# 555 on 24v

#### Wolframore

Joined Jan 21, 2019
2,545
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?

#### sghioto

Joined Dec 31, 2017
3,296
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.

#### Wolframore

Joined Jan 21, 2019
2,545
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.

#### sghioto

Joined Dec 31, 2017
3,296
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?

#### Wolframore

Joined Jan 21, 2019
2,545
Try the math, 4 LED with Vf of 1.6V in series at 20 mA, it’s actually 352 mW.

#### BobTPH

Joined Jun 5, 2013
4,950
Use the DC to DC converter. Driving them from 24V wastes .35W.

Bob

#### sghioto

Joined Dec 31, 2017
3,296
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.

#### Wolframore

Joined Jan 21, 2019
2,545
Looking for simple low part count, power wasted is not a problem, heat is.

#### Wolframore

Joined Jan 21, 2019
2,545
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.