[TangerineSDR] [HamSCI] RE: Notes from PSWS / TangerineSDR call of 07-26-2021

Ward Silver hwardsil at gmail.com
Wed Jul 28 22:08:41 EDT 2021


Recent editions of the Handbook offer several polarity protection circuits
in the Power Sources chapter (Chapter 7).  Some of them use P-channel
MOSFETS that have much lower forward voltage drops than standard diodes,
even Schottky barrier diodes.

Regarding common-mode chokes, I think it is better to determine the need
first - what frequency range and amplitude, for example - and then select a
type of choke to do the job.  For example, blocking RF common-mode at VLF,
HF, and VHF/UHF requires different ferrite mixes and the number of turns
also makes a difference in the choke's peak rejection range. (See Chapter 5
- RF Techniques - of recent Handbook editions.  Much of this was written by
K9YC and comes from his tutorials.)

I realize the original post was about power supply failures and not RFI but
nothing like a good digression :-)  Analog Devices also just published a
couple of articles about protecting inputs for low-power interfaces in the
Analog Dialogue - a little Googling will find the recent issues which are
free and public.  All good stuff.

73, Ward N0AX

On Wed, Jul 28, 2021 at 7:54 PM <tangerinesdr-request at lists.tapr.org> wrote:

>
>   Jonathan,
> You can use a standard silicon diode like the 1N4000 series (1A rating)
> for the polarity protection diode unless you want to clamp the reverse
> voltage at less than 0.7-0.8 volts, in which case a Schottky will
> provide a lower clamping voltage.? Since the reverse protection diode
> will clamp reverse transients, you don't need a bidirectional TVS.
> Jules
>
>
> On 7/28/2021 3:16 PM, Jonathan wrote:
> > Phil,
> >
> > This design (Integrated VLF antenna/preamp in PVC pipe with Raspberry
> > Pi box) was used for an MIT study and the author of vlfrx-tools built
> > a bunch of these antennas and boxes for those conducting the study.
> > This was in 2018. Since this might have fallen under your umbrella,
> > did you hear of it or know who the PI might be? The DC-DC converter
> > has an operating frequency of 100-600 kHz, so it doesn?t really affect
> > anything below that. Paul really liked the performance of his design.
> > He even wrote a guide on how to set up the Radpberry Pi as a VLF SDR
> > using his vlfrx-tools software.
> >
> > Jules,
> >
> > I really appreciate the supplied schematic. Should I use the Schottky
> > diode where you drew the regular diode, in parallel like you said? I
> > could also probably solder the MLCC across the leads of the
> > electrolytic and install it. Also, that TVS is bidirectional. Should a
> > unidirectional TVS be used so it clips any negative spikes closer to 0V?
> >
> > David,
> >
> > Tom mentioned the same. I?ll be working on trying to fit an
> > electrolytic and a ceramic capacitor.
> >
> > Khan,
> >
> > This is good news. Email me if you have issues.
> >
> > Jonathan
> > KC3EEY
> >
> >
> >
> > On Jul 28, 2021, at 2:27 PM, Julius Madey <hillfox at fairpoint.net
> > <mailto:hillfox at fairpoint.net>> wrote:
> >
> >> Jonathan,
> >> I still think you need to get rid of the series polarity protection
> >> diode and suggest the following schematic (with 22uf 50 volt Aluminum
> >> electrolytic)
> >>
> >>
> >> <eaeficdplljflikc.png>
> >>
> >> The PTC poly fuse will open either on overload, sustained voltage
> >> above TVS rating or reverse polarity.? This will also snub transients
> >> on the line the series diode may have permitted.? There might be some
> >> virtue in adding an MLCC cap of ~ 0.47uf in parallel with the
> >> electrolytic for lower overall ESR.
> >>
> >> Regards,
> >> Jules-K2KGJ
> >>
> >>
> >>
> >>
> >>
> >>
> >> On 7/28/2021 12:41 PM, Jonathan wrote:
> >>> Hi Tom,
> >>>
> >>> That makes a lot of sense. I?ll try fit a 0.1uF and a 22-47uF
> >>> electrolytic on the feedline loop.
> >>>
> >>> Thanks again!
> >>>
> >>> Jonathan
> >>> KC3EEY
> >>>
> >>> On Jul 28, 2021, at 10:55 AM, Tom McDermott <tom.n5eg at gmail.com
> >>> <mailto:tom.n5eg at gmail.com>> wrote:
> >>>
> >>>> Hi Jonathan - a concern is that the DC-DC converter input stage
