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Old 28th Oct 2020, 4:30 pm   #1
Diabolical Artificer
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Default SMPSU + Linear regulator combo?

Could I use a linear regulator like a LM317 to drop the voltage off a SMPSU? Good idea or a nightmare on a stick? I'm thinking something like a 12v SMPSU into a big cap, then into a LM317.

Why? Saves space and a tfmr.

Andy.
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Old 28th Oct 2020, 4:37 pm   #2
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Default Re: SMPSU + Linear regulator combo?

Should work, don’t forget two 100 nF caps to input and output and maybe two larger caps as well, 10 uF or so to prevent oscillation.
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Old 28th Oct 2020, 4:37 pm   #3
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Default Re: SMPSU + Linear regulator combo?

Why wouldn't it work? DC is DC, wherever it comes from.

Use a protection diode too if you're being careful, so that there's no opportunity for a cap to discharge 'backwards' into the regulator.
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Old 28th Oct 2020, 4:47 pm   #4
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Default Re: SMPSU + Linear regulator combo?

Hi Andy whats the big cap for?

SMPS usually have good regulation an respond pretty well to step changes in current.

Cheers

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Old 28th Oct 2020, 4:59 pm   #5
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Default Re: SMPSU + Linear regulator combo?

This was (maybe still is) a common technique when switched mode psu were less developed and/or there was a large I/O difference. The smps would do the “rough” work and provide just enough voltage for the linear to give good performance without too much heat.
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Old 28th Oct 2020, 5:01 pm   #6
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Default Re: SMPSU + Linear regulator combo?

Quote:
Originally Posted by Diabolical Artificer View Post
Could I use a linear regulator like a LM317 to drop the voltage off a SMPSU?
This is exactly how most modern appliances are built. Central SMPS followed by local linear regulators for microprocessors and whatnot.
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Old 28th Oct 2020, 5:04 pm   #7
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Default Re: SMPSU + Linear regulator combo?

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Originally Posted by paulsherwin View Post
Why wouldn't it work? DC is DC, wherever it comes from.

Use a protection diode too if you're being careful, so that there's no opportunity for a cap to discharge 'backwards' into the regulator.
Good point, thanks Paul.
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Old 28th Oct 2020, 5:14 pm   #8
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Default Re: SMPSU + Linear regulator combo?

Quote:
This is exactly how most modern appliances are built.
SMPS non isolated chips are used for this as well, saves power consumption, board space and the size of the initial isolated mains to DC SMPS. Some of the "just for the job" ones need only one more component then a linear reg, an inductor.
 
Old 28th Oct 2020, 5:24 pm   #9
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Default Re: SMPSU + Linear regulator combo?

some industrial smpsu's with multiple output voltages use IC regulators like this, built in already from the main output.
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Old 28th Oct 2020, 8:02 pm   #10
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Default Re: SMPSU + Linear regulator combo?

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Originally Posted by Ambientnoise View Post
... a common technique when switched mode psu were less developed ...
I remember it well. Back in the 1990's, an industry-standard and extremely well regarded audio mixing console had (outboard, rack-mounted) power supplies using that technology.

Wanting to get away from their traditional (and very heavy) toroidal transformer supplies, the manufacturer had decided to use bought-in SMPS modules followed by their own linear regulators. I think they got through three manufacturers before they found one that was reliable enough to avoid wrecking their reputation for a world-class product.

Because a failed mixer PSU would be a show-stopper, it was standard practice to run with two supplies, one being redundant, with diode combiners. But in this case we always made sure there were three... plus another spare stashed away in the flightcase of contingency (GOOS) kit.

Happily, SMPSU's are now a mature technology and normally reliable.
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Old 29th Oct 2020, 7:14 am   #11
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Default Re: SMPSU + Linear regulator combo?

Thanks chaps, I was worrying for nothing. I was wondering if the "noise" off the SMPSU might interfere with the 317 in some way, so thought a big cap might be needed before the 317, then wondered if SMPSU's might find a capacitive load hard to drive. I have seen ATX supplies with 78xx after regs, hence the idea.

I'll, of coarse be sure to include diodes, caps etc as per datasheet.

Andy.
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Old 29th Oct 2020, 4:17 pm   #12
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Default Re: SMPSU + Linear regulator combo?

I tried using a SMPS wallwart to power a small ClassD amp module and an associated chip based tone and preamp module.
It was very disappointing in that there was a lot of noise and hum introduced into the circuit. I tried a 317 based reg board on it in case it wasnt properly regulated and it made no difference. Some SMPS just arent up to the job if you need them for low noise audio or other critical applications. I dont really understand how they work but i was told it had something to do with noise injected on the earth/0V rail. I may have completely misunderstood that part, I find SMPS to be black magic and unfathomable and they have no place in any of my analogue amateur Audio or radio builds.

I imagine theres some professional members here who know the ins and outs and ways to filter the noise out. It must be possible as my Squeezebox touch relies on a SMPS.

Like you I was thinking how nice to have a use for a bunch of spare SMPS wallwarts. I seem to have amassed a set of BT modem and router wifi supplies, all 1.5A 12V jobs.
Hope all well with you Andy, I'm distracting myself trying to learn how to use Kicad design software and catching up on the roundtuit list.

T'other Andy.
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Old 29th Oct 2020, 4:55 pm   #13
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Default Re: SMPSU + Linear regulator combo?

