Transistor over heating while dimming power LED

[YIM bunchhat]

Hello,

I got a problem that I dim power LED 4-7W/50V, but I wonder why my Transistor TIP122 is over heating. I measure the current Ic = 70mA and Ib = 1mA only. I also try change to other transistor but I still got this problem....:sad:

 

[betwixt]

At 70mA the saturation voltage is only about 0.75V so it should only dissipate about (0.75 * 0.07) = 0.05W. That would barely heat it.

Is this a linear dimming circuit or are you using PWM? Please show your schematic.

Brian.

 

[Easyrider83]

Do you think 1mA is enought to open this transistor?

 

[FvM]

Do you think 1mA is enought to open this transistor?

It's a darlingtion with B > 1000, so Ic/Ib=70/1 would indicate saturation.

The applied dimming method is unclear though.

 

[YIM bunchhat]

Hello all,

Here is my circuit, I use linear not PWM with range 0v-10v. 1mA I just test not my dimming method. I wonder why it over heat. Sorry for my confuse I use H1061 not TIP122. When I use H1061 dimming smoothly but over heat, but if I use TIP122 little heat but not smooth.
View attachment dimming.jpg

 

[betwixt]

Giving wrong information = geting wrong answer!

H1061 is not a darlington device and you are starving it of bias current. I'm not sure what the op-amp circuit actually does, it would appear that if you connected the top of C5 directly to the base of the H1061 and removed the components between them it would work just as well if not better.

Brian.

 

[FvM]

Wether the base current is sufficient to switch the transistor fully on or not, it's a linear dimmer circuit that dissipates heat according to Iled*(50V-Vled), independent of internal circuit details.

If you want less transistor heating, go for a PWM dimmer or use a sufficient heatsink.