[hpsdr] ALEX - Input protection

[Henry Vredegoor]

Sorry Tommy, but I think you referring to Francis Garcia, Frank WA1GFZ,
instead of me??!!      ;-)

73's,

Henry.


> -----Original Message-----
> From: hpsdr-bounces at lists.hpsdr.org 
> [mailto:hpsdr-bounces at lists.hpsdr.org] On Behalf Of 
> Hollingsworth, Tommy
> Sent: dinsdag 7 augustus 2007 18:42
> To: Graham Haddock
> Cc: hpsdr at hpsdr.org
> Subject: Re: [hpsdr] ALEX - Input protection
> 
> 
> ***** High Performance Software Defined Radio Discussion List *****
> 
> Graham:
> I am trying to understand what your criteria are.  I just 
> completed an SDR
> 1000 clone and am trying to decide what kind of clamp to use 
> on the input.
> After spending 6 months building it, I really do not want the 
> smoke to get
> out of my working radio :)  Protek parts are not easy for me 
> to obtain.
> The parts below look interesting:
> 
> 		Semtech	Semtech	DiodesInc	Protek
> 		RClamp0502B	SL05		DLP05LC		GBLC05C 
> V standoff	 5		2.8		5		 5
> volts
> Vbr		 6		6		6		 6
> volts
> Vclamp	15		5.5		11		
> 18.3	volts
> I peak	 5		24		17		
> 17 	amps
> C		.6		2		1.6		
>  3 	pf
> 
> Package	SC75		SOIC8		SOT23		SOD323
> 
>    ***Note that Protek uses typical so I used typical for all C values
> Note that similar parts are also available from On 
> Semiconductor.  I really
> doubt that these are more expensive than the Protek part, considering
> Protek's $50 minimum per part and $500 minimum order. Note 
> that all of these
> parts are available from Digikey except for Protek. I assume 
> next that you
> have a target current/energy level higher than 5 amps so I 
> looked for some
> in that range.  The Diodes Inc DLP05LC looks really good to 
> me at 17 amps.
> You could have two in parallel for the capacitance of the 
> GBL05C .  Note
> that I see it as a good design idea to create a bidirectional 
> clamp from two
> unidirectional devices.  The spike really does ring, and in 
> doing so dumps
> energy into both device, not symmetrically but close enough. 
> 
> I am a little concerned that by trying to push the 
> capacitance to a really
> low value, a TVS is being chosen that will not really be 
> robust enough.
> Henry Vredegoor described using one or multiple 1N4148 diodes 
> in series with
> large TVS parts.  Note that the 1N4148 has a capacitance of 
> 4.0 pF(max) and
> the 1N4448 has a capacitance of 2 pF.  I am curious if we can 
> stack two of
> the TVS in series and cut the relative capacitance in half.  I am not
> certain that it would work that way.  I do have a transient 
> tester available
> and may have to build a test board to find out. 
> 
> 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.
> I am really curious what Henry and some of the others might 
> feel about this
> part, considering it is easy to obtain.  Is this clamp 
> voltage high enough?
> 
> 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 also plan on building the Mercury 
> and Penelope
> in the near future.  
> Cheers
> 73
> Tommy Hollingsworth KV5T
> 
> 
> -----Original Message-----
> From: Graham Haddock [mailto:KE9H at austin.rr.com] 
> Sent: Monday, August 06, 2007 8:50 PM
> To: Hollingsworth, Tommy
> Cc: hpsdr at hpsdr.org
> Subject: Re: [hpsdr] ALEX - Input protection
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