Led Resistors reach 110C!!

[Oznog]

Just use a current-mode converter like the HV9910 or the ZXLD1360.

These don't generate any significant heat, work with smaller power supplies, and regulate the current very accurately.

Otherwise, as you drop the supply voltage near the LED's forward voltage and lower the ballast resistance, the ballasting becomes less reliable. For one, the forward voltage of the device is not known exactly. Second, the forward voltage drops as it warms up which would increase the current when the voltage across the ballast resistor is low.

 

[dons21]

Wow I seem to have started a new debate on constant current.

The reason I used the resistors is because the leds I bought have a common anode so I can not connect them in series.

Question; If I did have 4 350mA Led's connected in series 2.2 * 4 =9.6v would the FET 2n7000 which Is rated to 700mA(i think) used to control them have to increase its switching power or will it still be switching at 350mA??

 

[audioguru]

The 2N7000 is a pip-squeak of a little-guy Mosfet.
Its max allowed continuous current is only 200mA.
When it has 10V gate-source its max resistance is 5 ohms.

Its resistance increases as it heats. At 350mA its max voltage loss is 1.75V when it is cold. It will try to dissipate 613mW or more which is much higher than its allowed max of 400mW.

Smoke, anyone?

 

[Hero999]

Build a BLack regulator with the current limit set at 300mA and connect all three LEDs in series then short them out to turn turn them off.

https://www.romanblack.com/smps/a04.htm

 

[justDIY]

Build a BLack regulator with the current limit set at 300mA and connect all three LEDs in series then short them out to turn turn them off.

said shorting to be done by a transistor of some sort?

 

[Hero999]

It doesn't matter, it could be a transistor or a simple switch. The current is limited by the switching supply so it won't damage anything, in fact the more LEDs you short circuit, the less current it will draw from the supply.

 

[Peter_wadley]

Why not..

Just attach the resistor to a large heat sink?... possibly add a computer fan aswell.. treat it like a CPU chip.

To increase contact and heat transfer dont forget to use heat sink compound.

My 2 cents