[TangerineSDR] [HamSCI] RE: Notes from PSWS / TangerineSDR call of 07-26-2021
Julius Madey
hillfox at fairpoint.net
Wed Jul 28 16:12:00 EDT 2021
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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>>>>>
>>>>>
>>> --
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>>
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