Calculating MAX485 DMX universe driver current draw

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DGK

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I am trying to find out how much current a 5V MAX485 RS-485 transceiver will draw when driving a DMX universe of about 5-8 RGB LED floods.
I have a hard time understanding the MAX485 data sheet where it discusses the driver current. https://datasheets.maximintegrated.com/en/ds/MAX1487-MAX491.pdf
My plan is to use 6-8 DMX outputs controlled by the Teensy 4.1 serial outs, using Shawn Silverman's TeensyDMX library.
The circuit will have an external power input for the optocouplers and the MAX485's but to know how much current it needs to supply I need to know about the MAX485's draw.
Maybe someone can help me understand this issue.

Thanks
David
 
Current demand on a RS485 net is usually driven by the termination resistors. The transceivers don't require enough to matter much.
 
Do you mean that the current required by the MAX485's to translate the input signal to the differential output signals isn't enough to matter much?
Isn't the current demand on the RS485 net to be met by the MAX485 supply? If so how can I calculate this demand?

Current demand on a RS485 net is usually driven by the termination resistors. The transceivers don't require enough to matter much.
 
If you have 100 Ohm termination resistors on each end of the cable, then driving 5V into that will require 100mA. Compared to that, everything else is small change.

The driver will not actually source 5V into that load. The minimum is 1.5V and the maximum is 5V so assume the worst case (from a power perspective) and use 5V.

It sounds like you should read "The Art and Science of RS485" by Bob Perrin. An old Circuit Cellar article but a pdf can usually be turned up by Google.
 
Thanks for your explanation! My brain was almost there, now even closer and I found the article to arrive at full understanding.
For those interested in the article: https://www2.htw-dresden.de/~huhle/ArtScienceRS485.pdf


If you have 100 Ohm termination resistors on each end of the cable, then driving 5V into that will require 100mA. Compared to that, everything else is small change.

The driver will not actually source 5V into that load. The minimum is 1.5V and the maximum is 5V so assume the worst case (from a power perspective) and use 5V.

It sounds like you should read "The Art and Science of RS485" by Bob Perrin. An old Circuit Cellar article but a pdf can usually be turned up by Google.
 
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