Using a bi-colour LED with separate Cathode

Thread Starter

1CM69

Joined Jun 12, 2017
56
Hi,

I am working on this: http://www.ti.com/lit/an/snva190b/snva190b.pdf circuit and have added two LEDs to show the current state.

A green LED that is only powered when the circuit has not been tripped & a red LED that is only powered when the circuit has been tripped.

See image IMG_0073.JPG & IMG_0074.JPG

I would really like to be able to use just a single bi-colour LED in place of the two but cannot find a common connection between the two LEDs.

Is there a way around this to utilise a bi-colour LED?

Thanks
 

MrChips

Joined Oct 2, 2009
23,955
A 2-terminal bi-colour LED has the two LEDs wired anode-to-cathode.
You need something like an H-bridge in order to be able to reverse the current.
Something like this, or two inverters/buffers that you can drive with complementary signals:

 

Thread Starter

1CM69

Joined Jun 12, 2017
56
A 2-terminal bi-colour LED has the two LEDs wired anode-to-cathode.
You need something like an H-bridge in order to be able to reverse the current.
Something like this, or two inverters/buffers that you can drive with complementary signals:

I forgot to mention in my initial post that I have some bi-colour LEDs, they're the 3 lead type using a common cathode & an anode each for the Green & the Red
 

Thread Starter

1CM69

Joined Jun 12, 2017
56
Then use the 3-lead LED. That will be simpler to get working.
I've been trying to use them but there in no common connection between the two existing separate LEDs, see the schematic in the PDF link I posted along with my diagrams showing where the current LEDs are placed in the circuit.

I have read about the possible use of a NAND gate but I know nothing about how these work or how they would fit the circuit
 

MrChips

Joined Oct 2, 2009
23,955
Your question is really about how to use the LMV431 precision shunt to turn on an LED in a circuit where the LED cathode is connected to GND.

Use a PNP transistor such as 2N3906 to drive the RED LED.
Select R3 and R4 values depending on your supply voltage.

Red-Green LED.png
 

Thread Starter

1CM69

Joined Jun 12, 2017
56
Your question is really about how to use the LMV431 precision shunt to turn on an LED in a circuit where the LED cathode is connected to GND.

Use a PNP transistor such as 2N3906 to drive the RED LED.
Select R3 and R4 values depending on your supply voltage.

View attachment 131044
OK, that's great thanks.

Are the R1 & R2 resistors in your diagram extra or are they the ones already shown in the PDF I posted in my opening post?
 

MrChips

Joined Oct 2, 2009
23,955
R1 is your resistor. R2 has been added to limit the base current to Q2. 1k to 10k should work.
What is your maximum supply voltage?
 

Thread Starter

1CM69

Joined Jun 12, 2017
56
If your input voltage is 13.8V then the circuit will never be able to trip at 14.17V.
You want to think is through over again.
But my circuit does work, I have it working.

The schematic in the PDF is set for 19.2v trip, I adjusted the divider for my particular purpose.

4.75K & 49.9K resistors in place of the 4.7K & 68K as shown in PDF

Sorry I was not clear on this point.

Regards..,
 

MrChips

Joined Oct 2, 2009
23,955
But my circuit does work, I have it working.

The schematic in the PDF is set for 19.2v trip, I adjusted the divider for my particular purpose.

4.75K & 49.9K resistors in place of the 4.7K & 68K as shown in PDF

Sorry I was not clear on this point.

Regards..,
Ok. I misunderstood the function of your circuit.
The supply voltage is nominally 13.8V and the GREEN LED is normally ON.
You want the RED LED to turn ON as well if the supply voltage rises above 14.17V.
 

Thread Starter

1CM69

Joined Jun 12, 2017
56
Ok. I misunderstood the function of your circuit.
The supply voltage is nominally 13.8V and the GREEN LED is normally ON.
You want the RED LED to turn ON as well if the supply voltage rises above 14.17V.
I want the GREEN LED to go out and the RED LED to illuminate when the circuit trips.

You can see that I do have this functioning using separate LEDs by the two sketches I uploaded but would really like to use just a single 3 lead LED in their place.
The issue I am finding is I cannot find a common cathode connection for the 3 lead LED to connect to.

Regards
 
I want the GREEN LED to go out and the RED LED to illuminate when the circuit trips.

You can see that I do have this functioning using separate LEDs by the two sketches I uploaded but would really like to use just a single 3 lead LED in their place.
The issue I am finding is I cannot find a common cathode connection for the 3 lead LED to connect to.

Regards
Wouldn't you simply connect the cathode to ground? Since it looks like you have the red led already drawn that way in your sketches, and you say your circuit already works it would make the most sense I would think. Or is there some sort of switching of the green led cathode you don't have drawn that would make more work? I kind of think you seem to be overthinking things if it already works with seperate leds.
 
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Thread Starter

1CM69

Joined Jun 12, 2017
56
Wouldn't you simply connect the cathode to ground? Since it looks like you have the red led already drawn that way in your sketches, and you say your circuit already works it would make the most sense I would think. Or is there some sort of switching of the green led cathode you don't have drawn that would make more work? I kind of think you seem to be overthinking things if it already works with seperate leds.
As in my sketches, both LEDs work separately.

The GREEN is on when power runs through the circuit and then is off when the circuit is tripped, as it should.

At the moment the RED LED is off with power running through the circuit and comes on when OV causes the circuit to trip, again as it should.

The GREEN LED does indeed have a connection to ground.

I have tried connecting the RED KED to ground also & while this does illuminate the LED, it is now on constantly no matter the state.

I need to find a common connection for both LEDs where the RED only receives power after the circuit trips.

I have tried connecting the RED to ground and then in turn, tried connecting the other RED lead to all points on the LMV431 & both Q1 & Q2 without any luck.

I could just stick with two seperate LEDs but it would be nicer to have the one in their place.
 
What about using a P channel FET with the gate connected to your 'D' in your sketches to drive the red LED. If 'S' is always powered your led will always be on. It shouldn't make a difference if you're using a two color or individual LEDS... it seems there's something else going on. Is it possible that the single red LED has different voltage and current ratings that cause it to appear off while the combined LED appears on?
 
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Thread Starter

1CM69

Joined Jun 12, 2017
56
Just been fiddling with this circuit again, trying to figure this out & didn't get anywhere but here are two sketches outlining the various connecting points in the POWER ON & TRIP states.

IMG_0083.JPG IMG_0084.JPG

I thought I had found a common connection on the ANODE of LMV431 and then connecting the POWER ON LED to D on ZXMP6A13F

and the TRIP LED to S on ZXMP6A13F

Doing this though caused the POWER ON LED to stay energised after the TRIP voltage level & most importantly the circuit did not TRIP, output voltage was able to continue to rise above set TRIP level.

It would be really nice to solve this, although my fallback plan is using two rectangular LEDs glued together to save real estate & connecting up the way I know does work.
 
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