[time-nuts] Fluke Thunderbolt Monitor Revisited

GandalfG8 at aol.com GandalfG8 at aol.com
Fri Jun 4 14:14:06 UTC 2010

Hi All
As there still seemed to be unanswered questions regarding the circuitry of 
 these I've separated the two PCBs from one of mine to determine exactly 
what is  going on.
Whether or not the regulator circuit is based in any way on the iCruze  
original processor board I don't know but it's now quite clear that the  
processor circuitry itself is an amost exact copy of Didier's version  using the 
20 pin DIP C8051F330 processor.
The only differences I've found so far are the values of the resistor  in 
the programming interface and those on the RS232 input but  the layout itself 
looks to be identical.
I'll go over it more thoroughly later and produce a complete  schematic.
Didier's original schematic, which includes the three series diodes for use 
 with a 5 volt supply, can be found here .....
As commented by Leigh earlier, all the diode positions on the PCB are  
occupied by zero ohm links so I think it's reasonable to assume this is probably 
 universal and ALL monitors are likely to need modifying to keep the 
processor  supply voltage at a safe level.
The absolute maximum rated supply voltage is 4.2 volts,  IO port  and RST 
pins are indicated as tolerating up to 5 volts, but the  specified operating 
supply voltage range is 2.7 to 3.6 volts so I would  recommend fitting all 
three diodes as per Didier's original circuit rather than  just two.
The supply to the display is taken directly from the output of the  
regulator and not via the diodes.
For anyone wanting to avoid hardware modification another option would be  
to run the unit from an external 5 volt supply, as was originally  
suggested, and let the onboard regulator just act as a dropper.
With a 5 volt supply the output voltage from the regulator is 3.6  volts 
but of course it isn't regulating and the supply to the display is  also 
I have found that my display at least will run ok at 3.6 volts  but the 
contrast pot may need adjusting.
However, if this route is followed there's little room for error and it's  
important to make sure the external supply is kept below 5.6 volts otherwise 
the  processor supply will again exceed 4.2 volts.
Fitting the diodes is a once and for all solution and is certainly the  
preferred option.

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