I just found something funny. I have been thinking "clean" power supplies that
connects to AC mains. Then I thought, what about lead acid batteries?? So I
went to my lab and took some measurement. This is a 12V 7A lead acid sealed
battery, the kind commonly found on UPS devices.
The result? Surprise? The battery is oscillating at 5MHz and noise level is
15mV peak-to-peak!!!!
Of course, not...! Battery is pure DC and while voltage might drift, this is
not that. For the record, a charger of any kind is not hooked up. It's one
battery all by itself. Battery is not oscillating but that's what the
measurement actually shows. That brings another point in my quest to "clean"
power source. It's not just the power supply but the whole lab eco system has
to be considered. Having one master 24V source (my original plan) is not the
answer if mV level noise is going to be a problem.
This "discovery" puts whole new layer to having a nice power supply.....
Just as a point of reference, I hooked up a common cheap float charger. The
charger itself has 2V p-p noise. Connected to battery, it still have 100mV p-p
noise. There goes battery = noise sponge theory....
---------------------------------------
(Mr.) Taka Kamiya
KB4EMF / ex JF2DKG
On Tuesday, December 24, 2019, 3:00:50 PM EST, Bob kb8tq <kb...@n1k.org>
wrote:
Hi
> On Dec 24, 2019, at 6:40 AM, Poul-Henning Kamp <p...@phk.freebsd.dk> wrote:
>
> --------
>
>> That again depends on topology and control type. The canned converters
>> are almost always optimized to have the lowest number of switches and
>> work with cheap magnetics (single coil) without easily entering
>> problematic operation modes, noise is only a secondary concern.
>
> That depends a LOT on which canned converter you decide to buy,
> if you only go after price, or W/mm³ capacity, then certainly yes.
>
> But for a one-off application like this, any money saved on a
> cheap model is easily lost many times over in the trouble it will
> cause.
>
> But returning to the original post: Has anybody ever characterized
> how much difference it makes to use two different PSU's for heater
> vs. electronics sides of telecom Rb's ?
The “old time” answer was that a poorly regulated / poorly filtered supply was
considered “ok” for a heater. For the active electronics you wanted something
nice and stable / clean. To your point, once you get around to *measuring*
this, stability wise that answer often does not hold up. Noise wise, you are
right
back to “what frequency?” ….
The somewhat more complex “old time” answer was that you don’t want the
honking big current of the heater coming off the supply you have tried so hard
to super-regulate. ( = it’s the supply that’s the issue not the Rb it’s self).
Obviously
that’s going to depend on how the supply was designed.
Of course next layer to the onion is …. where does the ground current go? ….
hmm….
Bob
>
> I'm sure there is a reason why they make it two different pins ?
>
> --
> Poul-Henning Kamp | UNIX since Zilog Zeus 3.20
> p...@freebsd.org | TCP/IP since RFC 956
> FreeBSD committer | BSD since 4.3-tahoe
> Never attribute to malice what can adequately be explained by incompetence.
>
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