[time-nuts] Thunderbolt serial port problem UPDATE

lists at lazygranch.com lists at lazygranch.com
Thu Apr 14 16:36:31 UTC 2011


Just a FYI, when a regulator goes into thermal shutdown, it oscillates in and out of operation. That is, once it shuts down, it cools off, then turns on again. Bench testing LDOs regularly puts the chips in this mode. The frequency is low, maybe 10Hz. 

What may have happened here is the thermal trip point for the LDO is a bit too low. The trip points aren't tested in production. At least not for the industrial grade parts. What you do when designing thermal protection is you set the limit above 150 deg C and hope it doesn't shift into the operating range. 


-----Original Message-----
From: Arthur Dent <golgarfrincham at yahoo.com>
Sender: time-nuts-bounces at febo.com
Date: Thu, 14 Apr 2011 09:04:47 
To: <time-nuts at febo.com>
Reply-To: Discussion of precise time and frequency measurement
	<time-nuts at febo.com>
Subject: [time-nuts] Thunderbolt serial port problem UPDATE

 What I have done with the displays I got from Fluke.L is lift the end of the 
interface board and hold it away from the display board just enough so I 
can heat and remove the 5 volt regulator and replace it with a jumper. It 
might be a little easier to just lift the input and output leads off their 
solder 

pads and leave the chip there but I removed the chip as well. I'm supplying 
the display with a regulated 5 volts so removing the regulator from the display 
assembly eliminates one of the possible problem area by removing one of 
the heat sources from the board. Some people have had problems with the 
regulator shutting down if they supplied it with 12 volts and the chip went 
into thermal shutdown.

 Next I remove the 3 zero ohm resistors (that are where the 3 diodes should 
have been installed to drop the 5 volts to ~3.3 volts for the CPU) and replace 
them with a red LED with leads long enough so it can be on the back side of 
the interface board because there wasn't enough room between the boards for 
the LED I am using. I have some SMD red LEDs but decided they might be 
too hard to solder between the two boards. The red LED makes a pretty good 
~1.7 volt zener diode so now I have the display powered by 5 volts and the 
CPU powered by ~3.3 volts which makes it happy.  

          -Arthur 
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