> >>>> would become unstable and oscillate
> >>>> with the cable inductance.? If that happens then there could be
> >>>> high frequency AC voltages present at the
> >>>> input of the converter.? Would those over-voltage the input??
> >>>> Possibly.?? Would it radiate RFI from the cable?? Likely.
> >>>>
> >>>> Good design dictates sufficiently low source impedance to the DC-DC
> >>>> converter.? Almost all linear regulators
> >>>> require it.? An electrolytic and ceramic bypass pair right at the
> >>>> DC-DC input would be considered good design by many.
> >>>>
> >>>> -- Tom, N5EG
> >>>>
> >>>>
> >>>>
> >>>>
> >>>>
> >>>>
> >>>> On Wed, Jul 28, 2021 at 7:30 AM Jonathan <emuman100 at gmail.com
> >>>> <mailto:emuman100 at gmail.com>> wrote:
> >>>>
> >>>>     John,
> >>>>
> >>>>     Thanks for the suggestion! I think I?ll start with the
> >>>>     overvoltage age spike of 50V for 100ms or less based on the
> >>>>     datasheet spec of the DC-DC converter. I was thinking of some
> >>>>     sort of fusable circuit protection now that I?ll be using a TVS
> >>>>     diode to mitigate stress from the follow current.
> >>>>
> >>>>
> >>>>     Tom,
> >>>>
> >>>>     The datasheet wasn?t clear on the need for input filtering. On
> >>>>     the output of both the IP2415S, I have 0.1uF ceramic
> >>>>     capacitors. In the datasheet, it only mentions a spec of ?Input
> >>>>     Reflected Rated Current? with a series inductor and shunt
> >>>>     capacitor. I don?t really understand exactly what this is, but
> >>>>     it lists ?20mAp-p through a 12uH inductor and 47uF capacitor?.
> >>>>     Nothing else is mentioned about input protection or filtering.
> >>>>
> >>>>     In both situations that the DC-DC failed, the cable length was
> >>>>     only 4? of cat 5. This circuit that was proved and tested by
> >>>>     Paul, including the receiver and schematic I attached, did not
> >>>>     have such an issue, but the only difference is that the voltage
> >>>>     was lower than 30V. That?s why I was thinking it was a
> >>>>     turn-on/off spike. I think that in a lot of typical
> >>>>     applications, the input voltage isn?t that high, so spikes
> >>>>     don?t normally present a problem. The DC-DC that feeds the
> >>>>     receiver DC-DC has never failed and it was always fed with 18V
> >>>>     or less.
> >>>>
> >>>>
> >>>>     Jules,
> >>>>
> >>>>     You recommend the Schottky diode in parallel with the TVS, then
> >>>>     a series PTC, so during a spike, the TVS will conduct and the
> >>>>     PTC would fuse, and during a reverse polarity condition, the
> >>>>     Schottky diode will conduct with limited current due to the PTC
> >>>>     fusing? I can see the benefit that the series PTC in normal a
> >>>>     normal circuit condition would drop much less voltage that the
> >>>>     Schottky diode.
> >>>>
> >>>>
> >>>>     All in all, I agree with adding some capacitance to filter
> >>>>     voltage spikes and reduce or eliminate them. I?ll need to be
> >>>>     sure not to exceed the DC-DC max load capacitance, which is
> >>>>     47uF in this case.
> >>>>
> >>>>     But, to me, the most plausible cause for the failures are turn
> >>>>     on/off voltage spikes, even with a short length of cable.
> >>>>     Adding electrolytics is a bit difficult due to the tight space
> >>>>     constraint as seen in my previous email, so I?ll try out the
> >>>>     TVS and a PTC. Other than voltage spikes, does anything else
> >>>>     come to mind?
> >>>>
> >>>>
> >>>>     Thanks guys.
> >>>>
> >>>>     Jonathan
> >>>>
> >>>>     KC3EEY
> >>>>
> >>>>
> >>>>     On Tue, Jul 27, 2021 at 9:34 AM Jonathan <emuman100 at gmail.com
> >>>>     <mailto:emuman100 at gmail.com>> wrote:
> >>>>
> >>>>         Hi Everyone,
> >>>>
> >>>>         Before the meeting ended last night, I wanted to discuss
> >>>>         the issues I?ve experienced with my VLF active antenna. As
> >>>>         I mentioned, the DC-DC converter (XP Power IP2415S) failed
> >>>>         after I applied power. I turned the volume up because I
> >>>>         wasn?t hearing sferics, and shortly after, I heard the
