I'm sure I have some 7805s lying around, maybe a 7812/7912. I'm interested
to see the 1/f noise of a classic regulator, what load current do you
expect? I can bias a 7805 for the same load and measure the 0.1 to 10 Hz
Also if you have a digital scope without a very good builtin FFT, octave
would be one solution.
On Tue, Oct 18, 2016 at 10:46 AM, Nick Sayer via time-nuts <
> Just an update. I’ve built the second prototype board (I skipped over the
> first design), and it’s powering my tbolt right now.
> The design calls for 15v in (though it would also work with 13.8v). The
> +12 output comes from a D2PAK 7812. For +5, there is an AP1509 buck
> converter to make around 6.5 volts, then a DPAK 7805. For -12, there is an
> MC34063 configured as an inverter to make around -13.75 volts and then a
> DPAK 7912.
> Steady-state, the system appears to be working just fine. The AP1509’s
> inductor and the D2PAK 7812 are just warm to the touch.
> I checked for noise and ripple on the outputs and it’s somewhere around ±2
> mV or so generally. From what I can see on the scope, there’s no ripple -
> it’s all high frequency noise. I am not absolutely certain that the noise
> measurement represents real noise or the limits of my measuring ability.
> I’m just using the scope probes the scope came with, and 2 mV/div is its
> lowest range.
> I haven’t compared the noise with the ex laptop supply that I was using
> before, but I’d have to believe it’s cleaner. I don’t really have a way to
> check the oscillator’s before and after ADEV. My only other reference is an
> FE5680A, and I think the thunderbolt’s going to be far better at lower tau
> (where this all matters).
> I know also that ±2 mV is still one and perhaps two orders of magnitude
> higher than some have called for. But before I attempt to reduce the noise
> further, I’d like to know that there are real gains to be had. Would
> someone with a Thunderbolt and better output noise measuring wherewithal be
> willing to take a prototype and compare it with something that does have µV
> levels of noise and ripple so I can get an idea of what there is to gain?
> If you like, you can make such comparisons public - no secrets here.
> > On Aug 30, 2016, at 10:37 PM, Nick Sayer <nsa...@kfu.com> wrote:
> >> On Aug 30, 2016, at 8:48 PM, Cube Central <cubecent...@gmail.com>
> >> I would be interested, I think. Planning ahead for if the one I have
> for my Thunderbolt fails, I guess. Are there different models or would a
> photo of the input ports on mine be useful?
> > Actually, what I had in mind is to just put a SIP4 header on the board
> for the output and people could wire the “last mile” themselves. The input
> is a 2.1mm barrel connector. You use whatever 15W 12VDC wall wart is handy
> and plug it right in.
> > What it really amounts to is that you get +12 volts directly from the
> input, then there’s a buck converter to drop the +12 down to +5 and an
> inverter to generate -12 from the +12. Those 3 voltages, plus a ground go
> to the SIP4.
> > So it’s just two switching power supplies to turn a +12 volt only supply
> into the three-way that the Thunderbolt wants.
> > It’d be good for around 1500 mA @ 5V and around 50 mA @ -12 (the +12
> spec is whatever is left from the source supply’s power spec) - more than
> enough for a Thunderbolt. Probably enough for a hard disk or a smallish PC.
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