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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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10 Replies

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)2012-06-20

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

steveastrouk (author)2012-06-20

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

rickharris (author)2012-06-18

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.

lemonie (author)2012-06-24

Thanks guys

rickharris (author)2012-06-18

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

lemonie (author)2012-06-20

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

L

ARJOON (author)2012-06-19

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???

mh76dk (author)2012-06-18

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 :-)

mh76dk (author)2012-06-18

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