| Electronics > Projects, Designs, and Technical Stuff |
| Driving an optocoupler LED at different supply voltages |
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| Yansi:
Minimum LED current is not determined experimentally, but defined by the manufacturer. If I remember correct, it is 8mA for 6N137 (please verify in the datasheet) You need a current source for the LED. Just ask google for "LED current source" and check the simple transistorized solutions. I'd bet on this two transistor one to work quite nicely across a decent voltage range up to some decent speeds. //EDIT: I think this may be a bit better than trying to source unobtanium depletion mode (J)FETs with just the right amount of current at zero Vgs. //Those 8mA I may have probably confused with this device? Not sure, been quite a while last time I've used such opto (there are better alternatives these days): https://media.digikey.com/pdf/Data%20Sheets/Sharp%20PDFs/PC410L0NIP%20Series.pdf |
| fenclu:
--- Quote from: Yansi on January 16, 2019, 05:58:43 pm ---Minimum LED current is not determined experimentally, but defined by the manufacturer. If I remember correct, it is 8mA for 6N137 (please verify in the datasheet) You need a current source for the LED. Just ask google for "LED current source" and check the simple transistorized solutions. I'd bet on this two transistor one to work quite nicely across a decent voltage range up to some decent speeds. //EDIT: I think this may be a bit better than trying to source unobtanium depletion mode (J)FETs with just the right amount of current at zero Vgs. //Those 8mA I may have probably confused with this device? Not sure, been quite a while last time I've used such opto (there are better alternatives these days): https://media.digikey.com/pdf/Data%20Sheets/Sharp%20PDFs/PC410L0NIP%20Series.pdf --- End quote --- The 6N137 is actually a different beast, which uses some logic at the output and also requires a supply of between 4,5 and 5,5V. The 6N136 (or 6N135) uses a photodiode with a transistor output. The datasheet of the 6N136 does not state a minimum supply current, or at least I can't find one. Nevertheless, with the decreasing current, the speed decreases. The 4 mA I determined resulted in a fall time of around 2us, which is acceptable in my case. |
| CJay:
--- Quote from: Yansi on January 16, 2019, 05:58:43 pm ---Minimum LED current is not determined experimentally, but defined by the manufacturer. If I remember correct, it is 8mA for 6N137 (please verify in the datasheet) --- End quote --- Indeed, I did some experimenting with optocouplers a while back and found forward current through the LED is very imprtant if you need to meet specs for transfer ratio but you can drive them with quite low currents if you don't need them to be high speed. --- Quote from: Yansi on January 16, 2019, 05:58:43 pm ---Just ask google for "LED current source" and check the simple transistorized solutions. I'd bet on this two transistor one to work quite nicely across a decent voltage range up to some decent speeds. --- End quote --- That's the bipolar version of the one I linked, it works well with a MOSFET in the T2 position, 2N7000 works. I built a few dozen for some PWM fading LED strings a few years back, they're good to really quite high frequencies, far higher than I needed. to stop the flicker |
| langwadt:
with only 100mV to work with, 1.8V LDO |
| Yansi:
Do you have (any) power supply available at the isolated (LED) side? |
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