I have a simple 4 led on 555 project but it needs to run on 24VDC.

1. power the 555 using a zener to drop the voltage down to 12V then switch the LEDs in series using a MOSFET and 4 220 Ohm resistors to distribute the heat.

2. Use a DC-DC to drop tp 8V and switch LED directly in series.

Anyone have a better idea?

 

I prefer #1. If one LED goes out it doesn't effect the others but it would require 5 watt resistors.
Why couldn't you combine 1 and 2? Wire the LEDs as in #1 but use a lower Vcc through the drop converter.

 

they are red LED with Vf of 1.6V so dropping 17.6v which is over 1/3 watts. So if I drive it directly from 555 at 8V, the drop is negligible and it can drive 20 mA just fine with maybe a 200 Ohm in series. They want 0805 passives.

 

they are red LED with Vf of 1.6V so dropping 17.6v which is over 1/3 watts.

That doesn't add up. How much current is required for each LED?

 

Try the math, 4 LED with Vf of 1.6V in series at 20 mA, it’s actually 352 mW.

 

Use the DC to DC converter. Driving them from 24V wastes .35W.

Bob

 

Correct if in series.
Using 8 volts the series resistor would be closer to 20 ohms if the output from the 555 is only around 7 volts.

 

Looking for simple low part count, power wasted is not a problem, heat is.

 

Follow up: I’ve come up with a number of options.

1. MC34063 - buck converter, Rcs generated so much heat, I looked for other options.

2. Looked into other buck converters but after finding some that cost too much I looked for other simpler solutions.

3. 7812 - using a pre-regulator to drop to 15V - this was a good option as peak dissipation across any one component never got over 300 mW. Avg dissipation less than 1/5 W.

4. 7812 - using pass transistor - this was just as good as pre-regulator but required one less part.


A modern buck converter will dissipate less heat but this would require large Inductor and capacitors. I wonder if I’ve missed any tricks.