First Linear Power Supply Build Questions

Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.
Hi All!

I'm sort of new to the DIY audio and electronics scene, and this site has been providing me with a wealth of useful information for my journey thus far.

I have decided to build my own linear regulated power supply to provide the power I will need for an upcoming headphone amp I will be building. Since I'm no pro, I was hoping I could get some feedback on my design before building it, just in case I made some catastrophic mistakes in my design or anything like that. Its pretty simple and based off the LM317 adjustable voltage regulator. Most of the schematic is based on the information found in the LM317 datasheet. The requirements I have for this build is a clean source of about 30VDC at the output. I did the calculations and came up with the details and design listed on the attached photo of the schematic.

If anyone has a moment to look the schematic over for any mistakes or to give any feedback or criticisms of the design, I would be immensely grateful. Thanks!
 

Attachments

  • ps-schematic.jpeg
    ps-schematic.jpeg
    751.2 KB · Views: 586
Moderator
Joined 2011
The DC voltage on C1 should be around 32.5VDC at nominal AC line.
This means there should be more drop across the LM317 for good regulation,
mfr says 3V minimum under worst case, with low line. As is, I would only
expect 27VDC out with low line.
 
Last edited:
There's something to watch out for with LM317 and that's not exceeding the maximum in-out voltage (40V) under short-circuit conditions. In this case you might, under worst case input voltage and an output short, destroy the LM. What's the regulation on your transformer?

Also - do you know what maximum output current you'll be needing?
 
Hi abraxalito,

Regarding your note on short circuit conditions, diodes D2 and D3 in the schematic are included based on the suggestion to include them on the LM317 datasheet. Specifically, in the event "the input is shorted to ground during a fault condition, protection diodes provide measures to prevent the possibility of external capacitors discharging through low-impedance paths in the IC" and D2 and D3 prevent capacitors discharging into the output of the regulator. Does this satisfy the concerns you mention?

Regarding the regulation of my transformer, I do not know if it does or doesn't have regulation. I intend to purchase either one of these to use for this project:

187F28 Hammond Manufacturing | Mouser
or
F8-28 Triad Magnetics | Mouser

As for maximum current output, I must admit I did not go through the calculations for current, and simply assumed between 1.5A or 3A would suffice for my purposes, at least for now. Still learning so this is one area I'm short on knowledge in.

Thank you for your comments!
 
This may be a stupid question, but what kind of headphone amp needs a 30 V single supply these days? That would have to be a seriously oldschool concept, AC-coupled output and all. Even the JLH uses a split supply. Good luck with turn-on thump then.

A 100 VA xfmr seems a bit excessive, even Class A concepts tend not to run more than about 150 mA per channel. 1 A output probably is on the generous side already. Honestly I would consider it a bit worrying if you have no idea what your circuit is actually drawing.
 
Disabled Account
Joined 2002
Why LM317? There are better ultra low drop regulator ICs availabe. I would use CLC filtering before the reg as well. With a modern IC you can probably use a 24V transformer and have less energy wasted in heat. Maybe not a design goal but cooler electronics also live longer. A key parameter is the desired output current.
 
Last edited:
This may be a stupid question, but what kind of headphone amp needs a 30 V single supply these days? That would have to be a seriously oldschool concept, AC-coupled output and all. Even the JLH uses a split supply. Good luck with turn-on thump then.

A 100 VA xfmr seems a bit excessive, even Class A concepts tend not to run more than about 150 mA per channel. 1 A output probably is on the generous side already. Honestly I would consider it a bit worrying if you have no idea what your circuit is actually drawing.

Fair question! So while the primary motivation for this supply is having enough juice for a beefy headphone amp, it will also sort of serve as a bench supply for other, non-audio projects.

When used for the headphone amp, I am using the Goldpoint VG2 circuit to split the supply up to +/- 15 VDC to power various versions of a cmoy, which I've found to serve me well as a learning model thus far.

