20-07-2011, 08:10 PM
The "50 times" comes from 50 being the reciprocal of 0.02. This is a useful numerical result to remember for another reason:
If you're doing power supply work with a 'scope, 50Hz should give 20ms. If you have hum on the output of the power supply, one of the first things to check is the ripple frequency. If, as is most often the case, you are dealing with full-wave rectification, the ripple frequency should be 100Hz (10ms). If you have 50Hz, you have an open-circuit diode in the rectifier...
I'd suggest that "100 times" (10mA) is a better starting point for most modern LEDs. The key words in that sentence, of course, are "starting point". These modern LEDs (blue, white, pure green) with a higher voltage drop are very bright indeed, and if they are being used on a front panel as a directly viewed indicator, 2mA (500 times) might be plenty. Older LEDs tend to need a bit more, but experiment (and enjoy!).
The higher the voltage, the less significant the forward drop of the LED is, so it can be ignored at say ~12 volts and above, as the 20% spacing of typical E12 resistors gives a similar margin. But care is needed at lower voltages.
Oh, and LEDs don't like reverse-voltage. They "zener" at relatively low and poorly defined voltages. If using LEDs to replace dial lamps, it might be as well to use a pair in inverse-parallel so that each one never sees more than the VF of the other.
Like many engineers, I enjoy LEDs just a little more than I perhaps should :-)
Cheers,
Mark
If you're doing power supply work with a 'scope, 50Hz should give 20ms. If you have hum on the output of the power supply, one of the first things to check is the ripple frequency. If, as is most often the case, you are dealing with full-wave rectification, the ripple frequency should be 100Hz (10ms). If you have 50Hz, you have an open-circuit diode in the rectifier...
I'd suggest that "100 times" (10mA) is a better starting point for most modern LEDs. The key words in that sentence, of course, are "starting point". These modern LEDs (blue, white, pure green) with a higher voltage drop are very bright indeed, and if they are being used on a front panel as a directly viewed indicator, 2mA (500 times) might be plenty. Older LEDs tend to need a bit more, but experiment (and enjoy!).
The higher the voltage, the less significant the forward drop of the LED is, so it can be ignored at say ~12 volts and above, as the 20% spacing of typical E12 resistors gives a similar margin. But care is needed at lower voltages.
Oh, and LEDs don't like reverse-voltage. They "zener" at relatively low and poorly defined voltages. If using LEDs to replace dial lamps, it might be as well to use a pair in inverse-parallel so that each one never sees more than the VF of the other.
Like many engineers, I enjoy LEDs just a little more than I perhaps should :-)
Cheers,
Mark
