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Switching power supplies
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lorencz



Joined: Oct 04, 2015
Posts: 18
Location: Detroit
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PostPosted: Thu Aug 31, 2017 12:17 pm    Post subject: Switching power supplies Reply with quote  Mark this post and the followings unread

A while ago I was looking at euro cases and noticed that some brands had what looked like two switching power supplies in them hooked up in series to create a bipolar power supply. I had seen those cheap Chinese SMPS intended for LED strips on eBay, but of course they're single rail.

I did a little research on whether or not it was feasible to hook up two SMPS in series, and read a lot of opinions saying everything would blow up, and a lot of opinions saying everything would be fine.

Anyway, I decided to do an experiment, and bought two small 3A LED power supplies, for less than $5 a piece, and hooked them up in series. I've had them under a small load, (a Yusynth Steiner filter,) for about an hour and everything seems fine. The voltage is nice and stable. The sound is fine and doesn't seem to be effected by the switching, (which is to be expected because it's well above audio range,) and the power supplies are nice and cool.

Figured I'd share because less than $10 for 3A of stable power is not a bad deal. I really wasn't sure it would work, and hadn't read any posts about anyone doing this before. As much as I love linear power supplies, switching supplies would be a cheaper alternative, especially when you factor in available amperage for larger cabinets.

I haven't put them under any significant load yet, (worried about damaging modules,) but I might hook up some DC motors later and let them run, monitoring the voltage output and whether or not the power supplies continue to seem healthy.

If you're curious about sound quality, I've attached a sample of a Wogglebug going into the Steiner filter, with the cutoff being modulated by a Sloth Chaos, then going into my computer with a little reverb, flanging, and EQ added.

Also, if one were really concerned about ripple, they could buy 15V supplies, put some beefy caps after the supplies, and then parallel some LM7x12 regulators to handle a few amps of current.


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Last edited by lorencz on Thu Aug 31, 2017 1:28 pm; edited 2 times in total
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wackelpeter



Joined: May 05, 2013
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PostPosted: Thu Aug 31, 2017 12:58 pm    Post subject: Reply with quote  Mark this post and the followings unread

You mean in series and not parallel, right? Wink

There's a thread over at Muffs regarding switched PSU's... From what i remember some DIY Shops do offer some of them. Those are as i remember correctly PSU's with two separate outputs.

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lorencz



Joined: Oct 04, 2015
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PostPosted: Thu Aug 31, 2017 1:03 pm    Post subject: Reply with quote  Mark this post and the followings unread

Ha, yeah, series. I should correct that.

I've looked in to dual rail switching supplies, (Meanwell makes a few as I remember,) but with the added cost it didn't seem worth it compared to a standard linear supply, which is why I tried this.
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PHOBoS



Joined: Jan 14, 2010
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PostPosted: Thu Aug 31, 2017 1:04 pm    Post subject: Reply with quote  Mark this post and the followings unread

I assume you mean the input (mains) in parallel and he outputs in series. It can work as long as the V- output of
the supplies isn't directly connected to the chassis/earth. I measured on a similar supply and that does indeed not
seem to be the case.

edit: ah you posted just before me

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lorencz



Joined: Oct 04, 2015
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PostPosted: Thu Aug 31, 2017 1:11 pm    Post subject: Reply with quote  Mark this post and the followings unread

Yeah, before I hooked it all up I made sure the earth and negative output were isolated, which they are, and which I should have probably noted for anyone else wanting to try this. And yes, mains in parallel, output in series.
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lorencz



Joined: Oct 04, 2015
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PostPosted: Thu Aug 31, 2017 1:40 pm    Post subject: Reply with quote  Mark this post and the followings unread

Actually, I may have spoke too soon regarding bipolar SMPS being significantly more expensive. There don't seem to be a lot of +/- 12V options out there, but +/- 15V is easy enough to find, which allows plenty of headroom for filtering and linear regulating, so that's probably actually better. And, the price difference is negligible. ($30 for 4 amps doesn't seem so bad. Could probably run 4 or 5 rows of 84HP off that.)


