I have obtained a small transformer with 7 windings, a 220V and 6 other individually isolated low voltages. How can I find out the current rating for the lower voltage windings? I was told these used to be made for "CD players". This is from the local market, so no datasheet. Transformer weighs about 500 grams and looks reasonably well made. If it was only one secondary 12-0-12V, I'd say it would be 1-1.5A. But, with 6 completely isolated secondaries, I'm not sure. The wires near the output seem probably slightly less than 0.5mm dia, but I'm not sure, it may be thinner. Output voltages were listed, and I measured the resistance with a DMM:
Volts Ohms 220 229 13.5 1.3 10 1.5 12 1.9 11.5 2.3 3.5 2.5 21.5 11.1
I got this to power op amps, and am currently building an electronic load (for a power supply that I will be building next). So, if an electronic load can be used to guess the output current rating (by plotting voltage drop etc...), that will also be a big help. Obviously op amps will only draw ~1mA, so I'm safe for now.... but circuits tend to get bigger...
This is my first question on stackexchange. Thanks in advance for all help and suggestions!
 Thanks Neil and Tony!
Assuming 20VA for the transformer (as it's not a toroid), I have the following calculations:
wire V R V2/R VA I red 220 229 ~20 blue 13.5 1.3 140 7.2 0.53 purple 10 1.5 66 3.4 0.34 green 12 1.9 75 3.9 0.32 white 11.5 2.3 57 2.9 0.25 black 3.5 2.5 5 0.3 0.08 yellow 21.5 11.1 42 2.2 0.10
I have managed to make an electronic load on the breadboard, and although it's not terribly accurate because of the wire resistances, it seems to be working reasonably. I used it to load up the blue winding. Firstly, the transformer is becoming warm even with no load (all secondaries disconnected). With some back an forth loading over 30 min (while testing the electronic load, mostly below 200mA, maybe 2 minutes @ 400mA), it became quite warm (under ceiling fan, room temp 30C, rel humidity 75% here in Kolkata/Calcutta). I didn't measure temperature as it wouldn't be accurate under the ceiling fan. Forgot to measure the hot resistance as well. Also, the voltage drop was a lot. DC voltage (measured after a 1N4007 bridge and 1000uf cap) went down quickly (open circuit voltage close to 18.5V, went down to even 11.5V under load):
V mA 18.22 0.32 17.0 33.5 15.8 150 15.0 180 14.88 245 13.8 350
All this for the blue winding. (Also I managed to fry an lm358 as I stupidly forgot that transformers put out AC, thankfully the IRF540 was spared). The open circuit AC voltage is 13.8V.
I'll see if I can load up all the windings simultaneously for the test you suggested, but I'm a bit scared considering how fast it's heating up. I'll probably have to measure the AC primary current to check what is happening. The other 9-0-9V transformer I have has 1K resistance on the primary winding, and it stays cool even with 175mA load.
Neil, thanks for the excellent answer, I'll accept it, and see if I can do the resistance test in future.
Added Feb 2018: After some more experience with transformers in the local markets in India... Transformers here are rated on the open circuit voltage (no load). i.e. a 12V 5A transformer will show 12V at no load. Super hyped up ratings? Customer beware? One needs to carry a multimeter and measure the secondary winding resistance to estimate voltage drop and heating before purchase.