×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Swapping Incandescent for Telephone-Slide LEDs in Panels

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?

RE: Swapping Incandescent for Telephone-Slide LEDs in Panels

It is not that complicated, LED could be rated at 24Vdc and has 10ma draw, so if you place it in replacement of your bulb, it will draw 10ma through the resistor and trip coil.

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

Just buy the 125VDC LED and don't over analyze the problem. They will have sized the resistor to limit the current to a manageable level for the LED lamp, 38mA in that particular case (but it will be slightly more since a 125VDC system rarely has a voltage that low). Add in the trip coil and the current will be just a bit less; far less than enough to operate the trip coil.

RE: Swapping Incandescent for Telephone-Slide LEDs in Panels

Slightly off topic, but we've been really happy with the Electroswitch control switches with integral red/green LEDs. We are specifying them for new panels and retrofits.

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources