General > General Technical Chat
Quick TVS diode question. Vclamp < Vbreakdown
<< < (3/12) > >>
mag_therm:
Thanks Benta and Thm_w

I had been thinking of GDT and TVS in parallel, but now I will do series. I can get some GDT from Surplus Sales.

I have put in a station ground close by, per ARRL recommendation consisting of two driven copper plated rods 2.5 m (8ft) long spaced 1200 mm.
 Before brazing on the 4AWG leads, I measured the DC resistance between them with tractor battery, at 36 Ohm.
Then when trying later with a Ohm meter, the ground had charged up between them to 0.4 V DC and stayed like that.
So I don't know if the grounds will conduct a surge.
T3sl4co1l:
Series is when you need a high turn-off voltage, such as for mains voltage protection.  For signal purposes such as telephone (POTS), antennas probably, Ethernet, etc., a raw GDT or thyristor will do.

Hmm, I wonder if parallel connection is reasonable anyway.  Let's see, they break down at a minimum voltage, but it takes time (some µs), and it's faster as you go up from there, I think up to an upper limit where breakdown is very fast.  Presumably, you could clamp that initial excess, given a TVS beefy enough (which isn't too bad, it's only a few µs), then the GDT kicks in and clamps the rest.

Looks like at least a few others have thought so:
https://itecnotes.com/electrical/electronic-two-gas-discharge-tube-in-parallel-a-fast-acting-gas-acting-tube-and-a-gas-discharge-tube/
so if that's a good enough reference, I guess that's something.  The case here with a series inductor between them, is nice: under the surge's dI/dt, the GDT gets much more voltage, triggering sooner and dumping less energy into the TVS.

Note if this is on something like feedline, treat the shield (ignore the signal inside it) as a single-ended line with respect to earth.  Mind you only get as good suppression as you have ground impedance.

Tim
mag_therm:
Thanks Tim,
I will get some devices and test with a 200 V DC supply.

However, I ran some qucs on the circuit. If a pulse of only 50 V  and 30 ns on antenna  gets into the tuned circuit,
it rings, being high Q and  is all over for the fet which is rated 35 V between all.
If a TVS on input fires some time later to an ideal short it (depending on timing) increases the ring up voltage as Q of tank is raised by shorting the antenna Z.

So I will also add  back to back zeners at output of tuned circuit before fet.

Later Edit: For HF radio application, the MicroSemi  SA5.0CA Bi-Directional series might be suitable.
No snapback, < 5 nanosec, with a nominal 500 Watt at 1 millisec. in DO-41

SA14CA 14 V standoff for receivers and SA150CA for up to 100 Watt transceivers.

https://download.datasheets.com/pdfs/diode/mcs/msc1400.pdf
Tomorokoshi:
I've been replacing damaged MOVs in Tripp-Lite Isobar power strips with the GMOV series from Bournes:

https://media.digikey.com/pdf/Data%20Sheets/Bourns%20PDFs/GMOV_Series_DS.pdf
kellogs:
Good thread!
And I am about to make a bunch of mistakes. Can you spot them ? :D

So I'd like to perhaps implement the solution from the link below:


--- Quote from: T3sl4co1l on July 28, 2022, 12:25:00 am ---
https://itecnotes.com/electrical/electronic-two-gas-discharge-tube-in-parallel-a-fast-acting-gas-acting-tube-and-a-gas-discharge-tube/
so if that's a good enough reference, I guess that's something.  The case here with a series inductor between them, is nice: under the surge's dI/dt, the GDT gets much more voltage, triggering sooner and dumping less energy into the TVS.

--- End quote ---

for automotive device protection. I am going to take a nasty load dump for the first impulse to mitigate. Attached its specs. Instead of a GDT I shall use this guy, an GMOV-20D450K - which by the way is a 49J part so I see no way for it to be able to absorb the load dump energy; let me assume I am wrong here, moving on:


--- Quote from: Tomorokoshi on July 29, 2022, 02:44:32 am ---I've been replacing damaged MOVs in Tripp-Lite Isobar power strips with the GMOV series from Bournes:

https://media.digikey.com/pdf/Data%20Sheets/Bourns%20PDFs/GMOV_Series_DS.pdf


--- End quote ---

Does V_c in the datasheet table stand for V_clamp ? I am going to assume that as well. For the GMOV to *only* begin conducting the inductor should build up a voltage spike of 150V

150V = U_ind < L * di/dt

For the impulse rise, worst case: di = (U_s - U_alternator) / R_i = (101 - 14) / 0.5 = 174A and dt = 5ms

150 < L * 174 / 5
L > 150 * 5 / 174 mH
L_min = 4.45 mH

For the impulse fall, worst case:

U_ind = 4.45 * 174 / 40 = 19.36V

 - which, with a proper TVS, will not cause problems. But I would probably want more L so the GMOV will actually have time to conduct that energy away, perhaps a tenfold value, 44.5 mH? Any possible issues with the TVS or the rest of the circuitry to be protected?

Now, please correct me. Thank you!
Navigation
Message Index
Next page
Previous page
There was an error while thanking
Thanking...

Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod