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I have a TTL output from an incremental linear encoder which also supplies a 5V output at 140 mA. I would like to use this 5V output voltage to power a camera IO board with RS422 Interface which has a specification of 5V/50mA.

How do I reduce the current from 140mA to 50mA. Can I just use a 150 Ohm resistor or have you got any suggestions ?

schematic

simulate this circuit – Schematic created using CircuitLab

Please find the camera IO board specifications below :
Pin......Level.............Description..........Default 1........RS422.............GPIO_P0(input).......Incremental encoder(high) 2........RS422.............GPIO_P1(input).......Lightbarrier(high) 3........LVCMOS............GPIO_P4..............Can be used for lightbarriers 4........LVCMOS............GPIO_P2..............Reserved 5........3.3Vover100Ohm....Power................Output 6........LVCMOS............GPIO_P3..............Master/Slave-interface 7........0V................Ground...............- 8........LVCMOS............GPIO_N4..............Master/Slave-interface 9........RS422.............GPIO_N0(input).......Incremental encoder(low) 10.......RS422.............GPIO_N1(input).......Lightbarrier(low)
11.......0V................Ground...............-
12.......LVCMOS............GPIO_N2..............Reserved
13.......0V................Ground...............-
14.......LVCMOS............GPIO_N3..............Reserved
15.......5Vover100Ohm......Powersupply..........Output

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  • \$\begingroup\$ I should have mentioned more clearly. There is a digital IO Interface Card within a camera and it is specified to use a external 5V power supply with a series 100 Ohm resistor . As the complete IO interface including power supply and the canal inputs are packed into 15 Pin D-Sub Connector, I would like to send the signals directly from the encoder itself. I hope this answers your question. \$\endgroup\$ – Robionic Jan 11 '18 at 14:09
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If the camera needs 5 volts and takes up to 50 mA then supply it with 5 volts and even if the supply is capable of proviging 140 mA (or more) it doesn't matter because the camera will take only the current it needs to operate. I expect that the 140 mA figure is a maximum rating for the device i.e. it is capable of supplying 5 volts to a load than can take up to 140 mA.

Regards your comment, this contradicts what your main question body appears to say so please provide a link to the camera interface card data sheet so this contradiction can be resolved.

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  • \$\begingroup\$ That is correct, 140mA is the maximum rating for the device. Please find the link in the question.The extra comment was made in reply to a person who had answered, but made a decision to revert back his answers. \$\endgroup\$ – Robionic Jan 12 '18 at 8:25
  • \$\begingroup\$ I don't have an account with whoever that link was for the data sheet. \$\endgroup\$ – Andy aka Jan 12 '18 at 11:24
  • \$\begingroup\$ I have added the IO Board specs in my question \$\endgroup\$ – Robionic Jan 15 '18 at 9:23
  • \$\begingroup\$ I would need to see a full data sheet to resolve this. \$\endgroup\$ – Andy aka Jan 15 '18 at 10:07
  • \$\begingroup\$ Datasheet is available here and the Digital IO Port configuration is in page 38 \$\endgroup\$ – Robionic Jan 15 '18 at 12:23

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