Have 24V need 5V 0.1A. Works fine with 78L05 (right size) but gets too hot. Smallest cool alternative ?

[OSLO]

The Zener gives a constant Vin to the regulator irrespective of load.

The resistor drops voltage in proportion to the load current and so the regulator can see the full 19 volts at low current and 9 volts at 100ma (and anything in-between).

Personally I would prefer to see a constant Vin rather than a voltage modulated by the load current.

How should the zener diode be connected ? Do I need an additional resistor ?

 

[Mooly]

The Zener is in series with the positive supply to the reg. The end with the stripe goes to the supply. If you get it the wrong way round it will only drop 0.6 volts instead of the marked Zener voltage.

 

[OSLO]

The Zener is in series with the positive supply to the reg. The end with the stripe goes to the supply. If you get it the wrong way round it will only drop 0.6 volts instead of the marked Zener voltage.

Thanks. Got it.

 

[Mooly]

That's looks correct 👍. You should see a voltage across the Zener equal to its value.

 

[OSLO]

Thanks. The zener and 78L05 runs much cooler now 😊

 

[Mooly]

That's good 🙂