I want to dim 24 LEDs which are powered by a transformer outputting 24V 6W at 0.25A

I am worried about temperatures, especially considering the circuit has to stay on for several hours keeping the LEDs off.

Would a TIP122 transistor be ok?

Any tips for keeping the transistor off until the dimming process starts?

  • I'd suggest using MOSFETs. The TIP122 has a 2V drop, so it will dissipate 0.5W worth of heat. MOSFETS will only dissipate a few milliwatts. To turn of the leds set the output pin to low. To make sure the led stays off on powerup, add a pull-down resistor to the PWM output-pin. – Gerben Oct 27 '15 at 10:06

I would suggest using the CAT4101 constant current sink PWM LED driver. It can handle up to 1A at up to 25V and deals with the constant current regulation for you - just provide at least the 24V you need and enough current capacity and connect the correctly calculated current setting resistor and the CAT4101 will do the rest for you. You can then control it through the PWM input pin direct from the Arduino.

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  • very interesting option, thanks! Would the NCP3065PG work as well? pdf1.alldatasheet.com/datasheet-pdf/view/196869/ONSEMI/… It's just because it would be easier for me to source it – Martin Oct 27 '15 at 22:03
  • I have not used that chip and not in a position to look right now. – Majenko Oct 27 '15 at 22:12
  • just got the CAT4101 but I am a bit confused about how to calculate the resistor setting. The datasheet states that the LED current is mirrored from the current flowing from the RSET pin and then there's a table listing a few values matches. So I guess having 250 mA means I need a resistor around 2000 Ohm. Is there anyway to calculate the exact resistance? – Martin Nov 10 '15 at 10:25
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    I use 1.2 / (led current / 400) - it's not exactly like the table but close enough. I have it as an online calculator: majenko.co.uk/?page=cat4101 The 400 comes from the datasheet: "RSET to LED Current gain ratio" on page 2. – Majenko Nov 10 '15 at 10:41
  • All working pretty good. I just get a minuscule amount (almost of light from LEDs when the PWM is set to LOW. Could this mean that I need a slightly bigger resistor? Also, having a problem of Arduino freezing after a short time, arduino.stackexchange.com/questions/18307/… could this be due to CAT4101? – Martin Dec 1 '15 at 11:10

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