Inclusion of a power resistor.

[davelectronic]

The psu i plan to repair, with the LM317K doing 300mA of the workload.  Note the smaller heatsink on the back.

[davelectronic]

So if i want the LM317K to do 300mA of the psu workload, what resistor value would be appropriate. In heros schematic it says a single 10 ohm resistor input to the regulator and transistor and emitter base junction would allow the regulator to do 66mA after that the transistor does the work. Its posted on page 1.

Is it really that easy, and if 10 ohms restricts the regulators workload to 600mA in one schematic, how can the same single value resistor of 10 ohms restrict it to 66mA in another schematic, but less the 1 ohm emitter bias resistor  ? All i know is it makes no sense.

[davelectronic]

I'm going to try ditching the 1 ohm ceramic power resistor, and on the input to the LM317K use a 10 ohm 2 watt resistor. See how that goes, the schematic on page i says anything above 66mA is handled by the transistor. I would have gone for 6 ohms 2 watts, but none to hand. I know there's little output protection, so i will add a simple crowbar to protect the output.

[davelectronic]

Thanks for the help from everyone I'm going with the schematic below, but ordered some 4.7R 3 watt resistors. I was thinking a bit more current through the regulator than 66mA as it would be with 10 ohms. Hopefully it will work ok.

[Zero999]

Well it does, in an Alti i restored after i found the internals wrecked. But that one is doing exactly as per the circuit resistors, that heatsink can handle 600mA  for the regulator, at the back of the unit.

The smaller one has a much smaller TO3 heatsink, so i will reduce thst 600mA down to about 300mA.

How much smaller is the heat sink?

The one in the picture should be enough for the full output current.

One thing to bear in mind is the output ripple. If the voltage across the regulator IC is too low, then the output may measure close to the correct voltage but it might have ripple on it, which can only be seen with an oscilloscope. The ripple will only be there at high load currents and when the mains voltage is on the low side.

[davelectronic]

The big heatsink on top has the MJ2955 on it, the rear heatsink is, i would say a quarter the size of the top heatsink.  This circuit from what i can make out, and varieties of it use different resistor combination.  You have some schematic with single base emitter resistor, and some with two resistors, the base and emitter resistor. Values vary from 0.3 to 0.7 to 1 ohm for the emitter resistor, and the base resistors range from 2.2 ohm up to 22 ohm and a far few values in between.
I figured leave out the two resistor schematic and use a single resistor. 3 watts was the cheapest i could find. A 3 watt 4.7 ohm resistor would give the regulator about 127mA if the maths is correct. It seemed a good compromise between 2.2 -  22 ohms and every value in between. If i use that 1 ohm emitter resistor then I'm still passing 600mA through the regulator.

[davelectronic]

Im not a million miles away from this Fairchild schematic below. I think 3 watt 4.7 ohms for an LM317K and MJ2955 will be fine. I've trawled through many forum threads. It should be fine.

[Zero999]

That should be fine but I advise moving the 0.33µF capacitor to the input side of R1. Otherwise, when the power is first applied, Q1 will turn hard on, passing the full input voltage directly to the output.

[davelectronic]

Thanks again all.