> >>>>         squeal of the DC-DC converter. When I pulled out the
> >>>>         smaller foam core partially, I heard a squeal coming from
> >>>>         the DC-DC converter itself.
> >>>>         <image3.jpeg>
> >>>>
> >>>>         I had a failure of the DC-DC converter when I first built
> >>>>         the VLF preamp. It was being powered by the Raspberry Pi
> >>>>         box (same power circuit as my text box). I had a power
> >>>>         failure and when the power was restored, the DC-DC
> >>>>         converter failed.? In both situations, it was being
> >>>>         supplied with 30VDC, with the limit is 36V. The reason for
> >>>>         using 30V is to overcome the voltage drop from long cable
> >>>>         lengths.
> >>>>
> >>>>         After both failures, there was a low impedance across the
> >>>>         input of the DC-DC converter causing the supply to be
> >>>>         pulled down. Because the input and output are isolated, I
> >>>>         figured the fault had to have been caused by a voltage
> >>>>         spike on the input due to power being turned on and off.
> >>>>         The datasheet mentions that it can withstand 50V spikes at
> >>>>         100ms, but I can?t think of anything else that could cause
> >>>>         the DC-DC to fail, especially that 30V is used to power it
> >>>>         and any voltage spike would ride on top of that 30V. I
> >>>>         contacted XP Power about the issue.
> >>>>
> >>>>         Here is the schematic of the power path. The blocks are the
> >>>>         IP2415S DC-DC converter.
> >>>>         <image1.jpeg>
> >>>>         I?m powering the DC-DC converter with 30V from another
> >>>>         DC-DC converter in the test box and Raspberry Pi box
> >>>>         (Raspberry Pi box not shown, but same circuit). Keep in
> >>>>         mind, the length is still relatively short in my testing,
> >>>>         so cable inductance isn?t the primary reason for the spike.
> >>>>         Plus, at long lengths, the voltage would be much lower due
> >>>>         to voltage drop across the length, and I think this was why
> >>>>         Paul might not have seen this issue when he originally
> >>>>         tried and built this design.
> >>>>
> >>>>         To mitigate this problem with the supply voltage at 30V, I
> >>>>         decided to use a TVS diode across the input of the DC-DC
> >>>>         converter. Here is the schematic: (my apologies for the TVS
> >>>>         diode being backwards)
> >>>>         <image2.jpeg>
> >>>>         I chose a unidirectional TVS so there is also protection if
> >>>>         the voltage spikes have any negative components. I picked a
> >>>>         TVS with a working voltage of 33V, a Littel Fuse P4KE39A.
> >>>>         According to the datasheet, it looks like this would be
> >>>>         adequate. TVS diodes have a PN junction that is more
> >>>>         rugged, with a larger surface area for greater current
> >>>>         density. One thing I overlooked with this type of circuit
> >>>>         protection is that once the TVS conducts, current from the
> >>>>         power supply will be shunted too, known as the follow
> >>>>         current, and in some situations, this may cause issues
> >>>>         requiring a fuse. In this case, since the spikes are
> >>>>         probably short duration, and since the DC-DC converter in
> >>>>         the text/Pi box has short circuit protection, this should
> >>>>         not be an issue.
> >>>>
> >>>>         So the questions I have are: do you also think it could be
> >>>>         voltage spikes that caused the failures, and if so, was my
> >>>>         choice of TVS diode adequate. Also, one important question,
> >>>>         if I connect the anode of the TVS diode to the receiver
> >>>>         side ground, will I lose isolation? I know that during
> >>>>         spike current conduction, I will. I?m sure I can still get
> >>>>         adequate protection with the TVS diode anode connected to
> >>>>         the negative of the isolated power loop too, but I just
> >>>>         wanted to be sure.
> >>>>
