Swapping Incandescent for Telephone-Slide LEDs in Panels
Swapping Incandescent for Telephone-Slide LEDs in Panels
(OP)
I'm trying to swap out some of the older incandescent bulbs for LEDs on relay panels to improve the life span. The control circuit is 125VDC. The incandescent lights have a series resistor. The light and resistor are wired in parallel with the trip contacts and this parallel section is wired in series with the trip coil. So, if the trip contacts close, the voltage across the light becomes zero and it turns off and the current flows through the trip contacts to the trip coil which opens the breaker.
Under normal conditions, I'm assuming that the full 125VDC should be dropped across the light/resistor combo since the resistance of this is much higher than the trip coil. The current flowing through the trip coil to neagtive DC is limited by the resistance of the light filament and series resistor, so it is a very small amount that doesn't cause tripping.
In this original circuit with the incandescent bulb (28V rating), how was the series resistor calculated? Did it require looking at the resistance of the incandescent filament at steady-state temperature and calculating the correct resistor needed to drop the ~28V across it on a 125VDC circuit?
This bit of confusion is making it difficult to order replacement LEDs. I know most isolated LEDs drop between 2-4V depending on wavelength. To calculate the series resistor, you subtract the forward voltage drop of the diode from the supply voltage and divide the remainder by normal operating current of the LED. The LEDs I'm looking at are shown here:
http://www.ledtronics.com/Products/ProductsDetails...
I'm looking at the 2nd bulb down which is rated for various voltages including 125VDC. With 125VDC, it shows a series resistor of 3300 ohms. How is this calculated? Are they subtracting the LED voltage drop from the control circuit voltage and dividing by the rated LED current?
Under normal conditions, I'm assuming that the full 125VDC should be dropped across the light/resistor combo since the resistance of this is much higher than the trip coil. The current flowing through the trip coil to neagtive DC is limited by the resistance of the light filament and series resistor, so it is a very small amount that doesn't cause tripping.
In this original circuit with the incandescent bulb (28V rating), how was the series resistor calculated? Did it require looking at the resistance of the incandescent filament at steady-state temperature and calculating the correct resistor needed to drop the ~28V across it on a 125VDC circuit?
This bit of confusion is making it difficult to order replacement LEDs. I know most isolated LEDs drop between 2-4V depending on wavelength. To calculate the series resistor, you subtract the forward voltage drop of the diode from the supply voltage and divide the remainder by normal operating current of the LED. The LEDs I'm looking at are shown here:
http://www.ledtronics.com/Products/ProductsDetails...
I'm looking at the 2nd bulb down which is rated for various voltages including 125VDC. With 125VDC, it shows a series resistor of 3300 ohms. How is this calculated? Are they subtracting the LED voltage drop from the control circuit voltage and dividing by the rated LED current?






RE: Swapping Incandescent for Telephone-Slide LEDs in Panels
what ever resistor you have, its power rating should be minimum I^2r. The voltage across the trip coil plus resistor will be supply - 24vdc. Division of voltage will be as per its DC resistances.
The main thing is to have a resistor in case the bulb housing / socket short circuit and supervisory current on the trip coil is limited by the resistor and does not operate the breaker when closed.
RE: Swapping Incandescent for Telephone-Slide LEDs in Panels
RE: Swapping Incandescent for Telephone-Slide LEDs in Panels