[time-nuts] Fluke Thunderbolt Monitor Revisited
GandalfG8 at aol.com
GandalfG8 at aol.com
Fri Jun 4 10:14:06 EDT 2010
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
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