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| Floating source on P-Channel |
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| Peabody:
--- Quote from: edavid on July 19, 2019, 04:31:29 am --- OP is using a 25mA segment current, and most (?) PIC devices have a VSS pin limit of 150mA, so that won't quite work. With lower current displays, sure... --- End quote --- If he really needs 25mA segment current, then the total current would probably be too much. But I would be very surprised if he actually needs anywhere near that. In any case, it would certainly be worth testing with maybe 680R resistors, or even 1K to see if they are bright enough at 1/8 duty cycle. Red also might be more efficient than green, but I'm not sure about that. I did a video on multiplexing by segment, which turns the segments on one at a time, with a refresh period of 2ms, and a 1/7 duty cycle, and I was able to use 2.2K resistors with 5V supply. But I used very high efficiency Vishay individual red digits which specify their mcd ratings at 1mA. I've never seen multi-digit modules with similar efficiencies. But even so, the OP's modules are rated at 25mA absolute maximum constant current per segment, with mcd output specified at 10mA, and my guess is that something closer to 10mA would work fine, perhaps even less, and then you would be within range on total current. And if anyone knows of very high efficiency 2- and 4-digit modules, please post part numbers or links. |
| advark:
--- Quote from: Peabody on July 18, 2019, 11:58:11 pm ---You may be using all these mosfets for a reason, but it seems you could do away with the segment driver mosfets altogether, and connect the segment lines to the PIC GPIO pins through resistors. While a segment driver is connected to the same segment on all eight digits, only one of the eight will be lit up at a time. A PIC GPIO should be able to drive one segment with no problem, but there might be an issue with the total current through the PIC when all eight segments are turned on on a particular digit. It depends on how much current you need for adequate brightness. --- End quote --- That is definitely worth trying! I'm using a PIC16F1518. According to the datasheet, the maximum rating for Vss is 350mA and 50mA for the I/O pins. So even if I put 25mA on each segment, which I doubt because it's basically all what the PIC is doing, will put 200mA on Vss. Just need to keep P-Mosfet to feed the anodes since the anodes are driven from a 74138. I'll dig down the PIC datasheet for the consumption of other peripherals (Clock, SPI and 7 status LEDs at 10-15mA) to be sure but that should be enough. |
| Peabody:
The efficiency of the displays and the status LEDs can be important considerations in your design. Some displays provide a lot higher brightness per milliamp, and let you get away with using less current, but are usually more expensive. And if you will have them on all the time, the status LED shouldn't need that much current. You'll just have to experiment with different resistors to see what works. |
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