need to build a opto-isolator for my dmx interface

Thread Starter

Jacquesl

Joined Dec 9, 2006
29
Hi, I need to build a opto-isolator for my DMX interface, it's a homemade oksidizer USB to DMX512, and it doesn't have a opto-isolator in it, and I have a problem, my dmx adapter freezes up because one of my DMX lasers makes a spike in the chain on Pin1(ground) and Pin3(positive), and therefor it makes my dmx adapter freeze up heavy.....:mad:
I do have a terminator.;)
So have anyone done this before..?

EDIT: So I'll need to have a opto-isolator on DMX out line, it's like a DMX splitter "witch have a build in opto-isolator in", but buying a splitter is extremely expensive and unnecessary
 
Last edited:

Norfindel

Joined Mar 6, 2008
326
I don't know what's a DMX, but i understand that the problem seems to be the USB power, so what about using a powered usb HUB and connect it there? Or make a sort of usb bridge that powers the device from an external supply?
 

Thread Starter

Jacquesl

Joined Dec 9, 2006
29
I don't know what's a DMX, but i understand that the problem seems to be the USB power, so what about using a powered usb HUB and connect it there? Or make a sort of usb bridge that powers the device from an external supply?
Ok, DMX is a protocol that is used to control DMX DJ Lighting, and my adapter is a USB to DMX convertor/adapter, so...hehe
 

bertus

Joined Apr 5, 2008
21,896
Hello,

The availability of components strongly depends on where you are located.
Where are you located? please update the location in the profile.

Bertus
 

JDT

Joined Feb 12, 2009
657
Do you want a transmitter or a receiver circuit?

Here is a receiver circuit that I have designed and used. Uses the 75176 but there are quite a few alternative I.Cs that you can use.
http://uk.farnell.com/national-semiconductor/ds75176bn/bus-transceiver-75176-dip8/dp/9487190

Try SP485E:-

http://uk.farnell.com/exar/sp485ecp-l/transceiver-rs-485-dip8-5v-485/dp/9387161

I get the isolated +5V supply by using a very small DC-DC converter:-

http://uk.farnell.com/xp-power/ie0505s/converter-dc-dc-1w-5v/dp/8727694

The circuit diagram shows input to a PIC microcontroller.
 

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Thread Starter

Jacquesl

Joined Dec 9, 2006
29
The one I need is a sender, from dmx dongle to dmx lights, I see the circuit diagram is for a receiver, I'll check it out, Have to go now
 

JDT

Joined Feb 12, 2009
657
Basically the sender is much the same. Same driver and opto-coupler and DC-DC converter - signal goes the other way.

Note that the sender (transmitter) has a terminating resistor across the pair, normally 120R. At the receiver end there are normally two connectors (always 5-pin XLR on "quality" equipment) so that the line can be looped to other receivers or a terminating plug fitted.

In my opinion, isolation is essential. If you are building a USB to DMX you need to protect your computer!
 

Thread Starter

Jacquesl

Joined Dec 9, 2006
29
interesting, I've got a terminator that is 100 Ohms, is that also ok? Currently to avoid that spike coming in my adapter, I'm running my dmx dongle without a pin1 attached, "Ground" and it work without any problems, is that safe?
 

JDT

Joined Feb 12, 2009
657
The value of the terminating resistor is not that critical. Open circuit causes reflections that can corrupt the signal.

Running without ground connected risks damaging the receiver/driver chip. It is not isolated because there are on-chip diodes that prevent the signal from going past the supply rail voltages. In fact, many circuits have zener diodes between the signal wires and pin1 to protect the chip.
 
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