[time-nuts] FE5680 missing PPS soln

[Rob Kimberley]

Hi,

You need to check your maths...

:-)
Rob K

-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of Attila Kinali
Sent: 03 February 2012 12:55
To: Discussion of precise time and frequency measurement
Subject: Re: [time-nuts] FE5680 missing PPS soln

On Fri, 3 Feb 2012 09:50:52 -0000
"Rob Kimberley" <robkimberley at btinternet.com> wrote:

> Mine was a green LED out of the junk box, but I did use 330R in 
> series. 1k8 seems rather large.

Then i'm not surprised that your FE-5680 refused to work :-)

Just calculate: 5V supply, approx 2V LED voltage -> 3V over the resistor.
3V/330R = 90mA. As i wrote already, the 74AC240 has it's absolute maximum
rating at 50mA. For a general logic output, i wouldn't recomend to draw more
than 10mA (as Bob Grant wrote, the lock signal doesn't seem to pass trough
the 74AC240). Hence, i would say that 330R seems rather small :-)

Keep in mind that you drove circuit at over current for quite a long time.
The circuit is most likely damaged (if they dont have an over current limit,
which i doubt). If for an unknown reason the lock signal fails in the
future, it might be due to this.

			Attila Kinali
--
The trouble with you, Shev, is you don't say anything until you've saved up
a whole truckload of damned heavy brick arguments and then you dump them all
out and never look at the bleeding body mangled beneath the heap
		-- Tirin, The Dispossessed, U. Le Guin

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