TLDR: I've made a short video about this, if you like looking at a video more than reading this. It's here: https://youtu.be/51235iFywI4
The problem is pretty straighforward.
I have a (toroidal) transformer that has two secondaries (completely independent), each giving 12V 10A as written on the label of the transformer. In reality, I can't even get close enough to this.
To test it, I've assembled the simplest circuit possible. One output of this transformer goes into a full bridge rectifier (rated 15A), then an ammeter, then a 2 mF capacitor for filtering (that's a 2 millifarad, as in 2000 uF) and then it's connected to a variable load device I've made earlier (inside this device there are only power resistors and a single power transistor, so the device itself is purely resistive load; no capacitors, no coils; although in DC I'm thinking it shouldn't matter.)
Initially, with no load, I have around 15V (which is normal). The more current I draw, the more the voltage drops and it goes below the level that I would expect. If I draw only 4A of current, the voltage is already around 9.5V. If I remember correctly, it goes down to 8V at 5A and so on.
The transformer is new, in perfect condition, almost never used. I've used this one only because it's got a clear label on it, but the exact same problem occurs on other transformers as well. I also have 2x classic transformers (meaning not toroidal) of 2x12V 3A and I can't even get 1A from them @ 12V.
So, basically, I have 2 questions:
- Why does this happen? If it's rated 12V 10A, why can't I even get half the current while still keeping the voltage?
- What kind of transformer should I buy in order to actually have 10A @ 12V (or higher?) Should i get a 30A, 40A?
To further test my setup a little bit, I've taken a switching power supply and put it to the test. Yes, I know, these are completely different things. Switching power supplies are complex electronic devices, transformers are passive components, they cannot be compared. I know and I'm not even making a comparison. I just wanted to test this experiment and have a different perspective. And the test was successful. I was able to draw 1.8A and the voltage dropped only to 11.7V (it's a 12V 2.1A supply). I didn't wanted to push it to the limits, the test was good enough for me.
P.S.: Please don't tell me to ditch the transformer and use a switching power supply. For what I want it's not an option for multiple reasons.