A single LED on an dual DC supply?

[teleman]

I've searched in text books & googled but coudn't find any information on connecting a bi or tri color LED to a dual (+/-) dc supply as a power indicator to show the status of both supply lines.

All equipment using LED power "on" indicators seem to use only a single LED connected either to the positive (or negative) dc line.I wonder if it's at all possible to use even a tricolor LED to achieve this?

Any "bright" ideas?

Thanks

 

[sofaspud]

It could be done, but you'll need to find a LED with the common lead connected to the cathode of one color and the anode of the other color.

 

[jan.didden]

You can also do it with a regular dual LED (which will have the cathodes common) with the cathodes to V- and either color a resistor to gnd to show neg voltage and the other resistor to +V to show that.

jan

 

[sofaspud]

That would work, but if you lose V- don't both colors then turn off?

 

[Robert Kesh]

Get a voltage between 0 and V- using a couple of resistors voltage divider, making sure enough current flows to power LEDs through it, and use this as your common cathode, with anodes to V+ and Ground via resistors.

You will have to jiggle with the resistors a bit, as when V- dies, it will affect brightness of V+ LED.

 

[DF96]

With a transistor or two to act as switches you can have whatever arrangement of colours you want. Not the sort of thing that textbooks are interested in, as there are no new principles involved. Decide what you want, then design a circuit to do it. Bi-colour and tri-colour LEDs are available with different wiring options.

 

[teleman]

Thank ye all for the excellent suggestions! Following "Sofaspud's" suggestion, I just found this 4 lead/3 color LED by "Ming Optics" on

www.mpja.com with the part no. 171370P

Detailed Description

5mm (T1-3/4) Tri-Color RGB LED. Transparent, diffused case. Common Anode with separate Cathode for each color. Red630/Green525/Blue430nM, R200/G300/B150mcd Min. output. Max forward current 20mA continuous. Max Forward Drop: R2.5/G4.5/B4.5V. Max reverse Voltage =5VDC. 30deg. viewing angle.
1" leads

1: Red
2: Anode Longest
3: Blue
4: Green


However, the "Ming" data sheet is showing a different lead configuration! for the same part no.! ie:17137?..(without the last 2 figures!)

1.Green
2.Blue
3.Anode/Cathode
4.Red

Do you think these 2 parts are the same?

 

[teleman]

Here is the link to the seller/part;


http://www.mpja.com/prodinfo.asp?number=17138+OP

 

[teleman]

17137-0P is a common anode LED while 17138-0P is a common cathode device!

 

[sofaspud]

Multicolor LEDs are usually available as both CC and CA. The cathode-to-anode types seem to be only 2-lead parts. So I can't think of an elegant way to do this. My drawing of Robert's suggestion will work, and here's an alternate way adding a 1N4148 diode that functions similarly. Otherwise, DF96's suggestion of some type of logic switching seems like the next step up.

 

[Elvee]

Multicolor LEDs are usually available as both CC and CA. The cathode-to-anode types seem to be only 2-lead parts. So I can't think of an elegant way to do this. My drawing of Robert's suggestion will work, and here's an alternate way adding a 1N4148 diode that functions similarly. Otherwise, DF96's suggestion of some type of logic switching seems like the next step up.

The circuit on the left will not work. The one on the right will, but it has a flaw: both LEDs will light as soon as the positive supply is present.
Here are two examples: the left one is based on common cathode LEDs, and they will light if and only if their respective supply is present.

On the right is the two-terminal version: the AC voltage from the transformer serves as a switching source.
The LEDs will light only if the AC voltage AND their respective supply is present.

 

[sofaspud]

The circuit on the left will not work. The one on the right will, but it has a flaw: both LEDs will light as soon as the positive supply is present.

I'm not seeing why the first circuit wouldn't work. Is the voltage difference too great?
For the second circuit, if the negative line is open why would that LED light? Something downstream? I had it figured that the change in color would be the fault signal. The flaw was that the third condition only caused a brightness change, not a color change.
You have interesting solutions. I had considered a variation of the left circuit, then began to wonder if the different Vf's of a RGB LED could be utilized to show three status colors.

 

[Robert Kesh]

I'm not seeing why the first circuit wouldn't work. Is the voltage difference too great?

You didn't put any resistors in series with the individual LEDs, as I suggested in my original post. Any extra components are not necessary however.

 

[sofaspud]

I drew individual LEDs only for clarity. It's supposed to be a single common-cathode bicolor, with the LED current flowing through "R2". I'm not seeing how putting individual resistors on the anode side makes all the difference.

 

[Robert Kesh]

I'm not seeing how putting individual resistors on the anode side makes all the difference.

In your circuit, what's the voltage drop across each LED?

 

[sofaspud]

It depends on the LED. Let's say 2 volts each.

edit: Ahhh...OK. That's what I meant by the voltage difference in post #12. That does throw things into a mess.

 

[Robert Kesh]

.

 

[Robert Kesh]

Only problem is the circuit will draw a lot more current than normal LEDs and one line dying will affect the brightness of the other LED a bit.

 

[teleman]

Just my 2 cents worth...will any of these combinations help?

The 1st LED is a 2 anode + 2 cathode device.....forgot where i found it! Is there anway the 3rd circuit could be modified for a +/- supply?

 

[sofaspud]

Only those that fit into one of the previous submissions.
I can add another: two individual LEDs side by side connected with heatshrink to a light pipe on the front panel. That should provide a 4-state psu status.