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I'm in the process of building a DMX bus repeater for a light control installation using the LTC490 bus driver in this configuration:

enter image description here

Now, this works well when using an external power supply (5V DC), but I was thinking if it would be possible to harvest some power using the DMX+ line of the input to power the rest of the device.

Two questions:

  1. Is this advisable or even possible?
  2. How should I do it?
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  • \$\begingroup\$ You will have to add extra circuitry. Capicitors and pull up resistors. this will cause the communications to be almost crippled as it has to wait for the caps to charge and send a small burst of information.. and it will have to be synchronised on all IC's! Just use an extra wire for your +5v if possible. \$\endgroup\$
    – Piotr Kula
    Jun 8 '11 at 11:10
  • \$\begingroup\$ I know that, I've made parasitic power with 1-Wire interfaces. You don't have to do what you're saying, when the signal is pulled up. RS-485 is a differential bus, with D+ being +3V to + 7V \$\endgroup\$
    – polemon
    Jun 8 '11 at 19:17
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Unlikely to be accepable, DMX permits a total load on the line of 32 unit loads where an RS485 unit is a 15k resistor to -3V or 5V and the LTC490 could draw more current than that, depending on its load. Although the datasheet quotes the supply current as only 500uA max, that is on no load and it could be up to 250mA if driving into a short circuit.

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  • \$\begingroup\$ Almost what I expected. I'll stick to the solution with an external power supply. Thanks! \$\endgroup\$
    – polemon
    Jun 8 '11 at 19:19
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While a device could, in principle, parasitically harvest some power from the input RS-485 signal wires and use it to drive its output RS-485 wires, a device can't ever transmit more power than it received.

So with 2 wires, a parasitically-powered repeater can't ever transmit a stronger continuous signal than simply passing the signal through with wire.

You might look at power over ethernet (PoE): Wikipedia: Power over Ethernet and Electronics Stack Exchange: PoE.

PoE "DC on Spares (mode B)" and USB and many other protocols send power through the same cable, on a separate pair of wires.

PoE "mode A" magically transmits power over the same wires while it simultaneously transmits data, which sounds similar to what you are trying to do. (Is it possible to somehow adapt the trick they use to your system?)

I suppose in principle another alternative is to parasitically collect power while the lines are idle. Later transmit that power in a brief burst when the occasional packet comes through. That might work OK squaring up edges that become too rounded off over long distances, as long as you give it time between packets to recharge, like the one-wire bus.

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I wouldn't recommend even trying as especially on the cheaper DMX kit the standard can be implemented quite loosely, perhaps the way would be to do it the way scrollers do it and put 24v down the unused wires in a dmx installation.

Personally though, if the cable is long enough to use a repeater I wouldn't, I would instead carry it across ethernet and/or fibre.

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