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:

  1. Why does this happen? If it's rated 12V 10A, why can't I even get half the current while still keeping the voltage?
  2. 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.

  • 2
    \$\begingroup\$ What's providing the input to the transformer primary? \$\endgroup\$
    – The Photon
    Commented May 31, 2020 at 15:21
  • 1
    \$\begingroup\$ Common factors: AC supply and an unknown box of tricks with resistors and a transistor. Just not worth suggesting anything until you prove that the unknowns are actually fit for purpose. \$\endgroup\$
    – Andy aka
    Commented May 31, 2020 at 15:28
  • \$\begingroup\$ And do you have a datasheet for the rectifier? \$\endgroup\$
    – The Photon
    Commented May 31, 2020 at 15:28
  • 2
    \$\begingroup\$ Please connect an AC voltmeter in parallel to the primary winding of the transformer to see what happens. I have doubts about the 220V cable. Connect a load without a rectifier. \$\endgroup\$
    – Peter MP
    Commented May 31, 2020 at 16:44
  • \$\begingroup\$ Also consider the wire gauge you are using : you can easily lose a volt or two in wiring losses. \$\endgroup\$
    – user16324
    Commented May 31, 2020 at 16:48

3 Answers 3


This is pretty straightforward: your transformer only supplies current during a small proportion of the line frequency cycle. This is the period when the transformer output is greater than the capacitor voltage. When the transformer voltage is less than the cap, the bridge diodes prevent current flow. The transformer is unable to provide the relatively high-current, low-duration pulses required to keep the cap charged. Essentially, a bridge only provides current to the cap for a fairly narrow window around the peak of the input sine wave. If it's not, this implies that the output current from the cap is so high that the voltage is being kept much lower than you think.

Another issue is the size of your filter cap - it is far too low. You need to learn to calculate the voltage/current relationship of capacitors. The base equation is $${dV}/{dt} = i/C $$ so, for instance, if you are drawing 10 amps from a 2000 uF cap, when the transformer voltage is less than the cap, the voltage at the cap will change at a rate of 10/.002, or about 5000 volts/second. Assuming you are using 50 Hz line frequency, the cap will completely discharge from 15 volts in 15/5000 seconds, or about 3 msec. Since your line frequency is 50 Hz and the output of the bridge will be full-wave rectified sine at 100 Hz, the load will only have current through it for about 30% of the time, and the average voltage will drop significantly, since it will be zero about 70% of the time. To make things worse, the transformer will be unable to fully charge the cap during each cycle,so the average will be even worse.

All of this will be obvious if you have an oscilloscope. I suggest you get a cheap PC-connected scope, which have a fairly simple A/D connector board and use the PC as a display. These are available from dBay quite cheaply, although as always caveat emptor.

A simpler approach is to ditch your load box entirely. To see what the transformer can do, load it directly with some power resistors, and use the meter to measure both the load voltage and current (although not both at the same time, of course). Simply put a known resistor/resistors on the transformer and measure the voltage. Dividing the voltage by the known resistance will let you back out the current. Just be careful about cooling the resistors. A 15-ohm resistor on a 12-volt transformer will dissipate nearly 10 watts.


Your capacitor is far from adequate to maintain the peak voltage, and you're probably exceeding the rating for the capacitor ripple current. To get 1Vp-p ripple at 10A out you need about 100,000uF, assuming 50Hz mains.

Also from a transformer rated at 12V 10A (RMS) you can expect to get a bit over 6A DC safely from the filtered DC output. That scales.


Your test setup is flawed. Try this: measure both the DC voltage AND AC voltage across the load. Then ask yourself why the AC voltage is so high.

The first step is to verify the transformer is performing to spec. Use a resistive load such high-power resistors or a bunch of incandescent light bulbs.

If you are using light bulbs, don't worry about the actual voltage rating of the bulbs so long as the actual voltage rating is equal to or higher than the output voltage of the transformer. If all you have available is incandescent AC-Mains bulbs, that's okay. You can will have use a lot of them, is all.

Set your DMM to the AC current 10 Amp range and connect the DMM in series with your load resistors or bulbs. Keep adding bulbs or resistors in parallel and observe the AC current. Stop at 1 Amp, then measure the AC voltage at the secondary of the transformer.

Keep increasing the load current while measuring the output voltage of the transformer. Make a table of measured current and voltage.

You should be able to determine pretty quickly if your transformer is working correctly.

Only after you have determined that the transformer is working properly should you continue your project.

  • \$\begingroup\$ Thank you very much for your sugestions, i will do all these measurements tomorrow and see what i get. \$\endgroup\$
    – AdrianNULL
    Commented May 31, 2020 at 22:58
  • \$\begingroup\$ I've done some tests today and i've clearly seen what i didn't understood before. Thanks for the help. \$\endgroup\$
    – AdrianNULL
    Commented Jun 1, 2020 at 18:51

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