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PHOBoS



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PostPosted: Thu Aug 31, 2017 2:39 pm    Post subject: Reply with quote  Mark this post and the followings unread

it's a nice compact solution, but I was looking at some specs and would indeed advice to add some regulators
as the ripple voltage seems to vary somewhere between 75mVpp and 150mVpp. But for eurorack the +/-15V
would do fine with some 12V regulators and you can add them per row to spread the dissipation. Not sure if
everything works the same at those high switching frequencies though.

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piedwagtail



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PostPosted: Fri Sep 01, 2017 10:25 am    Post subject: Reply with quote  Mark this post and the followings unread

I can't say I've got the full modular up and running on these yet, but on test they seem good.
I've built an offboard psu with +/-24 and +/-15 supplies. All the waveform electronics have onboard 15V bipolar regulators supplied by the 24V and the 15V supplies are unregulated for gate and pulse electronics with led currents.

Make the ground wire as big as possible ie.an aluminium bar

R
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halfshavedyaks



Joined: Jul 12, 2018
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PostPosted: Sat Jan 19, 2019 3:50 am    Post subject: Reply with quote  Mark this post and the followings unread

If anyone is doing this successfully please post details!

There are a bewildering number of cheap 12v and 15v power supplies availble, if theres a particular one that has been successful please share.

also if using 15v with a regulator down to 12v how are you doing it - regulator heatsinking and current capacity adds quite a bit of complexity, at that point one might as well almost use a transformer for a linear supply.

I also notice that most of these cheapo supplies have a power efficiency of about 74% (or maybe even less as many don't even say) wheras a decent meanwell power supply would usually have an efficiency of more like 85%

I'm wondering about doing this using better quality supplies such as meanwell and getting better efficiency and maybe not needing regulators as a result.

any thoughts?
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piedwagtail



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PostPosted: Sat Jan 19, 2019 4:53 am    Post subject: Reply with quote  Mark this post and the followings unread

The most important element in a modular is the ground, everything is referenced to that.
Secondly stiff power rails. Think that the power supply has impedances and rocks like an ac signal if you're not careful.

As I said I'm using 24V for audio signal electronics,with 15V regulators on or at each board. Logic and sequencers run from separate 15V supplies.

If you use a transformer, you will still be using regulators. 18V secondaries as 15V might not clear the regulator dropout.

Lot of board noise comes from big current swings from LEDs; actually saturation spikes in the switching transistor. I've posted constant current LED drivers in the lunetta forum.

Signal purity and conditioning always burns watts.

R
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halfshavedyaks



Joined: Jul 12, 2018
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PostPosted: Sat Jan 19, 2019 5:34 am    Post subject: Reply with quote  Mark this post and the followings unread

piedwagtail wrote:
The most important element in a modular is the ground, everything is referenced to that.
Secondly stiff power rails. Think that the power supply has impedances and rocks like an ac signal if you're not careful.

As I said I'm using 24V for audio signal electronics,with 15V regulators on or at each board.

R


so do you have any tips for a good ground? I'm not really sure what can be done other than the usual star grounding and thick wire.

24v -15v is a big drop when it is really just the regulator dropout voltage to consider, ripple should be fairly small from a switching supply compared to a transformer and there are no rectification losses to account for. Why do you need such a big drop?

I was considering using 15v supplies for regulated 12v output. 3v is more than the dropout voltage of many regulators. There are also LDO regulators with dropout voltages below 1v.

multiple regulators means the total current is effectively not shared so in a eurorack system you have load management issues to not exceed the max regulator current. I guess that's OK if you have lots of regulators all running below capacity. that's quite a complex setup though.
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piedwagtail



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PostPosted: Sat Jan 19, 2019 7:59 am    Post subject: Reply with quote  Mark this post and the followings unread

Quote:
thick wire

Rod is better or the chassis itself.
Quote:

Why do you need such a big drop?

Cost.
Big current 24V SMPS are cheap as are standard non-LDO regulators.
High frequency noise can be an issue with SMPS and buying multiple SMPSs to test is not an option.
Regulation at board level rather than at supply also keeps inter-board interference at bay.

