[f] AMB on Facebook

Powering the sigma25 off a switch-mode AC/DC wall wart?

sigma25, sigma26, sigma24

Powering the sigma25 off a switch-mode AC/DC wall wart?

Postby dfy » June 9th, 2013, 5:00 pm

I'd like to build a decent power supply for some DIY projects I'd like to try, and the signma 25 looks pretty nice :) . However, I have no formal training in electrical engineering, so messing with mains voltage is something I'd like to avoid.

As far as I understand, most DIY regulator circuits are meant to be powered by a standard transformer/transformer wall wart the transforms at mains frequency. Now, these wall warts are increasingly hard to come by (in Europe at least), while lightweight and efficient switch-mode supplies can be found easily. So:

1) Is it possible to power the sigma25 with a switch-mode supply?
2) It seems that some SMPS circuits use a high frequency transformer (thereby keeping the load side galvanically separated from mains voltage), while others don't (basically keeping output GND at mains null). I vaguely remember reading somewhere that the latter may hurt certain circuits or even be outright dangerous. Is this something I have to worry about?
dfy
 
Posts: 2
Likes: 0 post
Liked in: 0 post
Joined: June 9th, 2013, 4:51 pm
Location: Central Europe
Country: Germany (de)

Re: Powering the sigma25 off a switch-mode AC/DC wall wart?

Postby amb » June 9th, 2013, 9:54 pm

dfy wrote:1) Is it possible to power the sigma25 with a switch-mode supply?

Yes, you can, but instead of a transformer that outputs AC, you'll have a SMPS that outputs DC. The transformer (AC) voltage listed at the σ25 website assumes the former, and performs rectification onboard (i.e., does AC-to-DC conversion) with a bridge rectifier. While you could feed DC to the σ25 as input, you'll need to provide higher than the specified AC voltage (at least 1.41x) because the bridge would not be doing the AC-DC conversion (which normally provides a 1.41x voltage "boost" in a bridge rectifier).

Also, using a switch-mode supply may degrade the power supply performance due to added switching noise at high frequencies.

2) It seems that some SMPS circuits use a high frequency transformer (thereby keeping the load side galvanically separated from mains voltage), while others don't (basically keeping output GND at mains null). I vaguely remember reading somewhere that the latter may hurt certain circuits or even be outright dangerous. Is this something I have to worry about?

This depends on the circuit being powered. Having the PSU "-" output connected to AC ground is often problematic, and could cause such things as ground loops. With virtual-ground circuits (i.e., Mini³, M³ and other similar) a mix of different ground metaphores (where signal ground is not the same as power supply ground) may cause malfunctions.
User avatar
amb
Site Admin
 
Posts: 10379
Likes: 175 posts
Liked in: 482 posts
Joined: March 29th, 2010, 9:49 pm
Location: Sunnyvale, CA
Country: United States (us)

Re: Powering the sigma25 off a switch-mode AC/DC wall wart?

Postby dfy » June 10th, 2013, 1:20 pm

Thanks for your reply!
I know that I would need a higher DC voltage. (I'm not convinced I'd need as much as 1.41x though, the 78xx data sheet has a minimum V_in of about V_out + 2.5V, so an 18V DC supply should be fine for an output of 15V?)
Concerning the switching noise, I was thinking that C3 would filter most of that rather convincingly - or is there anything I'm missing?

About PSU- connected to AC ground, is a quick multimeter check enough to check whether a given SMPS is isolated against the AC line or not? Or should I simply forget about the idea of using an SMPS and see whether I can find an AC/AC wall wart somewhere (I can find PCB-mount transformers everywhere, but the only AC/AC wall warts I found were at Jameco in the US...) ?
dfy
 
Posts: 2
Likes: 0 post
Liked in: 0 post
Joined: June 9th, 2013, 4:51 pm
Location: Central Europe
Country: Germany (de)

Re: Powering the sigma25 off a switch-mode AC/DC wall wart?

Postby amb » June 11th, 2013, 11:53 am

dfy wrote:I know that I would need a higher DC voltage. (I'm not convinced I'd need as much as 1.41x though, the 78xx data sheet has a minimum V_in of about V_out + 2.5V, so an 18V DC supply should be fine for an output of 15V?)
Concerning the switching noise, I was thinking that C3 would filter most of that rather convincingly - or is there anything I'm missing?

You need to account for the voltage drop in the bridge rectifier, and you should operate the regulator with more than the minimum dropout voltage to make sure it stays in regulation.

Capacitors are effective in filtering noise only to a certain degree, because of its internal impedance and especially at the higher frequencies that the SMPS operates in.

