# DMX Decoder problem?



## Macabree (Aug 1, 2011)

Quick question. Is there any easy way to reverse the polarity in a dmx decoder. I have a common cathode lights and this is set up to have a common anode. Please be gentle on me, I hate dc current and leds!!


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## corey872 (Jan 10, 2010)

Assuming you can't just 'program it in' or flip a switch to invert the output, I suspect there are various circuits which could be used to make the inversion. [these would vary depending if you're looking for simple 'on/off' operation or something closer to a PWM / fading output.] But if you're going to seek instructions, buy components and assemble an inversion circuit, why not just buy common anode LEDs, wire directly in and be done with it?


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## hedg12 (Jul 6, 2008)

You could use an NPN transistor for each LED.








Something like the MPS2222A might work - depends on the load. I have an explanation of this application here - hope it helps.


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## David_AVD (Nov 9, 2012)

Most DMX DC controllers use NPN transistors or N-CH MOSFETs to do "low side" (switched ground) outputs and are used with common anode LED strings.

To use such a controller with common cathode lights, you'd need to convert it to "high side" switching. This requires the addition of one PNP transistor and two resistors per channel.

Unless the lights sets you have are particularly special, it would be simpler and cheaper to just replace them with common anode types.


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## pshort (May 6, 2008)

The circuit that Hedg12 shows makes the opposite conversion from what the OP asked for. David_AVD is correct, you need use a PNP transistor instead (with emitter and emitter resistor to +V instead of to ground, and the load between the collector and ground)


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## David_AVD (Nov 9, 2012)

I did a quick drawing of the circuit required, but can't seem to upload it here. (possibly because I'm < 10 posts)


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## HomeyDaClown (Oct 3, 2009)

Macabree said:


> Quick question. Is there any easy way to reverse the polarity in a dmx decoder. I have a common cathode lights and this is set up to have a common anode. Please be gentle on me, I hate dc current and leds!!


There are LED amplifiers/repeaters that will convert Common Anode input to Common Cathode output. They require 12 volts DC (wall wart) but are easy to wire up.

Here's an example:
http://www.electricalproducts.com.au/ele-6667-common-anode-to-common-cathode-led-power-repeater.html

Found a PDF Data sheet here with details for using with DMX controllers:
http://e-starled.com/resources/admin/uploads/brochure/brochure_1339257360.pdf


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## hedg12 (Jul 6, 2008)

pshort said:


> The circuit that Hedg12 shows makes the opposite conversion from what the OP asked for. David_AVD is correct, you need use a PNP transistor instead (with emitter and emitter resistor to +V instead of to ground, and the load between the collector and ground)


D'oh! Thanks Phil, not really sure what I was thinking.
(I really shouldn't drink and post - even if it is only iced tea...)


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## David_AVD (Nov 9, 2012)

HomeyDaClown said:


> There are LED amplifiers/repeaters that will convert Common Anode input to Common Cathode output.http://e-starled.com/resources/admin/uploads/brochure/brochure_1339257360.pdf


I hadn't seen those before. Would be a bit expensive if you had to convert a lot of channels.

I designed a 9 channel high side switching (common cathode) DMX controller a while ago to work with some oddball LED strips.


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