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Linear power supplies

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cnt:
My time machine is broken so I can't go back to 1980 and buy a linear wall wart.  Where you you guys source these for voltage reference projects?  Would a 24v switching supply + an lm317 work well (within a factor of 2 as noisy)?  I remember an eevblog video saying linear regulators don't reject noise that well but the reasons are unclear to me.

WattsThat:
A time machine wouldn’t help. Older wall worts had transformers with one or two transistor pass regulators. Nothing special there. Don’t kid yourself into thinking they were low noise.

Perhaps you should tell us what you need in terms of voltage, current and noise, perhaps even the intended application.

Kleinstein:
The main problem with SMPS is the injected common mode signal. So the secondary is not perfectly isolated but there is a capacitive coupling with 2 parts: one is the switching frequency and the 2nd is often half the mains voltage from a capacitor to dampen the higher frequency part.

Linear regulator would not at all help against the common mode part. The differential part can be filtered and an extra regulator can help.

Classic iron transformer based wall warts can sometimes still found with old equipment. With some luck one can sometimes still buy some left overs. Often they are non regulated and relatively low voltage, which can be a slight problem. Here an external regulator can help, usually more a low drop type as the voltage tends to rather low anyway. Old telephone routers a long time still came with classical supplies with a relatively high voltage.

cnt:
For context: Everything is for fun.  I have no NEED for anything I'm doing. 

I'm kinda hunting for best bang/$.  I'd rather spend 15$ on a power supply if the next 100$ is only going to get me -3db noise.

I need something in the 16-24v range, 20mA.  I don't have a concrete spec for noise and the only way I can measure really low noise is by seeing what the last digit on my DMM does.  I haven't tried, but I don't think my rigol DS1054 is that sensitive (am I wrong?).  This is the background for what I'm doing:


That said, when I read about the lm399 I was both impressed that it regulated temperature so well with a basic circuit and offended that it did so poorly in general (I was once a control theorist).  0.5ppm per degC ambient seems pretty bad when you can buy unheated zeners in the 5-10ppm/C range (1N82x). 

So I'm trying to make my own heated 1N825 with quality temperature control and a low frequency noise floor around the last couple of counts on my 6.5 digit keithley at work.

From my home linear bench supply, and done with jelly bean components (and a 1N825) in a breadboard it's currently stable on the last count of my 5.5 digit meter.
 

penfold:

--- Quote from: cnt on December 07, 2021, 10:16:27 pm ---[...] I remember an eevblog video saying linear regulators don't reject noise that well but the reasons are unclear to me.

--- End quote ---

The HF switching ripple is typically way above the control loop bandwidth of most linear regs and therefore the linear reg cannot functionally do anything about reducing it. The parasitic capacitances associated with the pass-transistor also tend to be low impedance at the fundamental (switching) frequency and harmonics thereof which make up the "noise". Which one of the two factors is the biggest problem would depend on the specific device, but each is at play and neither makes it better. I guess both those things came up in the video, may be different wording helps, or not, or makes understanding worse.

Bang-for-buck-wise, I wouldn't recommend the SMPS approach. The reason is that most wall-wart-style ones will probably have quite a high CM noise and be most difficult to tame. An alternative option for SMPS would be to build something up with an open-frame PSU in an enclosure with a little board with filtering, that way you get better control over the CM noise, have an earth connection, and access to the PSU chassis earth if needed - but going to that effort would still get you better more favorable noise from a mains-freq transformer for a similar price.

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