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Transformer voltage for dual power supply (+12V / -12v)

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Ian.M:
For transformers with two secondaries load *BOTH* secondaries!  If you don't, the losses in the primary are far lower, and its impossible to calculate the true equivalent resistance referred to the secondary.

You aren't going to damage the transformers running them at 100% load for 10 minutes in free air.  In fact they'd probably be OK with 150% load (50% overload) for that long starting from cold as there is a fair bit of thermal mass so no part of the winding is likely to exceed its maximum insulation temperature.    Its therefore good practice to aim for 80%-100% of full load when testing.

For the 50VA transformer please use 2x 20W bulbs, one on each secondary, as 20VA@12V is 1.67A RMS which, when you apply a factor of 0.62 for bridge rectification with capacitive input load, corresponds to a DC load current of 1A, which is right where we want to be operating as max load for a LM317/LM337 based PSU.

Normally you'd load the 100VA transformer with 2x 20W bulbs PER SECONDARY, but as you specifically are using regulators with a max rating of 1.5A DC, there is no point in going over 2.4A RMS which is 30W @12V.

Due to the non-linear resistance vs voltage curve of the bulbs, it would also be helpful if you measured the actual load current of one of the bulbs for each transformer.

Also, for our convenience and to guard against mains voltage fluctuations with time of day or week, please repeat the unloaded voltage measurements immediately before and after the loaded measurements and post all the results in the same post.

So far it looks like the 100VA transformer is likely to be the best bet. 

Turning on your heater or air-conditioner etc. is unlikely to have much effect unless your house's wiring is in particularly poor condition.  Problems start when a high proportion of the population turn on a high load appliance at the same time.  The classic case was the U.K's Coronation Street soap opera on ITV, which regularily aired late afternoon.   During its commercial breaks, you could bet that a high proportion of its viewers (over 9 million households in the '60's and '70's) would turn on a 3KW kettle.

https://en.wikipedia.org/wiki/TV_pickup
Dinorweg pumped storage powerstation in the UK was built to time-shift peak loads, but as it has the fastest spin-up time for a major power station in the UK, its currently mostly used to supply the rapid demand surges caused caused by TV pickup. https://en.wikipedia.org/wiki/Dinorwig_Power_Station
(Its pretty incredible - 12 seconds to go from near 0 to 1320 MW)

belzrebuth:
So to summarize:

For the 100VA transformer I need to attach a 20W bulb per winding and actually log the current draw as well as the voltage.
I'll leave it wired and test at 4-5 different times a day.
Correct?

I didn't quite understand what you've said about the 50VA transformer though..
The 50VA transformer is 1x12V so I only have one secondary.
I have two identical units so essentially they could be used as one 2x12 that's why I mentioned it.

I may need to focus on the 100VA transformer; having two transformers on such a small case might be inconvenient.
For reference this power supply will be mounted on a wooden case used for a modular synthesizer so there's no metal chassis or too much space for huge heatsinks etc..

I also wonder what kind of heatsinks I could use for my LM317/LM337:
Mounting them off the pcb will be difficult so I thought about getting somethink like this:
https://www.tme.eu/gb/details/sk104-38sts/radiators/fischer-elektronik/
Or this:
https://www.tme.eu/gb/details/hs-132-38/radiators/stonecold/

IanB:

--- Quote from: belzrebuth on August 27, 2018, 03:08:43 pm ---For reference this power supply will be mounted on a wooden case used for a modular synthesizer so there's no metal chassis or too much space for huge heatsinks etc..
--- End quote ---

This will be awkward since wood is an insulator. If you put a heatsink on a part inside a wooden box it won't be a heatsink any more since the heat will be trapped inside the box and can't escape. You might need to consider drilling vent holes in the box and having a small fan to move air through it and carry the heat away. Heat sinks always need some kind of air flow to work, either natural or forced convection.

belzrebuth:
Fan is not an option unfortunately.
It's awkward enough as it is.
This a studio instrument so no fans is absolutely essential..

Anyway, I've tested the transformer again with a 20W bulb attached to each of the secondaries.
Current draw is 1.75A and the voltage is 12.5V (per secondary).

I can try it with 2x 20W in series but wouldn't I exceed the rating?
Is the 3.5A rating per winding or for the whole transformer?!?

Ian.M:
For the 50VA transformer, 1x 20W bulb would be an appropriate test load for its single winding.  I expect the result will be somewhere around 12.9V loaded.

100VA/(2*12V)=4.17A so 3.5A per winding would be a pretty conservative rating, (or a 100% rating if its actually an 85VA transformer)
2x 20W in PARALLEL per winding (4 bulbs total) would put it right at or possibly slightly over its max rated current.  Not a problem for 10 minutes per hour, but not something you'd want to do 24/7.  However you only need to test it to about 50VA which you've already done.

For 1A out per rail, assuming your 100VA transformer, neglecting unloaded losses in the primary and core, input power is 2*12.9V*1A/.62=41.6W.   Output power is 2*12V*1A=24W.   The difference of 17.6W must be got rid of.   

That's enough to make a medium sized closed wooden box pretty toasty after a few hours running, and it gets a lot worse if a significant portion of that 24W is also used inside the same box.   I'm with IanB - its either going to need a fan or an externally mounted heatsink.  As you are dead set against a fan, the only real option is a rear mounted external heatsink with its fins vertical for better convection.

That PCB layout is very stupid from a heatsinking point of view.  The regulators and the bridge rectifier should have been mounted along one board edge so they could bolt to a vertical heatsink.   The best option, if its got plated through holes would be to mount the regulators under the board with their legs bent up.  However that doesn't mesh well with your possible heatsinking options, so you may have to bite the bullet and mount the regulators off-board, with 0.1uF to ground from input and output right at the regulator so it doesn't break into RF oscillation due to wiring inductance.

N.B. LM317 tab is Vout and LM337 tab is Vin so they both need isolated mounting kits if you want to use a common heatsink.   A minimum thickness mica washer, thermal grease, and an insulating bush on the screw is the best option.

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