Although there already exist many websites about such stuff, another simple DC-DC converter is described here. The output voltage equals twice the input voltage minus the voltage drop across the diodes.
Capacitors: 10 uF/25V, Diodes: 1N4148 (or any other general purpose diode). The IC is an ICL7662 which is rated to have a maximum output voltage of 38.6 V at 20 V input. The ICL7660A may also be used but can take only 13.5 V input voltage.
The plot shows the switch-on behaviour. When 13.8 Volt is applied (upper, grey trace) it takes about 2 ms until the output reaches 24 Volt (green trace).
I use this DC-DC converter on several SMA relais since about 2 years now, none has failed so far. As a test, I put one converter on a 10 Hz square wave generator and let it switch a 24 V relais on/off a whole week (abt 6 million cycles). It survived this treatment as well.
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