[time-nuts] Fluke Thunderbolt Monitor Revisited
brooke at pacific.net
Fri Jun 4 15:51:23 UTC 2010
I've added a photo of the component side of the 8051 PCB at:
J1-2 is connected directly to pin 6 of the uC which is also connected to
the output of the 78D05 regulator.
GandalfG8 at aol.com wrote:
> 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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