[hpsdr] ALEX - Input protection

[Graham Haddock]

Tommy:

Comments below.  I note you are in Leander.
I am in Austin, so not far away.

--- Graham

 >
 > Graham:
 > I am trying to understand what your criteria are.  
 >
Well, I need a bidirectional TVS, preferably in a single
small case, but could be two parts if cheap and small enough.

I don't want it to clamp below 3 volts, and want it to be in
hard, high current clamp by 10 volts or low teens.

Peak power - around 300 watts, peak current around 15 amps,
mostly because that seems to be what most of them can do.

Capacitance - low single digits.  The parasitic capacitance
of the PCB is about the same, so no reason to go much lower.


The ProTek GBLC03C or GBLC05C seems to fit this well,
but as you say, they are hard to get.  As I read the web site,
they will take a $50 minimum order on a credit card.  
If we can't find a suitable alternate part from a friendly
source, then I am willing to do a $50 minimum buy and parse them
out to the others that are interested.  There seem to be
at least four or five of us that need them for various projects.


The Diodes Inc DLP05LC would be my second choice, but would need
two of them back to back, (not in parallel), to form a
bidirectional clamp for receiver RF inputs.  The SOT-23 is
small enough that two of them is not a big deal.  A single
one would be a nice clamp for each 3.3/5 volt SPI bus line
inputs on ALEX.



 >
 > I am curious if we can stack two of the TVS in series and cut the
 > relative capacitance in half.
 >
Yes, but then I am sure you pay some other price, such as increased
series resistance.


 > This sounds like we would be just as well off putting several of
 > the DLP05LC TVS parts in parallel on the line and forgetting about
 > the smaller diodes.
 >
Actually in series not parallel, but yes.

 >
 > Is this clamp voltage high enough?
 >
The Mercury data converter will overload at around +/- 0.75 volts, so
pick a number several times higher to start clamping.  It needs
to be in heavy clamp by ten volts or so. In the case of the Hittite
switches I am using, the parts will fail somewhere around 15 to 30
volts.  Don't have an exact number.

 >
 > Please let me know if this makes sense, or if I am overlooking
 > something.  I am anxious to get my radio on the air, but I plan
 > on it lasting for a long time so I won't rush it.
 >
I think you make a lot of sense.  
Also put a resistor, if there is not already some other DC path
to ground, say 10K ohms, from center to ground on the receive
input lines, to bleed off any (slow) static buildup, without
triggering the TVS.


--- Graham / KE9H

==
>
>
>   



 1186522905.0