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Originally Posted by bikerhifinut View Post
I tried using a SMPS
It was very disappointing in that there was a lot of noise and hum introduced into the circuit.
I dont really understand how they work but i was told it had something to do with noise injected on the earth/0V rail.
I know exactly what you mean, and I have never been satisfied with that explanation. 'Injecting noise into ground' doesn't mean anything. To have noise, a voltage must be developed between two points, and the question must be answered, how does that voltage develop? Just 'injecting something into ground' is not an explanation. The problem is particularly mysterious because it can still happen even when the SMPS is isolated from mains earth, and the audio system is also not earthed, so there are no ground loops. In theory there would be no path for noisy switching currents to exit the PSU, enter the audio system, and return to the PSU. And yet the noise remains, far louder than obvious things like stray capacitance or simple PSU ripple should allow. I have yet to see the phenomenon explained.

Last edited by daviddeakin; 29th Oct 2020 at 5:01 pm.
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Old 29th Oct 2020, 5:28 pm   #14
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Default Re: SMPSU + Linear regulator combo?

Yes - it'll work - and is standard practice in many fields.

It's quite common to have a big SMPS that feeds its DC outputs to individual linear-regulators mounted on each of the various circuit-boards in the equipment.

This has several advantages:

1] The SMPS regulation doesn't need to be that good because the individual linear regulators on the boards will clean things up.

2] Less likelihood that noise from one board will feed-back down the power-supply rails and perturb the operation of other boards.

3] Heat dissipation is distributed more-evenly throughout the equipmene, making cooling simpler and reducing the number of 'hot spots'.

4] Can make "soft-start" arrangements easier.

I'm quite happy using a SMPS essentially as an 'electronic transformer' then using 317s or similar for higher-precision on-board-regulation.
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Old 29th Oct 2020, 6:57 pm   #15
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Default Re: SMPSU + Linear regulator combo?

Quote:
'Injecting noise into ground' doesn't mean anything.
That's because you are thinking in very LF terms.

The switching transients in these PSUs put them into the RF band. The typical source of emc are loops in the powerful part of the circuit. So what you get is the earh wire, or whatever, being driven like an end-fed aerial.

If you are looKing for your "loop" you have to think beyond wire. RF things are effectively completing their circuit by capacitance - no wire required!
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Old 30th Oct 2020, 9:12 am   #16
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Default Re: SMPSU + Linear regulator combo?

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That's because you are thinking in very LF terms.
If you are looKing for your "loop" you have to think beyond wire. RF things are effectively completing their circuit by capacitance - no wire required!
Hmm, then on a related subject, can you explain this phenomenon? A bluetooth receivier and an amplifier, sharing the same power ground but with audio isolation transformers (so no hard ground loop), STILL buzzes!

https://www.youtube.com/watch?v=n_b_0BWP0mI&t=371s
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Old 30th Oct 2020, 9:29 am   #17
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Default Re: SMPSU + Linear regulator combo?

Quote:
Originally Posted by daviddeakin View Post
Quote:
Originally Posted by GMB View Post
That's because you are thinking in very LF terms.
If you are looKing for your "loop" you have to think beyond wire. RF things are effectively completing their circuit by capacitance - no wire required!
Hmm, then on a related subject, can you explain this phenomenon? A bluetooth receivier and an amplifier, sharing the same power ground but with audio isolation transformers (so no hard ground loop), STILL buzzes!

https://www.youtube.com/watch?v=n_b_0BWP0mI&t=371s
Without knowing the details at all, I would say that LF systems can very much develop similar problems to RF ones - hum loops are one instance of them.

Essentially the problem here is thinking that something called "ground" actually exists - in the sense that it has zero potential across it. It can be hard to appreciate that a solid lump of metal, for instance an aluminium chassis can be at different potentials on opposite sides, something that most people's understanding of "ground" precludes. But with enough circulating current (for whatever reason) at any frequency some potential difference will exist, even if its only a few microvolts. And in some systems - particularly those demanding very high S/N ratios, that potential can lead to the sort of problem you are referring to.


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Old 30th Oct 2020, 9:38 am   #18
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Default Re: SMPSU + Linear regulator combo?

Quote:
Essentially the problem here is thinking that something called "ground" actually exists
YES!! People have an almost religious belief in "ground" - especially amateur radio folks.

Once you come to accept that "ground" doesn't actually exist, your designs usually improve considerably.

Quote:
audio isolation transformers
... or capacitors as they are also known to emc engineers.
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Old 30th Oct 2020, 10:46 am   #19
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Default Re: SMPSU + Linear regulator combo?

I routinely work with this configuration, and also with SMPSUs and DC-DC converters in analogue signal applications. As mentioned it is a popular technique but it is not a panacea for all SMPSU hash.

An important consideration especially where the load is floating, is that no amount of linear post-regulation will remove common-mode noise originating from the SMPSU, which is a frequent source of buzz and other maladies. The entire load circuit, with its nice smooth DC, can end up flapping about at hundreds of kHz referenced to something like half mains voltage wrt. true earth, complete with some AM at 100Hz from the switcher duty cycle. If the application circuit were truly symmetrical including all strays, the common-mode noise would not affect its operation; but as soon as there is a ground plane or other major assymmetry (even a linear post-regulator!) it resurfaces as single-ended noise within the circuit. Not unlike modulation hum in a radio that has perfectly smooth HT, due to RF side-effects at the source of that smooth HT.
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Old 30th Oct 2020, 4:30 pm   #20
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Default Re: SMPSU + Linear regulator combo?

In the end I knocked the idea on the head, but will file the idea away. A lot of folk over on DIYaudio use SMPSU's in valve amps with success, both for HT and heater supplies, but from reading the above it seems the idea is full of potential pitfalls. I suppose that it's further complicated by all the various SMPSU types and configurations. We're entering the world of voodoo and RF which to some of us is the same thing.

Thanks again for all your IP., Andy.
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