With regards to my failure to follow the current, if I set my limit to having 1.5a of currents, that would be playing it safe as I know that doesn't exceed the limitations of any of the downstream parts or ICs i'm using. Upwards of that would be nice to have for other projects, but perhaps I'm trying to accomplish too much with one thing. Otherwise, I would appreciate any pointers on helping me achieve the maths needed to calculate my specific current needs. Thanks
 
Why LM317? There are better ultra low drop regulator ICs availabe. I would use CLC filtering before the reg as well. With a modern IC you can probably use a 24V transformer and have less energy wasted in heat. Maybe not a design goal but cooler electronics also live longer. A key parameter is the desired output current.

The short answer? LM317 makes for simple circuitry and its cheap. The long answer - I have oodles of heatsinks which I'll dive into to find an appropriate fit for this one for which would alleviate some thermal issues although still not efficient.

That said, what ICs do you have in mind as better alternatives? :)
 
Taking into consideration concerns stated so far, I realize it would be wise to stay with my original calculations using 1.5A as the required current.

As for CLC filtering, information I am seeing so far mentions its applications in SMPS and DC-DC converters? Either way, while it may be beneficial I am trying to keep costs in check and may have to forgo including additional features for now.

I am under the impression that the general circuit I have is sound, albeit inefficient?

Thanks to everyone offering input, I appreciate it!
 
If you are watching the budget, I would get a 12-0-12 toroidal transformer, and power supply board (with your choice of regulation)from that auction site, and get some quality replacement caps from a known supplier, or even a well rated seller of surplus parts if they can provide date codes.
That will give you a real +\- power supply for headphone/preamp/dac use.

A fuse before the transformer at the least is good practice too. There are power cord sockets that have a fuse built-in for reasonable prices for low current applications.
 
Regarding your note on short circuit conditions, diodes D2 and D3 in the schematic are included based on the suggestion to include them on the LM317 datasheet. Specifically, in the event "the input is shorted to ground during a fault condition, protection diodes provide measures to prevent the possibility of external capacitors discharging through low-impedance paths in the IC" and D2 and D3 prevent capacitors discharging into the output of the regulator. Does this satisfy the concerns you mention?

Short answer - no. Those considerations with the diodes are to re-direct currents from charged up capacitors under short-circuit conditions and work well, but only with the LM317 used within the datasheet parameters. My concern was the potential of going beyond the datasheet's maximum, something those diodes don't address.

Regarding the regulation of my transformer, I do not know if it does or doesn't have regulation.
Regulation isn't a feature of a transformer, all have it to some degree. Its a measure of the transformer's windings resistance. Regulation is expressed as a percentage change in the output voltage between zero and full-load. I looked at the datasheets at the links you posted and neither gives any indication about the regulation - but for 100VA in an EI transformer I'd guess in the region of 10%. When the regulation figure is known then the maximum off-load voltage out of the trafo can be calculated.

Incidentally I commend your selection of a split-bobbin type of transformer. They generally have lower leakage current from mains due to lower inter-winding capacitance.
 
If you are watching the budget, I would get a 12-0-12 toroidal transformer, and power supply board (with your choice of regulation)from that auction site, and get some quality replacement caps from a known supplier, or even a well rated seller of surplus parts if they can provide date codes.
That will give you a real +\- power supply for headphone/preamp/dac use.

A fuse before the transformer at the least is good practice too. There are power cord sockets that have a fuse built-in for reasonable prices for low current applications.

While budget may be among the chief drivers for this project, its not the primary directive. Buying a kit would circumvent the learning process inherent in designing even a simple power supply design. And that would defy the primary directive. :)

This and the fact that in addition to this project, I already have the parts and pcb done and ready to assemble for the Goldpoint VG2 circuit to take the output from this power supply and split it into a dual supply for audio use. Which is a needlessly complicated way of obtaining a split supply sure, but it sure does satisfy the primary directive.

And I love the idea of using a power cord socket with a built in fuse - hadn't thought of that, thanks for the tip!
 
Short answer - no. Those considerations with the diodes are to re-direct currents from charged up capacitors under short-circuit conditions and work well, but only with the LM317 used within the datasheet parameters. My concern was the potential of going beyond the datasheet's maximum, something those diodes don't address.


