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redigeringar
Bigi LED display: Skillnad mellan sidversioner
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→Replacing hot transistors with MOSFETs
Didrik (diskussion | bidrag) m (Fixed typo) |
Didrik (diskussion | bidrag) |
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| Rad 75: | Rad 75: | ||
It was noted in an all pixels on stress test that the TIP120 transistors used for line selection could get as hot as 90 °C. This is not supposed to be a problem, and probably intended by design, as TIP120 is rated to a maximum temperature of 150 °C according to the datasheet, however it is a bit disconcerting. The transistors run a fair bit cooler when showing a typical image, though. | It was noted in an all pixels on stress test that the TIP120 transistors used for line selection could get as hot as 90 °C. This is not supposed to be a problem, and probably intended by design, as TIP120 is rated to a maximum temperature of 150 °C according to the datasheet, however it is a bit disconcerting. The transistors run a fair bit cooler when showing a typical image, though. | ||
Replacing the TIP120 transistors with MOSFETs would make it run cooler. However, this would require another mod as well. The 74LS138 | Replacing the TIP120 transistors with MOSFETs would make it run cooler. However, this would require another mod as well. The 74LS138 decoders used to select which line to draw are active low. This is then fed into ULN2003 line drivers to drive the TIP120 transistors. This effectively leaves the transistors pulled to ground by default, and allowed to go high when active through internal biasing. P channel MOSFETs would require the opposite polarity. Replacing the 74LS138 with its active high counterpart 74x238, making the ULN2003 output active low, would solve this. | ||
This mod would also require modifying the supply voltage in order to not fry the LEDs, since the voltage drop of 2-4 V of TIP120 would now be nearly gone. | This mod would also require modifying the supply voltage in order to not fry the LEDs, since the voltage drop of 2-4 V of TIP120 would now be nearly gone. | ||