How do I tweak this voltage regulator?

I pulled a power-board out of a 'phone charger (pic)
The transformer and bridge rectifier have been removed and it's powered from a 9V motor.
The charger says:
Input 230V~ / 50Hz  / 40mA / 9.2VA
Output 7V / 300mA / 2.1VA

I'd like it to regulate to 5.0 V (for USB devices) - what do I fiddle-with?


Picture of How do I tweak this voltage regulator?
sort by: active | newest | oldest
The original config is for a constant current charger, with end voltage termination. You could do with inverting the whole bloody thing. Constant voltage, end point current.
lemonie (author)  steveastrouk5 years ago
Hi (bad connection this week, intermittent)
Can you give me a basic 5V-out circuit diagram?
I'd rebuild it if I'd got a sound plan.

Using the 317, and Rick's math's all you need.
R1=220, R2=680
rickharris5 years ago
The resistors at the middle pin control the output - Taken from their data sheet

For 28 volts in the values given will give you anything between 1.2 and 25 volts. There is a 3 volt overhead to make the regulator work.

lemonie (author)  rickharris5 years ago
Thanks guys
I forgot to add the formula for working out the resistor values for a given voltage.
lemonie (author)  rickharris5 years ago
I'll see what steve' comes up with, but this looks like the sort of answer I was after, thanks.

ARJOON5 years ago
lots of power seems to be wasted in this circuit through the resistors, why not use a constant current along with a constant voltage circuit and tossing apart this pcb???
mh76dk5 years ago
This instructable might also be of use: (I feel quite stupid for not noticing it on the sidebar the first time around :-)
mh76dk5 years ago
Not being 100% sure, but I believe you need to fiddle with the resistors on the LM317T (it also seems this device is made by several companies, but i found a link with a list of datasheets, perhaps you can find out which one exactly it is):