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If I convert the AC from a transformer to DC using 4 diodes and one capacitor and I do it for another transformer and connect those DC out in series, what will happen? Is there any problem? How much current will it provide? How to calculate the current? All the dioes used here is 1N4007.

schematic

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    \$\begingroup\$ If you do that properly there is no issue. A transformer can deliver a certain maximum output current, you cannot exceed that current. If you connect a 1 A transformer and a 5 A transformer in series you can only draw 1 A as that is the limit of one of the transformers. You should really draw a schematic to show what you want to do and give details about the transformers. Also explain why you need to use two transformers and one is not enough. \$\endgroup\$ – Bimpelrekkie Jun 16 at 7:42
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    \$\begingroup\$ This feels a bit under-researched; have you tried drawing the schematic of what you describe? Then picked one arbitrary point in time and replaced all diodes that are "on" at that point with direct shorts? What happens? If you do that correctly, you'll be able to describe all things you're asking for yourself. If not: ask about where you're stuck! \$\endgroup\$ – mmmm Jun 16 at 7:44
  • \$\begingroup\$ I have added a schematic of what I am telling about. \$\endgroup\$ – Walid Jun 18 at 19:48
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The output current from two isolated power sources in series will be limited to smaller of the two constituent supplies.

A simple '4 diodes one capacitor' supply has a lot of ripple. The output ripple will be the sum of the two constituent supplies.

Depending on your application, the current limitation may be temperature rise in the transformers or diodes, or it may be voltage droop or ripple on the output capacitors.

Bear in mind that a '3 A' transformer is probably rated for RMS current, that is, for current into a resistive load. Driving a rectifier produces current with a very peaky waveform, and so the RMS (transformer heating effect) is disproportionately higher than the average (DC output to the load). This means a transformer has to be 'derated' when driving a rectifier. I don't have the figures to hand, but you should be able to search for 'transformer rectifier current derating'.

It would be worth pointing out that you could increase the output voltage by two diode drops (which alternatively means increase the output current for the same output voltage) by putting the two transformer secondaries directly in series, and using a single quad of diodes for rectification. That reduces the series string of diodes in the path of the current from 4 to 2.

It would also be worth investigating the output (as Frog suggests) from the 3A transformer alone driving a dual halfwave voltage doubler, but with bigger diodes than 1N400x. Curiously, two halfwave rectifiers in series still have ripple at the same frequency as a fullwave rectifier. While each 34 V output has only 50 Hz ripple, the 68 V output taken between them has 100 Hz ripple.

schematic

simulate this circuit – Schematic created using CircuitLab

If you are dealing with very high voltage output supplies, say more than your input mains voltage, then you may need to be concerned about stacking output voltages exceeding the rating of the secondary insulation.

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  • \$\begingroup\$ Looking at the OPs schematic, the output current should not exceed 1A, and a voltage of 68V can be expected if the load is minimal. You could consider using a half-wave rectifier which will give you the same voltage and more current (1.5A) using just the 3A transformer. At 1A you can expect a ripple of several volts, and with a half-wave rectifier it would be roughly double that, so perhaps consider using larger capacitors. \$\endgroup\$ – Frog Jun 18 at 21:39
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    \$\begingroup\$ @Frog That arrangement is sufficiently different from my answer that it's worth you working it up into a separate answer. However, it would be worth doing accurate calaculations of the current you can expect through half wave rectifiers (only 50% duty cycle), perhaps change 1N400x to 1N540x diodes. Perhaps using both transformers to put these two doubled halfwave outputs in parallel, in antiphase, to reduce the output ripple slightly, though currents might not split in the ratio of the transformers' current capability. \$\endgroup\$ – Neil_UK Jun 19 at 6:07
  • \$\begingroup\$ @Frog, I should use just two 3A 24Volts transformers and should use individual half wave rectifier for each transformer. Then I will get 48V or, 68v at 1.5A right? \$\endgroup\$ – Walid Jun 26 at 7:34
  • \$\begingroup\$ With a single 24V transformer you can generate +34, 0 and -34V so 68V total at 1.5A. With two transformers you have the option to use full-wave rectifiers as per your drawing, or use two half-wave rectifiers with the secondaries in antiphase as Neil_UK suggests; that gives the same ripple as the full wave rectifiers but with four diodes instead of 8. You’ll need diodes that are good for 3A such as 1N540x series. \$\endgroup\$ – Frog Jun 26 at 8:40
  • \$\begingroup\$ @Walid Bear in mind the derating that has to go with a rectifier load on a transformer. \$\endgroup\$ – Neil_UK Jun 26 at 8:51

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