Since I have 24VDC everywhere in the house I get more and more ideas of what I could connect there. A very useful thing would be USB chargers everywhere. So let’s build one.
I have a few MC34063 lieing around. With a few little changes to my standard 5V circuit I also get a charging LED and the SMPS will be stopped when not in use.
R13 pulls the output voltage up to about 7V. Then with R14 and R15 a feedback voltage of about 1.5V is generated which switches off the MC34063. Then the voltage over C12 drops and the LED goes dark.
When something is connected it will draw some current and pull the output voltage down. As soon as the feedback voltage reaches 1.25V the SMPS turns on and regulates the output to about 5V.
If something is connected which draws no current the output voltage will be more than 5V. But since some other chargers do the same thing (f.ex. Nokia) I think this is OK. With the 33k resistor a connected device has to use more than 0.2mA. I’ll try a 39k instead of 33k to get this minimum current further down. But maybe the MC34063 will then not switch off when the input voltage is just 21V (everything should work with voltages between 21V and 27.3V because of the lead batteries).
The circuit works fine. Time to install it somewhere in the house and make some PCBs.