| Electronics > Projects, Designs, and Technical Stuff |
| Driving an optocoupler LED at different supply voltages |
| << < (8/9) > >> |
| TrickyNekro:
How about using a 1.2V reference and a control transistor (either BJT or MOSFET) for the LED. Easy enough. You get the stability of the reference and the the fast switching of the transistor. Most optocouplers their LED is IR based so dropout is at around 0.8V - 1V. You can almost get away with using a BJT for switching. The collector - emitter saturation is around 0.2V. If you want to be sure you get to use a MOSFET like 2N7002. And there are dirty cheap references out there, you need it mostly for regulation anyhow. Edit: And there are parts that can work from 1.8V to 5V or maybe higher. Also you look for a voltage output reference not a shunt reference. |
| SiliconWizard:
--- Quote from: Benta on January 17, 2019, 04:12:00 pm ---In my experience, the JFET works as a current source down to appr. 1...2 V drain-source (depends on current). Below that it tends to go resistive, which is not really an issue in this application. --- End quote --- @Zero999 and Benta: indeed. Using a JFET was my first thought as well, but then I quickly realized this would not work properly in the op's use case. Beyond the variation of the current between JFETs of the same model, which may not be a big problem here (you'd just have to settle for a minimum current, the circuit would work fine for higher currents), you have to consider the characteristic curve of a JFET (Ids vs. Vds) @Vgs = 0 to see that below a certain Vds (which would typically be somewhere between 0.5V and 3V depending on the JFET), the current would drop a lot, defeating the purpose of driving the optocoupler's LED at a somewhat constant current regardless of the supply voltage. You could try selecting a JFET for a minimum acceptable current for the LED at Vds = 0.1V, but good luck finding a proper JFET for this while still limiting the max current to an acceptable level for the LED for the max. Vds (thus max. working voltage). As a side note, the Vfd of the LED is said to be 1.7V but in the 6N135/136 datasheet I have, Vfd is up to 1.9V max? So 1.8V may not even allow to light up the LED if you're unlucky... |
| TrickyNekro:
--- Quote from: SiliconWizard on January 18, 2019, 06:16:16 pm ---As a side note, the Vfd of the LED is said to be 1.7V but in the 6N135/136 datasheet I have, Vfd is up to 1.9V max? So 1.8V may not even allow to light up the LED if you're unlucky... --- End quote --- Do you have to use the 6N135/6? Cause I can find many parts that are much cheaper and have a lower forward voltage. Example: https://eu.mouser.com/Optoelectronics/Optocouplers-Photocouplers/_/N-6x5ji?Ns=Pricing%7c0&No=25&Rl=6x5jiZgjdhqbZ1yuoc3aZ1yuo3qbSGT And... a more specific example: https://eu.mouser.com/ProductDetail/Vishay-Semiconductors/VOH1016AB-T?qs=sGAEpiMZZMtd3yBnp8bAgLhQurE%252baQR29IOeeMf%252bGH1H6u1Y7D9ZWg%3d%3d I am sure if you search a little bit you can find better parts than 6N135/6. These are quite mature parts, that alone makes them expensive. |
| fenclu:
--- Quote from: TrickyNekro on January 18, 2019, 09:15:26 pm --- --- Quote from: SiliconWizard on January 18, 2019, 06:16:16 pm ---As a side note, the Vfd of the LED is said to be 1.7V but in the 6N135/136 datasheet I have, Vfd is up to 1.9V max? So 1.8V may not even allow to light up the LED if you're unlucky... --- End quote --- Do you have to use the 6N135/6? Cause I can find many parts that are much cheaper and have a lower forward voltage. Example: https://eu.mouser.com/Optoelectronics/Optocouplers-Photocouplers/_/N-6x5ji?Ns=Pricing%7c0&No=25&Rl=6x5jiZgjdhqbZ1yuoc3aZ1yuo3qbSGT And... a more specific example: https://eu.mouser.com/ProductDetail/Vishay-Semiconductors/VOH1016AB-T?qs=sGAEpiMZZMtd3yBnp8bAgLhQurE%252baQR29IOeeMf%252bGH1H6u1Y7D9ZWg%3d%3d I am sure if you search a little bit you can find better parts than 6N135/6. These are quite mature parts, that alone makes them expensive. --- End quote --- This one needs a supply voltage of at least 3V. --- Quote from: MasterT on January 16, 2019, 02:27:22 pm ---If you have spare digital pins on the uCPU, than pre-set 4-5 pins with different resistors value to a LED. Only one pin set as output at a time, others are inputs. Depends on the power source voltage selection of the appropriate resistor is made. --- End quote --- I think that MasterT's solution is the closest to what I had in mind. I will have to try that in practice and let you know how it went. Thank you everybody for your ideas ;). |
| Zero999:
Here's an implementation of MasterT's solution. |
| Navigation |
| Message Index |
| Next page |
| Previous page |