Current draw for a DMX 'universe'

Other Hardware General Electronics
1

Been trying to work out how you determine the 'load' of a DMX circuit.

I have an ESP32, driving a Max481 via an opto-isolator. This gives me basic DMX control output using an Arduino Library.

I intend to have up to 5 of these individual 'Universes' running off a bigger processor (probably a Teensy 4.1).
How do you determine the potential load to the system of each DMX universe? I am assuming this might have something to do with possible cable length and the EOL resistor? (130r).

I would assume the decoding of each light fitting would be negligible? Who knows!

300m is the maximum recommended run of DMX before requiring a booster.

The system all runs on 5v. Currently I have a 600mA PSU. I would like bigger, but that is all I can fit in my enclosure.

I suspect that isn't big enough for 5x Universes.
The drive circuit itself + processor won't be much.

Hmm. Me 'n' maths don't mix

Quickly drew this using an online circuit which is very similar to mine:

DMX transmit
DMX transmit1220×483 9.53 KB

I suppose I could wire one up and monitor the load on the PSU when I add a DMX fitting? Hmm

2

DMX uses RS485 as the physical layer. RS485 specifies that a unit load is 12KΩ and you can have up to 32 of them. Plus you have two 120Ω terminations. So the total bus load (all those resistors in parallel) would be about 52Ω.

With a 5V supply the current would be 5/52 = 96mA

3

Jim... I wish I could get my head around this stuff!

So my 600mA isn't going to run 5x Universes (or it's a bit near the limit).
I am going to see if I can source a larger PSU that will till fit in the enclosure.

Many thanks Jim

4

96mA is the worst case . 96x5=480mA If your 5V 600mA supply only provides current to the MAX481 ICs then it will probably be OK

5

Does anyone know if this RS485 transceiver is a direct drop in replacement for a Max481?

Been trying to compare the data sheets and my mind is frazzled.

I refuse to pay £4 for something when there might be a cheaper option available (plus I need 10 of them)

6

The MAX487 is much slower, only 250Kbits.
The SP481 looks to be the same.

7

Thanks Jim. Fresh eyes and all that.... been looking at datasheets for 3 days. Can't see wood for trees TBH