Rail-splitting a wall wart

Absolutely, if we let the noisy wires inside the machined box with no filtering.

Cheers

Phil Hobbs

Reply to
Phil Hobbs
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Reply to
John Larkin

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** Using a AC ( transformer) wall wart has a few notable advantages it you are going to use linear regs.

One with a single 12 volt secondary will supply +/- 16VDC or so using a ( 2 cap, 2 diode) voltage doubler. Highly reliable, no SMPS noise, very safe mains isolation.

.... Phil

Reply to
Phil Allison

And a user who owns both an AC wall wart and a DC wall wart is bound to stick the AC one into your DC-input device eventually, anyway.

Reply to
bitrex

At least in the old days it was common to use an AC wall wart (about

20 Vac) and in the load device use two half wave rectifiers to make +15 Vdc and -15 Vdc for op-amps.

What is wrong with this approach ? Is there also a requirement of running the device from a 24 V battery ?

Using an AC wall wart 50/60 Hz iron core transformer with special connector will discourage many experimenters and their devices will not work with AC, possibly releasing some smoke :-)

Reply to
upsidedown

An AC connector can be unpolarized, but no DC connector commonly is. So, you can fit an AC wart with reversible plug, and no DC wallwart is likely to be compatible. I've got a subwoofer that takes AC/DC input, it has a DC-like connector and full wave bridge... can't get the polarity wrong either.

Reply to
whit3rd

Even when the voltage and power matches, it can be difficult to find a compatible connector. This thirty-four assortment of different sizes and shapes to connect a nominal 19.5VDC adapter to a laptop fails to cover all possible variances:

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Danke,

Reply to
Don

That works great in two quadrants, and has the nice feature of dumping most of the waste heat into the zeners, which are cheap and can work at high temperature. From a belt-and-suspenders POV, it's comforting if it works in all four quadrants. (These boxes don't have motors in them, but it's been known to happen even so.) An extra couple of zeners would fix that up reasonably OK.

The op amp gizmo doesn't protect against both outputs being pulled outwards simultaneously, but is good in four quadrants otherwise, and a single unipolar TVS diode on the power input fixes that. (That would be a very uncommon occurrence anyway.)

The main high-dissipation scenario for our stuff is if somebody shorts the output when it's trying to produce a large signal. We generally solve that by using 22- or 33-ohm/10 uf RCs in the power leads of the output amp and a 2520-size 50-ohm series termination on the output.

With a properly terminated load, 10V output costs 100 mA, which drops the positive supply by a couple of volts. We specify +- 10V outputs, and generally use +-14Vish supplies, so nothing weird happens at that level. The maximum dissipation in the output amp occurs at half-supply, where it dissipates about half a watt--not awful for a beefy part such as a THS3091. (We used to use more of those, but they're getting expensive these days--AD8045s and THS4631s are good. Even LM6171s are pretty good, if noisy, and they have thermal protection too.)

Cheers

Phil Hobbs

Reply to
Phil Hobbs

Ac warts aren't common any more. We do try to design units that will work, or at least not blow up, when someone grabs the wrong cord off the tangle on the bench and plugs it into our box.

+24DC is our default input. A box might not work at 12, but at least won't blow up. Warts above 24 are uncommon.

I guess eventually everything will be USB-C with smart voltage selection. But still positive only.

I guess a USB-C power source is not generally isolated. Maybe never?

Reply to
John Larkin

mandag den 6. februar 2023 kl. 05.43.40 UTC+1 skrev John Larkin:

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Reply to
Lasse Langwadt Christensen

Similar idea, namely that a splitter before the regulators can be sloppy, with a reasonable deadband.

Reply to
John Larkin

I thought of that one too. It's reasonable in some cases.

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but the zeners have other virtues.

Why don't you design a rail splitter and post it for discussion?

Reply to
John Larkin

...

The 'common' AC wart at 9VAC was ideal for an external modem (remember those?) in the '90s, but today it's 19.5VDC for a laptop. If you want to futurize, that might be the pattern to work to.

One (ASUS, if it matters) adapter here is 19V 2.37A from 100-240VAC input, in a 2.0 x 2.0 x 1.125 inch package, and next year's model will be an improvement on that, though I can't imagine how.

Maybe a 50 kHz 20 VAC sinewave? Nah, I'd generate harmonics with any attempt to rectify. Better to keep those in the distal end of the power lead.

Reply to
whit3rd

Not any more. I have a few 48V and 54V power bricks used for PoE and PoE++ respectively.

The trouble with that is that the standard requires the negotiated output to be less than or equal to the requested output, so you need to check whether you actually got what you asked for.

Its isolated if it is a wall-wart, but not if its part of a PC. John

Reply to
John Walliker

A vivisectioned, so to speak, Sijosae rail splitter?

Danke,

Reply to
Don

LT1166 for the cash-flush, LM386 for the cash-strapped.

Reply to
bitrex

I have one of these older Radio Shack aftermarket laptop power bricks, the only laptop I've ever owned in recent history that any of the included connectors fit is a 2016 Fujitsu Lifebook.

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Reply to
bitrex

mandag den 6. februar 2023 kl. 16.27.11 UTC+1 skrev John Larkin:

48V is beginning to show up for POE
Reply to
Lasse Langwadt Christensen

In the US, the limit will be the NEC (National Electrical Code), which defines the max voltage for a low-voltage system that does not need to all the stuff needed to handle mains power (125, 240, 300 Volts).

There will be a European equivalent to the NEC, likely coordinated as much of this stuff is used worldwide.

Joe Gwinn

Reply to
Joe Gwinn

That's a flawed certainty. Here is a counterexample; the center is negative, and it isn't just the label, my multimeter says so too.

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What's really spooky, is that I recall an identical manufacturer and part number unit with center positive (also correctly labelled).

Reply to
whit3rd

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