A little TEA before bedtime.
So, I got that AC power supply/transformer enclosure finished; I'll post some tidy pictures another time, this is more a "traps for young players" post. How many people when picking out a transformer notice, and take note of, the bit of the specification that says "Temp rise"? If you don't pay attention it can bite you on the bottom. The Vigortronix 50VA transformer I used here has a "Temp Rise" specification of 42ºC, which means that at full output and a typical lab temperature of 23ºC it's going to get up to 65ºC. Yes, transformers can get quite a bit hotter than you might expect.
I generally reckon that 50ºC is "touch safe" for dry things. Now, mine is in a case so it's going to be touch safe because you can't touch it, but is the enclosure going to be touch safe? Is the enclosure going to get rid of enough heat? Better err on the side of caution and test it, we like to be thorough here at the Cerebus TEA Works and Catslave Market. So, 2 x 25Ω aluminium clad resistors, a hefty 0.4 K/W heatsink with integral fan to clamp them to (with a spot of mineral oil to ensure good thermal contact) and we're good to go. Transformers have long thermal constants, they can take hours to get up to their steady state temperature.
So this has been sitting here for a bit over two hours, dumping 50W into a 50Ω resistive load (so the transformers full rated 50VA), and has reached steady state. Here's what it looks like:
The resistors are at 70ºC, the heatsink 43ºC, and the enclosure 43.8ºC in an ambient 22.3ºC.
It's comforting that the theoretical 20ºC rise for the heatsink is 20.7ºC, probably attributable to a little extra heat gain from the 240V mains on a 230V transformer, combined with a little extra direct convective loss from the resistors.
So everything's safe and the transformer can't be too hot because it hasn't tripped its thermal fuses. I didn't for one moment expect that it even might, experience tells me this was going to be fine, but it's always a good idea to test your assumptions when you can't easily build a simple thermal model. I mean, where would you start with a transformer mostly relying on convection and radiation inside a Hammond 1590 box? Hardly easy things to calculate thermal resistances for. You could if you really had to, but sometimes "seat of the pants" is good enough.
Now I have to figure out if I can get an actual temperature for the transformer without extra holes to thread a thermocouple through. Worst comes to the worst I'll let it get it up to temp, whip the top off quickly and measure it quickly - I doubt that it'd drop more than 1 or 2 ºC in the interim. That's a problem for another day.