Variable Power Supply

[Bibio]

looking at the spec sheets for Q1 Q2 on the schematic below i have put together a variable power supply but i must be honest i know nothing so i would like advice as to if i have done it right.

An externally hosted image should be here but it was not working when we last tested it.

Thanks

 

[AndrewT]

why dual bridge rectifiers when the lower loss single bridge rectifier would do?

The adj to out resistor can be used to guarantee the minimum current draw of the second chip.
This should then be sized to pass the 5mA to 10mA that is usually specified for these 3pin regs.
R2 is usually the resistor, or voltage reference, that gets adjusted to trim the output voltage.
Why are you using a Vin of ~35Vdc and reducing this with a 7808 to ~8Vdc to feed the second regulator?

 

[sofaspud]

The two biggies to achieve a variable 3-6V DC output are 1) replacing the transformer with a single 12VAC secondary and 2) moving R1 in series with R2, and putting a (R3) 240-ohm resistor in its place, paralleled with D1.
What is left then is just some component optimization. Adjusting R1 & R2 values for tighter control of variable Vout (using Q2's datasheet formula). D1 might be more useful forward-biased on Q1's GND pin.

 

[Bibio]

rectified power i already have so going to tap into it for power.

using 7808 to drop voltage and make it more linear to feed the lm317 so i can fine adjust using r1 or thats the idea.

so if i read you right adjusting R2 will give me more/less mA, if i drop the resistance to 1k then i will get more mA?

 

[sofaspud]

You need to review the LM317 datasheet.
I rarely bother with "that fancy Vout formula" included there. Instead remembering the 1.25V maintained between the adjust and output pins, and Ohm's Law.