TL;DR: Read the bulleted list below.

I am designing a simple constant current source to power up some LEDs inside arcade buttons. My project consists of 20 of these buttons. Every button is internally composed by this circuit: enter image description here

I can't change this circuit because it is locked inside the button's plastic body, so swapping the resistor/LEDs isn't a possibility. Also, I couldn't determine the LEDs specific part number; the manufacturer of the buttons didn't gave me any info about it's circuit. I believe they are 0805 white LED's with 2.8V Vf and 20mA If. And the resistors also seem to be in 0805 packaging.

So to drive them safely from 12V jack plugs with over voltage protection and some brightness control I designed the following circuit, using components I already have lying around: enter image description here

The J1 and J2 are the connectors where I will wire up all buttons in parallel. The V1 voltage source is placed there only for simulation purposes. So, now I make these questions:

  • Is this current source okay to drive my button's LEDs? Considering the 1/8W supposed limitation of the internal resistors.
  • Does this over voltage protection circuit works?


  1. http://www.ti.com/lit/ds/symlink/lm158-n.pdf
  2. https://my.centralsemi.com/get_document.php?cmp=1&mergetype=pd&mergepath=pd&pdf_id=2n5060_series.PDF
  3. https://www.infineon.com/dgdl/irfz44n.pdf?fileId=5546d462533600a40153563b3575220b
  • 1
    \$\begingroup\$ The built-in resistors are providing the current limiting already. Why do you think you need additional current limiting? On a 12 V supply the 470 Ω resistor will limit the current to a little under 20 mA for the pair of LED or about 10 mA each. \$\endgroup\$ – Transistor Dec 30 '18 at 20:05
  • \$\begingroup\$ Yep, I noticed it. But my design focus was in brightness control and overvoltage protection. The reason for the over voltage protection is that it will be powered by 5.5mm barrel jack, those are very common in a wide range of different voltages/current ratings power supplies, so plugging the wrong supply is a real possibility. I did it myself with some electronic devices and the ones without over voltage protection created some magical smoke. \$\endgroup\$ – KawaungaXDG Dec 30 '18 at 20:13
  • \$\begingroup\$ Also, I don't want to be so close of the power limitation of these resistors as I would be by just applying 12V directly. For safety reasons, since I don't know much about this circuit, beside my speculations. \$\endgroup\$ – KawaungaXDG Dec 30 '18 at 20:22
  • 1
    \$\begingroup\$ In that case I'd be more worried about reverse polarity from a random PSU and would add a series diode. I can't really figure out what you're doing with the schematics provided but using an SCR would be unusual in a DC circuit as they won't switch off until the current drops below the hold-on value. The normal way of dimming LEDs is by pulse-width modulation (PWM) as this is more efficient on the controller. 12 V directly or indirectly is still 12 V. \$\endgroup\$ – Transistor Dec 30 '18 at 20:28
  • \$\begingroup\$ The SCR is for a crowbar-alike circuit, in the over voltage protection block. And about reverse polarity, I don't think it is possible since barrel jacks always have the same polarity, but it is something to consider, I will add a diode to prevent this. \$\endgroup\$ – KawaungaXDG Dec 30 '18 at 20:31

I would not choose any of these 3.

  • This R value seems to be for 12V operation across the switch.

    • 12V-2.8=10.2V /470 = 21.7mA assuming Vf is correct
  • Possibly these LEDs are ~ 2.8V @ 10mA.

Thus simple solution is PWM or Pot control OA or transistor to reduce voltage and/or current.

  • The button can drive the LED at the same time if it is 12V
  • or any transistor and 10k Rb Pullup to + or pulldown to Gnd.
  • \$\begingroup\$ I will move towards PWM instead of using a linear current source. Could you help me with the overvoltage protection circuit? It is the second block of my schematic. In my testing conditions it worked fine, but I'm not sure if it's the safest option (fuse + crowbar) Also, Thanks for the suggestions! \$\endgroup\$ – KawaungaXDG Dec 30 '18 at 21:07
  • \$\begingroup\$ If your supply is 12V , there is no need but if you fear supply failure, set threshold to 12.5V and use PTC polyfuse before SCR \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Dec 30 '18 at 21:08
  • \$\begingroup\$ I, unfortunately, cannot guarantee the jack plugged in this board will be a 12V one, so it would be important to have this feature. But if it would be too complex to implement correctly I could drop it from the project. \$\endgroup\$ – KawaungaXDG Dec 30 '18 at 21:09
  • \$\begingroup\$ Choose PTC in series with source voltage for I hold etc. and SCR with 12V Zener is .13.2V OVP \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Dec 30 '18 at 21:14
  • \$\begingroup\$ Okay, I will replace the fuse by a PTC, replace the Zener and move to a PWM based solution in the next design. \$\endgroup\$ – KawaungaXDG Dec 30 '18 at 21:16

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