High power LED switching

Other Hardware LEDs and Multiplexing
1

Hello all,

I am trying to control a 20W LED using a tip122 transistor. I am using PWM to control the brightness of the LED. The transistor gets dangerously hot after switching the LED for a few minutes. I have a 1KOhm resistor at the base for the control signal (attached circuit diagram). I believe the transistor cannot handle the load current and hence is dissipating heat.

My questions: what is a good way to control high power LEDs? and particularly a 20W LED.
and how is the PWM frequency related to the heat dissipated by the transistor?

circuit diag.PNG
circuit diag.PNG932×501 113 KB

2

54f26f8a35f052a8b2fa07cc5985738fce5f90e5.png

Bad choice of transistor - Darlingtons are pretty much obsolete.

You need a logic level - note that - power FET. I haven't used them myself yet, but others will offer a lot of choices if you cannot research that yourself.

3

Post a link to the 20watt LED.

It could be a 20watt cob LED that needs a proper LED driver module, not just a transistor.
Leo..

4

Well, if it is a bare COB LED, then the TIP122 will be just fine, will just need to add a heatsink and use this circuit:

Where the TIP122 substitutes for the second transistor and the base resistor is your 1k.

For 2 Amps to the COB, "R" would be about 0.33 Ohm.

5

20watt with a LED Vf of about 10volt is 2Amp.

The TIP122 has a Vce(sat) of ~1volt at that collector/base current.
That's a dissipation of 2watt in the transistor.
No surprise it gets sizzling hot.

Datasheet recommends a base:collector current of 1:250 for full saturation, so changing the base resistor to 470ohm could lower dissipation somewhat.

The above circuit is easier on the LED (limits current to a safe value), but could be harder on the transistor.
If the transistor has to drop 2volt, dissipation could be 4watt.

A proper switching LED driver would barely get warm with those currents.
Leo..

6

Thanks guys. But no luck yet.

@Wawa this is the LED I'm trying to control: https://www.amazon.com/gp/product/B00RCSAQWK/ref=oh_aui_detailpage_o03_s00?ie=UTF8&psc=1
it goes to 1.78amps at 12V.

Though it doesn't say it anywhere I'm pretty sure it is NOT a COB. I tried switching the TIP122 with an RFP30N06LE n-channel power MOSFET now the LED doesn't turn on. I'm using this kit SparkFun MOSFET Power Control Kit - COM-12959 - SparkFun Electronics with a 10Kohm resistor. My power source is a 12V Li-ion battery pack.

7

That's a floodlight, most likely with a LED driver inside.

Dimming that floodlight externally could be impossible.
The internal LED driver (if it has one) will fight that.

And yes, that yellow square is a cob LED.
Leo..

8

Oh damn! never considered the fact that there could be a driver inside it. Thanks Leo..!

9

Link corrected.

Wawa:
That's a floodlight, most likely with a LED driver inside.

Dimming that floodlight externally could be impossible.
The internal LED driver (if it has one) will fight that.

It is specified 12 to 24 V.

That means it does have a switchmode power converter inside with capacitors that most certainly will prevent you using PWM. The internal driver might be able to be modified but as you gather - it's simply not worth it.