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| Does this Digital output design correct? |
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| Danesh_SA:
Hi all, Happy new year , I design this circuit,which is a 4 channel Digital output. Maximum acceptable voltage is 28V .I use ULN2803 for this design. I use optocoupler to have isolated digital output from my MCU (this is an industrial design). My question is: As the ULN2803 is darlington pairs,does it necessary to have pull up (R2,R5,R8,R11) and pull down (R3,R6,R9,R12) resistors for TLP521-4 emitters and collectors? Thanks! |
| oPossum:
All of the 4k7 resistors can be removed. D3 must also be removed to allow the diodes in the ULN2803 to work. |
| MarkF:
--- Quote from: oPossum on January 01, 2020, 01:57:36 pm ---All of the 4k7 resistors can be removed. --- End quote --- No. That would cause a short between VIN and GND. One of four inputs: OR |
| SiliconWizard:
--- Quote from: MarkF on January 01, 2020, 02:35:21 pm --- --- Quote from: oPossum on January 01, 2020, 01:57:36 pm ---All of the 4k7 resistors can be removed. --- End quote --- No. That would cause a short between VIN and GND. --- End quote --- Indeed. *All* was weird. Maybe that was some alcohol in play there. ;D --- Quote from: MarkF on January 01, 2020, 02:35:21 pm ---One of four inputs: (...) --- End quote --- Yep, the version to use would depend on what polarity is needed from input to output - of course we don't know for sure what the OP intended. As to D3, I guess it provides simple protection against polarity inversion. The internal diodes of the ULN2803 are meant to act as freewheeling diodes I guess? Problem here is that if using D3, the supply voltage for the outputs should be taken at the cathode of D3, and not at its anode as we can guess from the schematic and the "Digital out" connector. Finally, I'm guessing the whole idea is just to make an isolated level shifter, but we don't quite know what it's supposed to drive. Knowing that may help finding an alternative to the ULN2803. |
| Danesh_SA:
Thanks all for replys, This circuit is just an industerial digital output card ,it used for driving 24v relays coil , low power selenoid's,signal lamps and ,... . As @SiliconWizard mention it,D3 is simple reverse polarity protection in my design. More information:MCU is 3.3V and D4 is TVS. --- Quote --- --- Quote from: MarkF on January 01, 2020, 02:35:21 pm ---One of four inputs: (...) --- End quote --- Yep, the version to use would depend on what polarity is needed from input to output - of course we don't know for sure what the OP intended. --- End quote --- 1-would you please explain me on both of the @MarkF suggestions? and why place both pull up and pull down resistors is wrong design? 2-As ULN2803 has 2.7K series resistor in all of the inputs,No need to add current limit resistors between optoucouplers and ULN2803 inputs.Is it right? |
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