew that there was an average rectified DC voltage and the formula belonging to it!
I don't really pay attention to that formula because it depends on filter capacitance and loading, so it's not very meaningful.

I haven't really tested the current formula, but it has been cited by a number of members.

Here's the complete document.
 

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Thread Starter

Jean1320

Joined Oct 11, 2019
13
I don't really pay attention to that formula because it depends on filter capacitance and loading, so it's not very meaningful.

I haven't really tested the current formula, but it has been cited by a number of members.

Here's the complete document.
Thanks, I'm into learning all of those formulas.
 

sparky 1

Joined Nov 3, 2018
404
The first schematic of the original post shows isolated secondary. The link shows some typical components used in a high end.

The inexpensive modules photo below to show some differences. One board nice double sided has large heat sinks and trim pots.
The capacitor arrangement are different. The original post schematic #2, #3 did not use parallel bridge like these in foreground.
 

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Last edited:

Thread Starter

Jean1320

Joined Oct 11, 2019
13
The first schematic of the original post shows isolated secondary. The link shows some typical components used in a high end.

The inexpensive modules photo below to show some differences. One board nice double sided has large heat sinks and trim pots.
The capacitor arrangement are different. The original post schematic #2, #3 did not use parallel bridge like these in foreground.
Ok, I see, thanks.
 
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