Mike - K8LH
Well-Known Member
Wilf updated that other forum thread to say his 1/32nd duty cycle design won't work because the 500-ma peak ULN2803 pins won't handle 2.5 amps peak. Now he's recommending his earlier 1/16th duty cycle design which scans the 16 anodes instead of the 32 cathodes and we're back to 2.5 amp P-FET anode drivers.
May I ask what you paid for that 20-amp PSU?
The more I think about it the more it seems that you may need a higher supply voltage for your 1/16th duty cycle display. For example, if we could somehow measure the peak voltage across one of those 10 ohm current limiting resistors and it was 50.0-mv then we'd know we have 5.0-ma peak current going through the LED. Now to get more current through the LED don't we need more voltage across the LED? But a 50.0 ma peak current going through that same LED and resistor would result in less voltage to the LED because the voltage drop across the resistor would increase from 50.0 mv to 500.0 mv.
I think I need to apologize for mistakes and my misleading design suggestions. I remembered enough from my earlier experiments to recommend 1/8th or higher duty cycles but I don't think I realized why. Now it seems (to me) that you might need a higher supply voltage when using extremely low duty cycles.
What do you think?
BTW, I went back to one of my earlier posts and added a qualifier to that 1/16th duty cycle design which states that a higher supply voltage may be required on extremely low duty cycle designs. I just want to make sure that I don't mess up anyone else who may stumble across this thread and that design.
May I ask what you paid for that 20-amp PSU?
The more I think about it the more it seems that you may need a higher supply voltage for your 1/16th duty cycle display. For example, if we could somehow measure the peak voltage across one of those 10 ohm current limiting resistors and it was 50.0-mv then we'd know we have 5.0-ma peak current going through the LED. Now to get more current through the LED don't we need more voltage across the LED? But a 50.0 ma peak current going through that same LED and resistor would result in less voltage to the LED because the voltage drop across the resistor would increase from 50.0 mv to 500.0 mv.
I think I need to apologize for mistakes and my misleading design suggestions. I remembered enough from my earlier experiments to recommend 1/8th or higher duty cycles but I don't think I realized why. Now it seems (to me) that you might need a higher supply voltage when using extremely low duty cycles.
What do you think?
BTW, I went back to one of my earlier posts and added a qualifier to that 1/16th duty cycle design which states that a higher supply voltage may be required on extremely low duty cycle designs. I just want to make sure that I don't mess up anyone else who may stumble across this thread and that design.
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