DMX Terminator Question

[ejsandstrom]

I have found the thread here about the home made terminators with the LED's.

I tried to build them 2 ways.
1. Parallel 2 270 resistors and a parallel Bi color LED. Resistance from pin 2 to 3 is 130 ohms and the LEDs seem to act properly. The ohms read correctly and the LED works fine.

2. Resistors are soldered together in pin 3. R1 is soldered to pin 2. The other side of R2 is soldered to 2 of the pins on the LEDs and then the other 2 pins for the LEDs are soldered in pin 2. The LEDs appear to work correctly, but the resistance between pin 2 and 3 is 260 Ohms. This is how they tell you to build them in the online page. This does not seem correct to me.

What am I missing? Thanks for the input.

Diagram to hopefully make it easy to understand.
http://www.lucidchart.com/invitations/accept/52a4e93a-7a38-4d5b-afae-76430a009f85

 

[MNicolai]

See here for a tested and true schematic. Appears to be what your 2nd build option matches.

With no voltage present, you'd ideally get 270Ω between pins 2/3 as the ohmmeter will only read the resistor directly connected between those pins. 260Ω is an acceptable value as resistors are built within a certain tolerance which is stated in the resistor color code. For the purposes of this, let's assume your resistor is a tolerance of +/-5% (which would be a gold band in the color code). In that case, the acceptable range of values for each of your resistors is 256.5-283.5Ω. Thus, your resistor reading 260Ω is built within the stated tolerance range.

The reason your ohmmeter will not read the bi-color LED and resistor in series with it is that LED's require a minimum bias voltage before the LED will activate and allow current to pass. With no voltage present in the circuit, current does not have a path through the LED's, thus your ohmmeter connected to pins 2/3 will not include measurement of the resistance of the bi-color LED nor that of the resistor in series with the LED.

 

[JD]

Yea, you don't want that LED directly across pins 2 & 3. The second diagram would be the one to use.
Although the receivers look for "any" difference, your transmitter (the board) would have it's signal loaded down too far. You can't really consider an LED to have a specific resistance because if you meter it, you only read a number that is the equivalent of the drop occurring in the Wheatstone Bridge in the meter, not an actual resistance.

 

[ejsandstrom]

I was using the Big Clive diagram. But this makes more sense, I have to apply voltage to turn on the diode and then the other resister will be in the circuit. Kind of like trying to get an ohm reading on a diode, it reads as OL until it has voltage applied to it.

 

[JD]

Exactly. If you use a meter that runs off a 1.5v battery, it will read "open." If the meter has a 9v battery then you will get some arbitrary reading. Regular diodes drop about .5 volts across the junction. LEDs are more like 1.5 volts. High power LEDs are a bit more as they stress the junction to get that kind of output. In any case, the value on a meter will only tell you the diode isn't blown open.

Although the tester in the second figure works well, treat it with caution. If your DMX goes whacky, replace it with a regular terminator. When the diode switches on it produces a hard edge which adds noise to the line. If the signal is strong, shouldn't be a problem. However, in marginal cases, this may be the "straw."