I think it would be a good exercise to plot measurements of the transistor case temperature (at equilibrium) versus various output voltage and loads.
You can start small -- e.g. 16V at 100mA, 200mA, 300mA, 400mA and then move up to higher power dissipation, e.g. 12V and 8V. Record the transistor case temperature after it has plateaued -- this will probably take only a couple of minutes at the most.
The graph of temp vs. output current for each choice of output voltage should be a straight line and from the line you can determine the thermal resistance between the transistor case and ambient air -- i.e. the effectiveness of your heatsink.
The main issue (assuming you are below the transistor's maximum junction temperature), is the hotter the power transistor gets to, the shorter its life time. It is best to not run it too hot, otherwise, it will fail eventually.
As a rule of thumb, the transistor life, drops by a factor of 2, for every 10 deg C, junction temperature rise.
So, if a transistor had a life of (ultra approximate figure, as I don't know how long it would last, really, at 100 deg C) 10,000 hours at 100 deg C, it would be more like 5,000 hours at 110 deg C and 2,500 hours at 120 deg C. Etc. I'm referring to Junction (internal) temperatures.
They (power transistors) also tend to fail short circuit (but not always!), which can put the full output voltage onto your circuit, with potentially unpleasant results (or pleasant firework demonstration, for around 5 seconds, then unpleasantness ).
but I don't think I would have guessed a ~33 C difference from one end of the resistor to the other end.
One question that comes to mind is should (for example) an 18 gauge solid copper wire heat to a lower temp than 18 gauge stranded wire for the same current put through both? (I haven't found a table comparing solid vs stranded temps by AWG, or even a current carrying comparison table by AWG showing solid vs stranded.... in case anyone has a link.) Thx
One question that comes to mind is should (for example) an 18 gauge solid copper wire heat to a lower temp than 18 gauge stranded wire for the same current put through both? (I haven't found a table comparing solid vs stranded temps by AWG, or even a current carrying comparison table by AWG showing solid vs stranded.... in case anyone has a link.) Thx
https://www.engineeringtoolbox.com/wire-gauges-d_419.html
If that's the case the chart is pretty/very interesting. For example, 16 AWG solid copper is rated for a load of 15 Amps but when it's 3 strands it drops to 10 Amps and to 7 Amps if it has 7-24 strands. Do you think that's what it's saying, that cores = strands?
Either way, thanks for finding and posting the link.
I'm not sure where the threshold is in the rest of the PS and I don't want to blow up the PS but maybe there is room to move upward toward 18-24 Volts and more current handling capacity, or maybe it might also be worth looking at a toroid transformer?
If that's the case the chart is pretty/very interesting. For example, 16 AWG solid copper is rated for a load of 15 Amps but when it's 3 strands it drops to 10 Amps and to 7 Amps if it has 7-24 strands. Do you think that's what it's saying, that cores = strands?
Either way, thanks for finding and posting the link.
Yes. Strands = cores. Those amperage ratings are for a 30oC ambient , for PVC-insulated wire . I believe PVC softening point is around 85 to 100oC . Other wire will have different current ratings based on the insulation . The type of wire you use depends on application and design . Datasheets for the type of wire you use will tell you the except able current ratings and temperature .
You take fewer core losses with a toroid than with other types of core . 5- 10% where EI cores can be as high as 15% .
The current I in amps is equal to the apparent power S in volt-amps, divided by the voltage V in volts:
I(A) = S(VA) / V(V)
For example if you need 3 amps at 40VAC on your secondary the core will need to be 120VA . This also applies to the primary as well . A 120VA core at 230VAC will draw about 500mA . Like volts current is proportional between primary and secondary .
Not sure where you live but here's another place you can look for toroid transformers :https://www.antekinc.com/transformers/
I have a few of these and they are built really solid .
I'm not sure where the threshold is in the rest of the PS and I don't want to blow up the PS but maybe there is room to move upward toward 18-24 Volts and more current handling capacity, or maybe it might also be worth looking at a toroid transformer?
Here is one a member did, which does 40 V at 4 amps.
https://www.eevblog.com/forum/beginners/success-prototype-40v-4amp-mc1466l-power-supply-thankyou-everyone-that-helped-!/
I'm pointing it out to you, to hopefully illustrate the work, and changes needed, to cope with higher capabilities.
Note well, that it has FOUR output transistors. That is because a single output transistor (like you have), can only handle a limited amount of power dissipation.
At a quick glance at the circuit, I just linked to, it seems reasonably straight forward (to make).
EDIT: Also, it seems to use 2 separate secondary windings/taps. Which allows for the much higher output voltages, without having to panic about a design which doesn't really want to handle too high an output voltage.
Analogy: You just bought a 5 watt output audio amplifier, and now want it to do 7 watts.
So, some people carefully help you to achieve this.
In 2 weeks , you now want it to go to 10 watts, output. Which turns out to be a very tricky modification and needs lots of calculations and changes.
What I am trying to say, is that you would be better off, buying (or getting the circuit for) a 50 or 100 watt power amplifier. Problem solved. Rather than trying to start with a 5 watt one, and increasing the voltage/current/power, watching what gets too hot, and trying to continually modify it, all the time.
What you have fundamentally, doesn't handle higher voltages, without a number of modifications. Also, the output current is also fundamentally limited, because it only has one output transistor.
Can you tweak it and get a bit more from it ?
Yes.
But really, I would suggest thinking about getting a kit/circuit or whatever, which can handle higher output voltages and currents, that you are happy with.
Opinions can vary. Some may think tweaking it, to get a bit more is worthwhile (theoretically, you could tweak it and tweak it, to very high voltages and currents). But, I don't really agree.
Better, to use a design which is designed for suitably high voltages and currents.
EDIT2: Or read through the various threads about your psu, and perform ALL the higher voltage modifications. There seem to be at least 2 or more. I suspect the higher voltage mods will rule out increasing the output current rating, as it will make the output transistor, significantly hotter.
The output transistor, is already getting rather hot, I'm not sure you would get much more from it, as it stands.
As long as I have two excellent power supply teachers here, what would you and Jwillis recommend for a circuit board for the design he posted? I'm not ready to use a software tool to layout a PCB and send it out for manufacture, so what's the next best approach?
Thanks again!
Edit/Update: looks like the lineage traces to here:
http://www.learningelectronics.net/VA3AVR/circ/ps4002/ps4002.html
I think I'm going to twiddle a bit more with the current design - I'm sure there is more I can learn from it - and then I'm inclined to embark on a new PS project.
They have been discontinued since 2002 . If they look like they have brand new shiny plastic with really bright white labelling I would question authenticity . Also they came in a ceramic package . If they don't have a white band around the perimeter where the pins are then they are not authentic.
Not to say they won't work , just don't expect longevity or accuracy .
Also check for MC1566L
One question that comes to mind is should (for example) an 18 gauge solid copper wire heat to a lower temp than 18 gauge stranded wire for the same current put through both? (I haven't found a table comparing solid vs stranded temps by AWG, or even a current carrying comparison table by AWG showing solid vs stranded.... in case anyone has a link.) Thx
https://www.engineeringtoolbox.com/wire-gauges-d_419.html