Is it "circuit safe" to use signal diodes on the cathode of a Bi-Led?

Question

I have a 3 pin Bi-LED, two anodes and one cathode. I am using it with two transistors, each transistor controls a anode. The only way I can sucesfully use this Bi-LED is if I attach 2 signal diodes to the cathode (gnd), thus creating two seperate cathodes, connecting them to the collector of the transistor. How will this effect the formula in choosing a resistor on the Bi-LED's anodes? I would assume that a smaller resistance would be chosen. It should be noted that one of the transistors are also powering a

150mA relay.
150R x 2 Bi-LED V+ ---------/\/\/\/\/---------==|\___ Diode1 V+ ---------/\/\/\/\/---------==|/ \---|\___TO COLLECTOR OF TRAN 1 \ \ \ | |/ | | Diode2 |--|\___TO COLLECTOR OF TRAN 2 |/ Heres a readable schematic:

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Walter Harley · Accepted Answer

As far as I can tell, your readable schematic is incorrect. Your above statement ("each transistor controls an anode") is not true, and indeed the circuit does not appear to do anything useful at all.

I would suggest re-posting on sci.electronics.basics to discuss the details of your circuit.

With specific request to your question about switching an LED, though: the voltage drop across a forward-biased signal diode is in the vicinity of

0.7V. (More or less depending on current and temperature, but 0.7V is a good rule of thumb.) The voltage on the resistor will be (5V - Vled - Vdiode - Vce), where Vce will be about 0.4V and Vled depends on the color of the LED.

Eeyore · Answer

No, that won't work !You'll need to switch the anodes.Whatever is Q2 meant to be doing btw ?Graham

JedOs86 · Answer

I fixed it, I am going to instead using a common anode Dual-LED

Is it "circuit safe" to use signal diodes on the cathode of a Bi-Led?

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