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

<JedOs86@xxxxxxxxx> wrote in message
news:1155430685.589551.318860@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
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:
http://www.myfilehut.com/userfiles/155002/BiLED.JPG


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.


.

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