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I have this momentary push button with built in LED. I cannot figure out whether it has a built in series resistor.

How can I check if it already has a series resistor without damaging it? If I use 12V to power it how can I size a series resistor? The voltage drop of it is not given.

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  • \$\begingroup\$ Due to the voltage rating of 12V and a contact rating of 36V, I'd assume it has a built in current limiting resistor suited for 12V operation. \$\endgroup\$
    – Klas-Kenny
    Commented Oct 10, 2022 at 11:08
  • \$\begingroup\$ You'd think for the princely sum of fifteen quid they could bother to copy more than the absolute minimum dimensions from the Chinese datasheet. Sad! \$\endgroup\$
    – Spehro 'speff' Pefhany
    Commented Oct 10, 2022 at 11:53
  • \$\begingroup\$ To make matters worse, there is no product on the datasheet that is iluminated and has a 12mm body. I do not like it at all \$\endgroup\$
    – Anas Malas
    Commented Oct 10, 2022 at 11:56
  • \$\begingroup\$ You should be able to use a simple multimeter to measure the resistance of the button when it is closed. If it gives a value close to zero than it does not have a resistor. If the resistance value is in the hundreds or even thousands of ohms then it most likely has a built in resistor. \$\endgroup\$
    – cats are the best
    Commented Oct 10, 2022 at 13:02
  • \$\begingroup\$ @catsarethebest that will not help with the LED also being in series \$\endgroup\$
    – Criticizing Israel not allowed
    Commented Oct 10, 2022 at 15:02

3 Answers 3

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The general formula for finding what resistor you need for an LED is: $$ R_{LED}=\frac{V_{source}-V_{LED}}{I_{desired}} $$ But in your case, we dont know the LED voltage because we dont have the datasheet, but also because we dont know if there is an internal resistor. If you ever are faced with an LED that you dont know if you can light up, these assumptions should be safe: the LED is red (V_LED = 1.8V) and can pass 10 mA of current (I_desired = 0.01). Your V_source is 12V, so even if it doesnt have protection, you can safely use a 1k resistor with it. Connect it through a 1k resistor to 12V, and either use it that way if you dont mind adding the resistor and it's bright enough, or read on if you want to know if you can remove the resistor and make it brighter.

Lets take the different possibilites, which allows you to know what the LED needs:

  • Case 1: LED is red, and is drawing around 10 mA: No internal resistor
  • Case 2: LED is white or blue, and is drawing around 8.8 mA: No internal resistor
  • Case 3: LED is green, and is drawing around 8.8-10 mA: No internal resistor
  • Case 4: LED is dimly lit even indoors, less than 5 mA: Either there is a resistor or multiple LEDs in series. Reduce resistance until you get to 10 mA. That would almost for sure be safe. You can probably reduce resistance until you either get to 20 mA or remove the resistor.
  • Case 5: LED draws more than 10 mA: there are multiple LEDs in parallel. I dont see how you would decide whether it needs an external resistor or not, or if it will work long term even, unless you can count the number of LEDs and try to guesstimate.

Note: the LED configuration could be weird, there is no way to know for sure, and I certainly wouldnt do any of this for something I want to sell or install for a long while. Buy from a switch manufacturer that tells you the data you need.

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    \$\begingroup\$ While powering the LED via your 1k resistor, measure the voltage across the LED. If it's more than about 3.5V, then it's got a resistor (or several LEDs in series) \$\endgroup\$
    – Criticizing Israel not allowed
    Commented Oct 10, 2022 at 15:03
  • \$\begingroup\$ @user253751 Excellent point! Still, it allows you to know even if they are in parallel. Question is, how to know how many in parallel if any \$\endgroup\$
    – Anas Malas
    Commented Oct 11, 2022 at 5:28
  • \$\begingroup\$ you don't need to know \$\endgroup\$
    – Criticizing Israel not allowed
    Commented Oct 11, 2022 at 10:31
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Based on these facts:

  • Contact current rating is 15mA
  • Button only illuminates when pressed

I think the LED is in series with the button, and you are responsible for limiting the current.

I think a built-in resistor wouldn't make sense because the light is in series with the switch. You measure a pressed switch by detecting the voltage between the switch and the pull-up/down resistor. If that resistor was inside the switch you wouldn't be able to measure the voltage there. Or you'd have to add another one outside the switch anyway, and now you have two resistors in series and the light is dimmer than it needs to be and the voltage is lower than it needs to be.

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    \$\begingroup\$ "Button only illuminates when pressed" I don't think that is true. That would make the LED completely useless as I think the user knows when he/she is pushing the button even without LED indication. \$\endgroup\$
    – Klas-Kenny
    Commented Oct 10, 2022 at 11:19
  • \$\begingroup\$ There are separate terminals on the back of the switch to access the LED connections. LED terminals and button terminals are independent. \$\endgroup\$
    – user1999
    Commented Oct 10, 2022 at 11:20
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    \$\begingroup\$ @user1999 well if you already have it why don't you tell us your results? why do you need to ask us? either just put 12 volts on the LED pins and see if the LED stops working, or (less destructively) test it with different resistor values and draw an I/V curve (for real or just in your mind) \$\endgroup\$
    – Criticizing Israel not allowed
    Commented Oct 10, 2022 at 11:30
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    \$\begingroup\$ Im not asking about the terminals are u blind? \$\endgroup\$
    – user1999
    Commented Oct 10, 2022 at 11:36
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    \$\begingroup\$ @user1999 ?????????? you are asking about the LED which is connected to the terminals! \$\endgroup\$
    – Criticizing Israel not allowed
    Commented Oct 10, 2022 at 11:37
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From the description on the RS web site that you linked to it describes the switch as having the LED light when the button is pressed. That implies that there may very well internal connections between the switch contacts and the LED.

Hopefully that is not the case and there are separate terminals on the back of the switch to access the LED connections. One way to determine if there is already an internal resistor is to use some clip leads and an external resistor and a variable bench supply. Connect up the resistor, LED and power supply as if an external series resistor is required and measure/observe the result. If the current measured with the setup is way less than what you would expect for the external resistor value used it leads to the conclusion that there is an internal resistor.

Another thing to consider is that some lighted LED switches that are designed to work on AC or DC will have multiple LED chips used in side that are wired up in a way that some of the LEDs act as rectifiers so that the rest of the LEDs will light up regardless of the applied polarity or if it is AC. The ones like this that I have used also had internal resistors. They were sold in several varieties with different resistor values to permit some to be used on 12V, some on 48V and some on 120V AC.

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  • \$\begingroup\$ There are separate terminals on the back of the switch to access the LED connections. LED terminals and button terminals are independent. \$\endgroup\$
    – user1999
    Commented Oct 10, 2022 at 11:19

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