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# How do I tweak this voltage regulator? Answered

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?

L

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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.

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.

L

Using the 317, and Rick's math's all you need.
R1=220, R2=680

The resistors at the middle pin control the output - Taken from their data sheet https://www.national.com/ds/LM/LM117.pdf

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.

Thanks guys

I forgot to add the formula for working out the resistor values for a given voltage.

I'll see what steve' comes up with, but this looks like the sort of answer I was after, thanks.

L

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???

This instructable might also be of use: https://www.instructables.com/id/Versatile-voltage-regulator-with-LM317/ (I feel quite stupid for not noticing it on the sidebar the first time around :-)

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): http://www.datasheetcatalog.com/datasheets_pdf/L/M/3/1/LM317T.shtml