06-09-2023, 07:13 PM
Hi all,
Most parts for the prototypes (except the batteries) arrived and I've been busy building a (really ugly :-) breadboarded 'concept-test' prototype.
The DC/DC step-up converter quiescent current (as noted by Garth) was a concern. The claimed figure from the vendor was 1mA, but once the "ON" diode in the DC/DC converter was removed it drops to the 0.6mA range (+/-20uA); which I think is pretty acceptable.
The converter was reconfigured to provide 6.1v when the battery is above 3.3v and a comparator is in charge of detecting lower voltages and modify the reference resistor value to provide 3.65v to the HP-41. This is low enough to turn the BAT annunciator ON but still keep the calculator working.
The test was done using a standard LM393 (the only one I have handy), and since it's a bipolar comparator its quiescent current is higher than expected on the final design, which will use a CMOS (probably MSCP6546 from Microchip) in the smallest package available: SOP8 or SOT23-5 (preferable) to save room in the PCB.
In the pictures, the BAT annunciator is ON when the PS provides 3.3v and OFF when it reaches 3.4v; the Voltage to the HP-41 is shown in the DMM on the right.
The 0.66v in the DMM on the left is the diode drop used for reference in the comparator.
Will keep posting as the project progresses.
All the best from the Caribbean.
Diego.
[attachment=12226][attachment=12227]
Most parts for the prototypes (except the batteries) arrived and I've been busy building a (really ugly :-) breadboarded 'concept-test' prototype.
The DC/DC step-up converter quiescent current (as noted by Garth) was a concern. The claimed figure from the vendor was 1mA, but once the "ON" diode in the DC/DC converter was removed it drops to the 0.6mA range (+/-20uA); which I think is pretty acceptable.
The converter was reconfigured to provide 6.1v when the battery is above 3.3v and a comparator is in charge of detecting lower voltages and modify the reference resistor value to provide 3.65v to the HP-41. This is low enough to turn the BAT annunciator ON but still keep the calculator working.
The test was done using a standard LM393 (the only one I have handy), and since it's a bipolar comparator its quiescent current is higher than expected on the final design, which will use a CMOS (probably MSCP6546 from Microchip) in the smallest package available: SOP8 or SOT23-5 (preferable) to save room in the PCB.
In the pictures, the BAT annunciator is ON when the PS provides 3.3v and OFF when it reaches 3.4v; the Voltage to the HP-41 is shown in the DMM on the right.
The 0.66v in the DMM on the left is the diode drop used for reference in the comparator.
Will keep posting as the project progresses.
All the best from the Caribbean.
Diego.
[attachment=12226][attachment=12227]