About PSU- connected to AC ground, is a quick multimeter check enough to check whether a given SMPS is isolated against the AC line or not? Or should I simply forget about the idea of using an SMPS and see whether I can find an AC/AC wall wart somewhere (I can find PCB-mount transformers everywhere, but the only AC/AC wall warts I found were at Jameco in the US...) ?

A multimeter check is enough.
You could always "make your own wallwart" by putting a transformer in a box.
User avatar
amb
Site Admin
 
Posts: 10379
Likes: 175 posts
Liked in: 482 posts
Joined: March 29th, 2010, 9:49 pm
Location: Sunnyvale, CA
Country: United States (us)

Re: Powering the sigma25 off a switch-mode AC/DC wall wart?

Postby MisterX » June 11th, 2013, 12:44 pm

Or should I simply forget about the idea of using an SMPS and see whether I can find an AC/AC wall wart somewhere (I can find PCB-mount transformers everywhere, but the only AC/AC wall warts I found were at Jameco in the US...) ?


How much current do you need?

RS has a couple of 15 volt AC to AC adapters but they're rather pricey.....however they have a few 18 volt ones that are reasonably priced.
Example:

http://uk.rs-online.com/web/p/plug-in-p ... y/7300192/
1352.5 and counting.
User avatar
MisterX
 
Posts: 1143
Likes: 52 posts
Liked in: 36 posts
Joined: March 31st, 2010, 2:00 am
Location: Kalamazoo, MI
Country: United States (us)

Re: Powering the sigma25 off a switch-mode AC/DC wall wart?

Postby nayaanika » January 22nd, 2014, 9:52 pm

Is the SMPS fan affecting my gaming? I am a game freak.. and I have many games running on my system... It was going great until I had this problem... When I play games, they open and my system gets hanged and a message is shown in a blue screen... I've been facing this problem for quiet a bit of time... Recently I have seen that the SMPS fan is not rotating... I tried to repair it thrice but cannot do that... is the SMPS fan affecting my gaming?
nayaanika
 
Posts: 1
Likes: 0 post
Liked in: 0 post
Joined: January 15th, 2014, 2:02 am
Location: njmhcjun vbnbn
Country: Anguilla (ai)

Re: Powering the sigma25 off a switch-mode AC/DC wall wart?

Postby amb » January 22nd, 2014, 11:47 pm

Many power supplies vary their fan speed based on need, there are internal thermal sensors and circuitry to automatically adjust the fan speed. Are you sure that your fan isn't working at all? If so, then it's likely that something is overheating due to lack of cooling. Whether that's the cause of the message you see on screen or not, I don't know. You didn't even say what the message is. And how does this relate to the σ25 anyway?
User avatar
amb
Site Admin
 
Posts: 10379
Likes: 175 posts
Liked in: 482 posts
Joined: March 29th, 2010, 9:49 pm
Location: Sunnyvale, CA
Country: United States (us)

Re: Powering the sigma25 off a switch-mode AC/DC wall wart?

Postby MagnumOpus » March 18th, 2014, 4:31 pm

I would like to use the S25 to power a Pimeta v2, and I am pleased to see that is accepts DC from a wallwart. Do I need to do what the quote under suggests, or can I just build it per amb site instructions and hook it up?

"Sigma 25 can be used without any ac voltage. If you look at the schematic, you would simply leave c1 and d1 unpopulated, then solder your + input to the spot where the bridge would go that is marked with a plus and your ground could be put into the ground pad next to where c1 goes.

If you want to do your own any of the 1n400x diodes are perfectly fine, and you would connect it from the output voltage pin on the 78xx to the input pin."
MagnumOpus
 
Posts: 25
Likes: 0 post
Liked in: 0 post
Joined: March 31st, 2011, 7:45 am
Country: Norway (no)

Re: Powering the sigma25 off a switch-mode AC/DC wall wart?

Postby amb » March 18th, 2014, 4:52 pm

You can do it either way. If you use the method described at the σ25 website, you'll need a DC wallwart of at least 5V higher than the desired regulated output voltage.
User avatar
amb
Site Admin
 
Posts: 10379
Likes: 175 posts
Liked in: 482 posts
Joined: March 29th, 2010, 9:49 pm
Location: Sunnyvale, CA
Country: United States (us)


Return to σ25 and σ26 regulated power supplies, σ24 transformer board

Who is online

Users browsing this forum: No registered users and 0 guests

Quick Links

AMB audio DIY resources
AMB audio shop

cron