Suggest you turn the oscilloscope to mV range and do the empirical study as you build. Seeing the noise/ripple is the best research.

R
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Grumble



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PostPosted: Sat Jan 19, 2019 9:49 am    Post subject: Reply with quote  Mark this post and the followings unread

I use this in my very modest euro rack: click
Using this for nowadays opamps shouldnt be a problem since these opamps have a psrr (power supply rejection ratio) of around 80bB.
Problems may arise when a reference voltage (or a virtual GND level) is made with just resistors and not with a voltage/current reference circuit.
And also be sure to use capacitors close to ic’s for filtering high frequency noise (standard practice)

edit: just measured the noise on my diy busboard with my hp54601a digital 100MhZ oscilloscope and it measures ~10mV RMS on the 12 volts both pos and neg.

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halfshavedyaks



Joined: Jul 12, 2018
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PostPosted: Sat Jan 19, 2019 12:22 pm    Post subject: Reply with quote  Mark this post and the followings unread

Hi Grumble, yes I'd seen that one from one of your other posts - looks better than average for sure - but I'm looking for 2A per rail or more.

I can't find a bigger version, which seems odd.

I'm torn between trying a 12v supply or a 15 with regulation.

this looks like good value for 2x15V at 4A

https://www.aliexpress.com/item/15V-15V-120w-dual-output-switching-model-power-supply-AC-TO-DC-4A-4A-type-can/32836083662.html

but the quality and efficiency are unknown. I use efficiency as a rough measure of quality for switching supplies since better made supplies tend to be more efficient.
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halfshavedyaks



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PostPosted: Sat Jan 19, 2019 12:23 pm    Post subject: Reply with quote  Mark this post and the followings unread

12V version

https://www.aliexpress.com/item/D-120F12-12V-12V-volt-120w-5A-5A-dual-output-switching-power-supply-can-be-customized/32834080782.html
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halfshavedyaks



Joined: Jul 12, 2018
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PostPosted: Sat Jan 19, 2019 12:57 pm    Post subject: Reply with quote  Mark this post and the followings unread

A potential advantage of using two single rail supplies as described at the beginning of this thread is that then each rail is individually voltage adjustable. The dual rail supplies only seem to have an adjuster for the positive rail!

The adjustability is typically +/- 10% or more so if you were to need an extra volt for regulator headroom (ie 16v instead of 15) it should be no problem.
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piedwagtail



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PostPosted: Sat Jan 19, 2019 2:50 pm    Post subject: Reply with quote  Mark this post and the followings unread

More inclined to covered terminals

https://www.aliexpress.com/item/S-75-15-Factory-outlet-75W-single-output-ac-dc-15V-5A-switching-power-supply/32327407523.html

x2 + Regs.

there's £6 versions but wouldn't trust.
Here's the icing for the cake
https://www.aliexpress.com/item/Mini-Digital-Voltmeter-Voltage-Tester-Meter-0-28-Inch-2-5V-30V-LED-Screen-Electronic-Parts/32854836483.html
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halfshavedyaks



Joined: Jul 12, 2018
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PostPosted: Sat Jan 19, 2019 3:23 pm    Post subject: Reply with quote  Mark this post and the followings unread

while I am also suspicious of the very cheap supplies how do we actually know they are any different? The $15 units could well just be $6 ones with more markup, and doubtless some are.

is there a clue in the specs or anything? I look for efficiency as a key spec for to guide me for that but many sellers don't give it, or else give a spec that isn't for the actual variant I want to buy.

