# Thread: Neon indicator lamp question

1. ## Neon indicator lamp question

Hi folks,

I am in the process of building a new power supply for an upgrade I am carrying out on my CNC router. I decided I wanted an indicator lamp on the power supply to show when it was powered up and picked up a couple of neon indicator lamps from Radio Shack yesterday (the type with the built in resistor). Unfortunately there were no wiring instructions on the package and extensive searching on the web has not turned up any useful information so I am hoping someone on here has an answer for me.

I have wired neons on a power supply for a ham radio but it was a long time ago and I can't remember now just how I did it . Does the neon go in series with the 'hot' wire after the switch or does it go in parallel between the 'hot' wire and neutral after the switch?

My gut feeling is telling me that neons were always wired in parallel whereas incandescent bulbs and LEDs are wired in series but I would like to double check before heating up the soldering iron. Any suggestions greatly appreciated.

Larry

2. Generally they are wired across the AC line....just tells you that AC is applied....I would opt for LEDs on the Power Supply output for an indication that the outputs are present....

3. Originally Posted by ViperTX
Generally they are wired across the AC line....just tells you that AC is applied....I would opt for LEDs on the Power Supply output for an indication that the outputs are present....
Thanks for that. I have an LED on the output from the current powersupply but the new one is going to be putting out somewhere between 30V and 44V DC (depending on what transformer I can find) and that is a lot of voltage to drop for an LED. I could drive an LED off the board I guess if I can find a suitable place to tap off 5VDC.

Larry

4. ## Use an LED

To use an LED on any voltage use the following equation:

AC or DC voltage divided by .010 gives a resistor value in Ohms.

Put the resistor in series with the LED.

Example 48 Volts divided by .010 = 4800 Ohm Resistor

4800 Ohms is not a standard value, but 4700 is standard. Use 4700 Ohms.
If LED burns TOO bright - increase resistance - If too dim - decrease resistance. The enemy of LED's is TOO much current - they smoke.

On some of the neon lights sold by Radio Shack, you will want to check the voltage. These neon lights are known as "grain of wheat neon's". You can change the resistance to work with almost any voltage.

Hope this helps.
Jerry

5. Thanks Jerry, I was familiar with the calcs for LED resistors and have a number of online calculators bookmarked that do just that. My biggest concerns about using a LED were the heat generated in the resistor to drop from 35-40V to around 2V for the LED and the fact that I could be drawing as much as 10A from the supply (don't want to let that magic smoke out ).

The neons are the small panel mount type with the built-in resistor and they are rated at 110V.

Larry

6. Originally Posted by The Wizard
My biggest concerns about using a LED were the heat generated in the resistor to drop from 35-40V to around 2V for the LED and the fact that I could be drawing as much as 10A from the supply (don't want to let that magic smoke out ).
An LED typically draws 10 milliamps and therefore so would a resistor wired in series with it (current is constant in a series circuit). The LED drops 1.5v leaving the resistor to drop 46.5v in the 48v supply example. Using R=E/I (Ohm's Law) R=46.5/.01=4650 ohms. The power loss using a 4700 ohm resistor would be P=I^2*R. That would be .01^2*4700=.47 watts. Use a 1/2 watt resistor. The resistor/LED series combination should be placed across the output terminals.

Chris

7. Thanks for that info Chris.

Larry