Regulation isn't a feature of a transformer, all have it to some degree. Its a measure of the transformer's windings resistance. Regulation is expressed as a percentage change in the output voltage between zero and full-load. I looked at the datasheets at the links you posted and neither gives any indication about the regulation - but for 100VA in an EI transformer I'd guess in the region of 10%. When the regulation figure is known then the maximum off-load voltage out of the trafo can be calculated.

Incidentally I commend your selection of a split-bobbin type of transformer. They generally have lower leakage current from mains due to lower inter-winding capacitance.

So I spent some time reworking this project based on some of the feedback given, which led me to replace the LM317 with the similar LM338, distinguished by its ability to output upwards of 5A and 7A surge current.

With 30vdc @ 3a output required, the LM338 should forgo some of the inefficiency concerns with the LM317. As such it doesn't exceed the datasheet specifications and therefore the protection diodes would indeed maintain their protective faculties, is this correct?

With a 28vac @3.57a trafo, assuming the appx. 10% regulation figure you provided, achieving a 30vdc shouldn't be an issue. Unless I'm misunderstanding what you are saying, that is. I appreciate you taking the time to look over those datasheets and provide some pointers for me!
 
Why LM317? There are better ultra low drop regulator ICs availabe. I would use CLC filtering before the reg as well. With a modern IC you can probably use a 24V transformer and have less energy wasted in heat. Maybe not a design goal but cooler electronics also live longer. A key parameter is the desired output current.

You were right, LM317 was a poor choice and current was a necessary output parameter to consider. With this in mind, required current is 3A @ 30vdc output which is better served using the LM338. I have also replaced the transformer with one rated at 28vac @3.57a. The LM338 was the only IC I found capable of handling upwards of 5A current in addition to handling over 35vdc input. To keep things cool ill be adding a fan(s) as well as the obvious heatsinks.

CLC filtering was a good idea, thanks for sending me in that direction. At this time I will include a CRC filter, while less effective at reducing ripple than CLC, its still better than just C.

Having calculated that I'll need 60,000uF for 0.5v of ripple at 3a, I'll be taking the rectified DC signal from the bridge through 30,000uF (as 3x 10,000uF in series), followed by R (~0.17 oHms), and 30,000uF after that. Does that sound about right?
 
http://www.hammondmfg.com/pdf/5c007.pdf

Great guide for calculating current.

AS-2215 - 200VA 15V Transformer - AnTek Products Corp

Good for 30V regulated or +/- 15v

Sounds like you need to define your design needs. In general power amps aren’t regulated. Maybe you’ll do better with a simple pass transistor voltage stabilization.

Isn't that what I'm doing here - designing a regulated, single output, power supply based on the (now) LM338?

Design needs have been established as 30vdc at 3a output.

Forgoing a toroid tranfo for now to keep cost in check, but thank you for providing a link for what appears to be a well priced option in the future. Also, the Hammond calculation pdf is very helpful, thank you!
 
With 30vdc @ 3a output required, the LM338 should forgo some of the inefficiency concerns with the LM317. As such it doesn't exceed the datasheet specifications and therefore the protection diodes would indeed maintain their protective faculties, is this correct?

The protection diodes aren't addressing the issue I've been referring to, which is potentially exceeding the DS max input to output voltage under short-circuit conditions.

With a 28vac @3.57a trafo, assuming the appx. 10% regulation figure you provided, achieving a 30vdc shouldn't be an issue.

Assuming 10% regulation with a 28VAC trafo, your offload voltage under nominal mains input will be 28*sqrt(2)*1.1 - 1.2V = 42.4V. A short-circuit applied under this condition is potentially going to destroy the regulator. With higher than nominal mains the likelihood of device destruction is going to be greater.

Unless I'm misunderstanding what you are saying, that is.

It does indeed look that way.
 
While you're experimenting, you could also take a look at the current boost configuration commonly used with these devices. Basic example here:
LM317 Adjustable Voltage current Boost Power Supply
You said you have fan in your design - and you're going to need it (!) - the 317/337 will get extremely hot you will discover. It's dissipating power (lots of heat) at the current you are drawing in the load (I) x (Vin - Vout).
And as you experiment you'll then realise that you don't want the fan on all the time...so a little thermistor control circuit needed next. Then you'll perhaps want some current limit control. Welcome to experimenting with electronics :)
 
Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.