I am often surprised at how price isn't always a good guide to quality at this low end of the market of mass produced goods.
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Grumble



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PostPosted: Sat Jan 19, 2019 4:15 pm    Post subject: Reply with quote  Mark this post and the followings unread

piedwagtail wrote:

Here's the icing for the cake
https://www.aliexpress.com/item/Mini-Digital-Voltmeter-Voltage-Tester-Meter-0-28-Inch-2-5V-30V-LED-Screen-Electronic-Parts/32854836483.html

Cool


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halfshavedyaks



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PostPosted: Sat Jan 19, 2019 4:21 pm    Post subject: Reply with quote  Mark this post and the followings unread

I think I'd find an ammeter more useful. after all the voltage is constant unless there is something major wrong.

https://www.aliexpress.com/item/Mini-Digital-Voltmeter-Ammeter-DC-100V-10A-Voltmeter-Current-Meter-Tester-Blue-Red-Dual-LED-Display/32947073204.html
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PHOBoS



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PostPosted: Sat Jan 19, 2019 6:26 pm    Post subject: Reply with quote  Mark this post and the followings unread

Being able to see the current draw or in this case even better both voltage and current is great. But I wonder if it could affect the
voltage since you need to place it in series* with the power supply. Just voltage is useful too though, if it starts to drop you know
the current requirements are higher than what the supply can handle. It might not drop a lot but it is definitely measurable.

* unless it uses induction to measure the current.

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halfshavedyaks



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PostPosted: Sun Jan 20, 2019 3:40 am    Post subject: Reply with quote  Mark this post and the followings unread

PHOBoS wrote:
Being able to see the current draw or in this case even better both voltage and current is great. But I wonder if it could affect the
voltage since you need to place it in series* with the power supply. Just voltage is useful too though, if it starts to drop you know
the current requirements are higher than what the supply can handle. It might not drop a lot but it is definitely measurable.

* unless it uses induction to measure the current.


I think those cheap voltmeters are not very accurate, but I guess it will still show a drop even if the absolute values are not accurate. (the precision is better than the accuracy).

I saw a spec for the ammeters resistance somewhere once IIRC it was less than 100milliohms.

If the ammeter volatge drop is a problem it could be placed before the regulator - still useful even though it isn't the actual draw from modules.
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halfshavedyaks



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PostPosted: Sun Jan 20, 2019 4:06 am    Post subject: Reply with quote  Mark this post and the followings unread

by the time I buy 2x 15v power supplies at about GBP15 each I could have bought a 120va transformer. like this one https://cpc.farnell.com/multicomp/mcfe120-12/transformer-encaps-120va-2x12/dp/FF01615?st=transformer%2012%20v

So using switch mode is only cheaper if you use a cheap one, and/or if you don't add regulators.

However, maybe there are some benefits to SMPS over a transformer:

* less sag under load than a transformer - so less voltage headroom needed for the regulator.

* don't need a rectifier so total voltage drop is less.

* can adjust the output to get the minimum voltage drop across the regulator that is enough for it to work well - so less heat dissipation in the regulator.

* more efficient (though i don't know what the efficiency of a linear supply actually would be)

If you can get a way with using cheap SMPS and no additional regulation as Grumble is doing - then it is marvelously cheap. In that case I'd be tempted to add a beefy but very low resistance inductor and some big caps to clean up the signal without dropping the voltage.
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piedwagtail



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PostPosted: Sun Jan 20, 2019 5:03 am    Post subject: Reply with quote  Mark this post and the followings unread

I use those voltmeters on each rail; on lights with an additional safety feature.
If there's a fault, accuracy is not the issue, immediacy is.

For Grumble's application I like these
https://cpc.farnell.com/webapp/wcs/stores/servlet/ProductDisplay?urlRequestType=Base&catalogId=10001&langId=69&storeId=10180&partNumber=PM11273
I found a 270k from input to positive and negative to ground, gave a nice +/-15V swing.

R
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PHOBoS



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PostPosted: Sun Jan 20, 2019 7:16 am    Post subject: Reply with quote  Mark this post and the followings unread

halfshavedyaks wrote:
I saw a spec for the ammeters resistance somewhere once IIRC it was less than 100milliohms.

I was more thinking about possible high frequency noise as they are digital or maybe problems if something goes wrong with them.
Not really something to worry about it though. I actually looked at those meters when I was making my own supply but they were
too large to fit on the panel so I settled for some small digital volt meters.

piedwagtail wrote:
I use those voltmeters on each rail; on lights with an additional safety feature.
If there's a fault, accuracy is not the issue, immediacy is.

I agree that accuracy isn't that important it just has to indicate if there is a drop in the supply voltage.

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