[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
> >>>>>
> >>>>>
> >>>>>
> >>>>>
> >>>>>
> >>>>>
> >>>>>
> >>>>>
> >>>>>
> >>>>>
> >>>>>
> >>>>>
> >>>>>
> >>>>>
> >>>>>
> >>>>>
> >>>>>
> >>>>>
> >>>>>
> >>>>> 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
> >>>>>
> >>>>>
> >>>>>
> >>>>>
> >>>>>
> >>>>>
> >>>>>
> >>>>>
> >>>>>
> >>>>>
> >>>>>
> >>>>>
> >>>>>
> >>>>>
> >>>>>
> >>>>>
> >>>>>
> >>>>>
> >>>>>
> >>>>>
> >>>>>
> >>>>>
> >>>>>
> >>>>>
> >>>>>
> >>>>>
> >>>>>
> >>>>>
> >>>>>
> >>>>>
> >>>>>
> >>>>>
> >>>>>
> >>>>>
> >>>>>
> >>>>>
> >>>>>
> >>>>>
> >>>>>
> >>>>> --
> >>>>>
> >>>>>
> >>>>> 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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> >.
> >>>>>
> >>>>>
> >>> --
> >>> 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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