[Lowfer] TX output protection.

Sun, 31 Aug 2003 14:18:20 -0400

Kurt,

I don't buy the idea that an MOV is just a large capacitor. It's true that 
they have a large amount of capacity, and sometimes it's too large to use 
them to protect a certain circuit, but they are very fast in sensing an 
overvoltage and are capable of absorbing a large amount of energy without 
damage to themselves (something a capacitor cannot do).

Two years ago I was responsible for re-qualifying our complete product line 
of controllers, pressure sensors, and chillers to a new power line 
resilience specification EN60126 required for CE sales (virtually all of the 
non-USA world). The main provision of this spec was to assure that any 
equipment inside a industrial building would withstand a lightening hit to 
the building. The EN60126 test requirement was something like a spike of 
1500v at 50A with a duration of ~50 microseconds. The product had to 
withstand this spike applied to power AND signal inputs. The final testing 
was performed by a EMI/ESD test house but I had to, first, design the 
changes needed to make the circuits pass. I used a 2000v, 5uf capacitor with 
a series 30 ohm resistor to simulate the $20,000 equipment used at the test 
house and found the test results were similar.

It turned out that the MOV (along with a small series resistor) was by far 
the best way to eat up a pulse of high energy without passing it on to other 
components. Check out the specifications for MOVs (also called varristers, 
ZNRs, and surge absorbers) on the Web. BTW, Digi-Key has a good selection of 
these.

If the MOV capacity (1000pf for the ones I was using) was too large for the 
application (such as the input and output of a Lowfer final) I used a 1N4004 
to ground and Vcc, and then a MOV from Vcc to ground. Also I had to assure 
sufficient series resistance so the current resulting from the conduction of 
the diode did not damage any other component. I later adapted this approach 
to my Lowfer final and I've taken a number of indirect hits without damage. 
Since the Lowfer setup has become a lightening 'test bench', of sorts, I 
never disconnect anything when a storm approaches.

Bill A

>From: "KD7JYK" <[email protected]>
>Reply-To: [email protected]
>To: <[email protected]>
>Subject: [Lowfer] TX output protection.
>Date: Sat, 30 Aug 2003 22:06:44 -0700
>
>I hear the horror stories of zapped finals due to lightning and ESD, has
>anyone considered MOVs on the output?  I have seen them available from just
>a few volts to several hundred.  If not these, what is used?
>
>Kurt
>
>---
>Ross Technologies Signals Intelligence Division
>Rosetta Proving Grounds
>
>_______________________________________________
>From the Lowfer mailing list
>Send messages to: [email protected]
>To sub/unsub visit: http://mailman.qth.net/mailman/listinfo/lowfer

_________________________________________________________________
Get MSN 8 and enjoy automatic e-mail virus protection.    
http://join.msn.com/?page=features/virus