Hi, I posted a couple weeks ago about a project I was thinking about; a large array of alphanumeric LED displays.
I got them today and took some current measurements:
With a 1.2 kHz refresh rate and 1/32 duty cycle, I can get a decently bright display with a 56 ohm resistor per segment. Current per segment (strobed) is about 2.5 mA. This is quite reasonable. But... what if the microprocessor froze, leaving one character stuck on? If all 18 segments are lit (worst case), then the current through the character's common cathode would be
(1.6V segment forward voltage / 56Ω) * 18 segments = 514.286 mA
If I have a 4-row display, with the rows multiplexed simultaneously and the 4 cathodes of a column tied together, the current draw would be over 2 amps!
So, should I use a power transistor to drive each column (to be safe), or just stick with general-purpose transistors and replace them if they ever get toasted?
If it's better to err on the side of caution, I'm kinda reluctant to use TO-220 MOSFETs because they're huge and would stick out like sore thumbs. Do transistors that can handle that much current come in smaller packages?
My guess is that it's fine to just use transistors rated for 500mA, and if I blow one, the flow of current will stop and no damage will be done to the display. I know it's nitpicky, but I'm being super-careful since these displays are over 20 years old, very rare, and very expensive.
I got them today and took some current measurements:
With a 1.2 kHz refresh rate and 1/32 duty cycle, I can get a decently bright display with a 56 ohm resistor per segment. Current per segment (strobed) is about 2.5 mA. This is quite reasonable. But... what if the microprocessor froze, leaving one character stuck on? If all 18 segments are lit (worst case), then the current through the character's common cathode would be
(1.6V segment forward voltage / 56Ω) * 18 segments = 514.286 mA
If I have a 4-row display, with the rows multiplexed simultaneously and the 4 cathodes of a column tied together, the current draw would be over 2 amps!
So, should I use a power transistor to drive each column (to be safe), or just stick with general-purpose transistors and replace them if they ever get toasted?
If it's better to err on the side of caution, I'm kinda reluctant to use TO-220 MOSFETs because they're huge and would stick out like sore thumbs. Do transistors that can handle that much current come in smaller packages?
My guess is that it's fine to just use transistors rated for 500mA, and if I blow one, the flow of current will stop and no damage will be done to the display. I know it's nitpicky, but I'm being super-careful since these displays are over 20 years old, very rare, and very expensive.