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

Jonathan emuman100 at gmail.com
Wed Jul 28 10:29:57 EDT 2021


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> 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.
> [image: 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.
> [image: 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)
> [image: 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> wrote:
>
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> Thank you, Tom.
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> The recording will be available later today at
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> https://youtu.be/AammohuygMw and hamsci.org/telecons.
>
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> 73 de Nathaniel W2NAF
>
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>
> *From:* TangerineSDR <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>
>
>
> *Cc:* Tom McDermott <tom.n5eg at gmail.com>
>
>
> *Subject:* [TangerineSDR] Notes from PSWS / TangerineSDR call of
> 07-26-2021
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> Notes from PSWS / TangerineSDR call of 07-26-2021
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> 1. Bill is using chart.js for magnetometer charting. He is setting up a
> database using Django web and database framework for Python.
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> 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.
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> -- Tom, N5EG
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> --
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>
> Please follow the HamSCI Community Participation Guidelines at
> http://hamsci.org/hamsci-community-participation-guidelines.
>
>
> ---
>
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