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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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steveastrouk
steveastrouk

Best Answer 8 years ago

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.

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lemonie
lemonie

Answer 8 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.

L

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steveastrouk
steveastrouk

Answer 8 years ago

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

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rickharris
rickharris

8 years ago

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.


LM317.gif
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lemonie
lemonie

Answer 8 years ago

Thanks guys

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rickharris
rickharris

Answer 8 years ago

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

Capture.JPG
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lemonie
lemonie

Answer 8 years ago

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

L

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ARJOON
ARJOON

8 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???

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mh76dk
mh76dk

8 years ago

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 :-)

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mh76dk
mh76dk

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