R1 sets the amount of current available from the regulator. In the schematic you posted the value is shown as 3 ohms which limits the available current to 233mA, however the load can only draw about half of this for the regulator to work correctly. If you know how much current the load will draw you can calculate the value R1= .7/(Iload x 2)
I have changed the resistor to 5k and it will go to 5v into 5R. Q2 gets very hot quickly even after fitting larger heat sinks .... I will continue
It's burning about 20 watts of heat (20V@1A) so a large heatsink is going to be needed. Unfortunately it's going to have to burn a bit more to make the reg operate as it should bringing Q2 right up close to its SOA. Make R1 .47 ohms and leave R2 at 5k and strap a big ole heatsink to it and it should meet your requirements for the 5V line but it's going to run very hot.
I did have a play with it over the weekend, but I am not going to use it now for the same reason, too hot running.
I am not sure how they rated the supply above 1 amp ….
I will use it for the rectifier and capacitor to provide me with the raw supply and select another regulator circuit
Just come across this thread, had the same problem and solved it by changing Q2 to P type TO 220 Darlington (eg TIP137) which has the same pin-out, it is then a straight replacement for the MJ15031. R1 needs to be around 0.5ohm to give you 1A at 5v. You obviously need a suitably rated 9v transformer.
Reason it does not work at that voltage/current with MJ15031 is that its vbe is insufficient to work with Q1, so a Darlington effectively multiplies the vbe available.
You can also substitute a P type enhancement Mosfet such as IRFP520, which is also a straight pin for pin replacement.
I have used both so can vouch that they work.
Hope this helps even if it is late to the party!
Reason it does not work at that voltage/current with MJ15031 is that its vbe is insufficient to work with Q1, so a Darlington effectively multiplies the vbe available.
You can also substitute a P type enhancement Mosfet such as IRFP520, which is also a straight pin for pin replacement.
I have used both so can vouch that they work.
Hope this helps even if it is late to the party!
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