> >>>>         Lastly, since the magnetometer board will undergo another
> >>>>         revision, I feel it may be worth it to add some sort or
> >>>>         adjustable regulator or use of an external power source
> >>>>         (not connected to the 5V pin of the Pi header) because even
> >>>>         with paralleling wires or pairs, some installations will
> >>>>         require very long lengths (especially since
> >>>>         interference-free locations are often far away from the
> >>>>         shack) and the voltage may drop to where the LDO cant
> >>>>         regulate. Using a higher voltage at the Pi end (like I do
> >>>>         with the VLF preamp) may be required for some
> >>>>         installations. I think this should be considered if it
> >>>>         already wasn?t addressed.
> >>>>
> >>>>         Jonathan
> >>>>         KC3EEY
> >>>>
> >>>>
> >>>>
> >>>>         On Jul 27, 2021, at 7:21 AM, Dr. Nathaniel A. Frissell
> >>>>         Ph.D. <nathaniel.frissell at scranton.edu
> >>>>         <mailto:nathaniel.frissell at scranton.edu>> wrote:
> >>>>
> >>>>>
> >>>>>
> >>>>>
> >>>>>
> >>>>>
> >>>>>
> >>>>>
> >>>>>
> >>>>>
> >>>>>
> >>>>>
> >>>>>
> >>>>>
> >>>>>
> >>>>>
> >>>>>
> >>>>>         Thank you, Tom.
> >>>>>
> >>>>>
> >>>>>
> >>>>>
> >>>>>
> >>>>>         The recording will be available later today at
> >>>>>
> >>>>>         https://youtu.be/AammohuygMw
> >>>>>         <https://youtu.be/AammohuygMw> and hamsci.org/telecons
> >>>>>         <http://hamsci.org/telecons>.
> >>>>>
> >>>>>
> >>>>>
> >>>>>
> >>>>>
> >>>>>         73 de Nathaniel W2NAF
> >>>>>
> >>>>>
> >>>>>
> >>>>>
> >>>>>
> >>>>>
> >>>>>
> >>>>>         *From:* TangerineSDR <tangerinesdr-bounces at lists.tapr.org
> >>>>>         <mailto:tangerinesdr-bounces at lists.tapr.org>>
> >>>>>
> >>>>>         *On Behalf Of *Tom McDermott via TangerineSDR
> >>>>>
> >>>>>
> >>>>>         *Sent:* Monday, July 26, 2021 10:02 PM
> >>>>>
> >>>>>
> >>>>>         *To:* TAPR TangerineSDR Modular Software Defined Radio
> >>>>>         <tangerinesdr at lists.tapr.org
> >>>>>         <mailto:tangerinesdr at lists.tapr.org>>
> >>>>>
> >>>>>
> >>>>>         *Cc:* Tom McDermott <tom.n5eg at gmail.com
> >>>>>         <mailto:tom.n5eg at gmail.com>>
> >>>>>
> >>>>>
> >>>>>         *Subject:* [TangerineSDR] Notes from PSWS / TangerineSDR
> >>>>>         call of 07-26-2021
> >>>>>
> >>>>>
> >>>>>
> >>>>>
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> >>>>>
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> >>>>>
> >>>>>
> >>>>>
> >>>>>         Notes from PSWS / TangerineSDR call of 07-26-2021
> >>>>>
> >>>>>
> >>>>>
> >>>>>
> >>>>>
> >>>>>
> >>>>>
> >>>>>
> >>>>>
> >>>>>
> >>>>>
> >>>>>
> >>>>>
> >>>>>         1. Bill is using chart.js for magnetometer charting. He is
> >>>>>         setting up a database using Django web and database
> >>>>>         framework for Python.
> >>>>>
> >>>>>
> >>>>>
> >>>>>
> >>>>>
> >>>>>
> >>>>>
> >>>>>
> >>>>>
> >>>>>
> >>>>>
> >>>>>
> >>>>>
> >>>>>         2. Scotty is looking at the Intel (Altera) Arria 10 GX
> >>>>>         FPGA 10GX270 for the version 2 Data Engine (supporting
> >>>>>         10GE). These FPGAs appear to be more available than the
> >>>>>         MAX10 FPGAs. The intention is to develop DE Ver 1 and DE
> >>>>>         Ver 2 in parallel
> >>>>>
> >>>>>         while awaiting FPGA component availability. The 10 GX
> >>>>>         development boards are pretty expensive.
> >>>>>
> >>>>>
> >>>>>
> >>>>>
> >>>>>
> >>>>>
> >>>>>
> >>>>>
> >>>>>
> >>>>>
> >>>>>
> >>>>>
> >>>>>
> >>>>>         -- Tom, N5EG
> >>>>>
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> >>>>>
> >>>>>         --
> >>>>>
> >>>>>
> >>>>>         Please follow the HamSCI Community Participation
> >>>>>         Guidelines at
> >>>>>         http://hamsci.org/hamsci-community-participation-guidelines
> >>>>>         <http://hamsci.org/hamsci-community-participation-guidelines
> >.
> >>>>>
> >>>>>
> >>>>>         ---
> >>>>>
> >>>>>
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> >.
> >>>>>
> >>>>>
> >>> --
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