Will get pulsating DC on a transformer's secondary if I apply pulsating DC to it's primary?

[Johan96]

I've been busy with an electrolysis project for quite some time now with some great results...the only problem now is that I've rewound an old welding transformer where the output on my secondary is only 5V with a short circuit current topping off at 900A-ish. It can handle 6-700A with ease( a dedicated cooling system...LOL) but now another even more expensive obsticale is that I don't have the money to rectify that amount of current.

So my question is, if I put my AC source through a full bridge rectifier and apply the resulting pulsating DC to the primary winding of my transformer, will I get the same waveform on my secondary which I can then just run through a bunch of capacitors?(by a "bunch of capacitors", I mean splitting the current to prevent heating)
If so, then I won't need such beefy semi conductors.....right?

Update: I'm not a very patient person so I went ahead and tried it anyway...
I attached my mains directly to a rectifier and the "DC" output to the transformer...it immediately trips my circuit breakers, no matter if I'm pulling current on the secondary or not...any reason as to why that may be?

 

[kellys_eye]

120 or 240V?

Either way, the rectification results in a higher DC voltage than the rated primary. Pulsing the primary will result in an AC output - the voltage, as it increases, induces voltage in the secondary and then, as it collapses, induces voltage in the opposite direction (a simplification) so you get AC anyway.

If you are using 120AC then the 900A at 5V is 4500W - even off-load the switch-on surge could be tripping the breaker.

You can get 5V DC power supplies (and 12V) rated at 1000W - so 5V @ 200+A - for a few dollars. They are listed as ex-server PSU's (computers) and you can parallel the outputs for as many amps as you want. I have a 12V 100A version that cost me £8 + postage (UK price - around 15 bucks delivered) that I modified (a simple tweak) to deliver 13.8V for charging and powering radio equipment.

Play safe, get the right stuff.

 

[Alec_t]

I attached my mains directly to a rectifier and the "DC" output to the transformer...it immediately trips my circuit breakers, no matter if I'm pulling current on the secondary or not...any reason as to why that may be?

It is likely the transformer core saturated, so the primary coil inductance dropped, so the coil drew a lot more current than it was rated for.

 

[hevans1944]

So my question is, if I put my AC source...

Yeah, AC source should have been a CLUE concerning WHY your scheme didn't work as planned. The transformer, consisting of two inductors (primary and secondary windings) magnetically-coupled through a common core, should have been a second CLUE, if only you understood how transformers actually work. And impatience is NOT what most of us here, who are involved in practical applications of electronics, embrace. Impatience almost always leads to errors if not outright disaster.

So, here is your "answer": transformers will never generate a direct current output, and transformers will not perform as intended if an excessive direct current component is present in their primary excitation. All materials (except vacuum and most gasses) will magnetically saturate when the magnetic field induced in the core, by current in the primary winding, reaches a certain large value. This saturation is demonstrated by plotting the induced magnetic field against the magnetic field created by current in the primary, a so-called B-H hysteresis curve for the transformer core.

When magnetic saturation of the transformer core occurs, any further attempts to increase the induced magnetic field by increases in the primary current will fail. As a result, when the transformer core goes into magnetic saturation, the reactive part of its complex impedance to the AC component of the primary excitation decreases to essentially zero. The real or resistive component of the primary impedance is determined by the copper resistance of the winding, but the primary current is mainly determined by the imaginary component of the impedance. If the transformer core magnetically saturates, the primary impedance decreases to a dangerously low value and the low copper resistance means the mains current increases to dangerously high levels.

Core saturation is actually used in some types of pulse transformers that depend on the rapid collapse of the magnetic field to create a high-voltage pulsed output in the secondary. This pulse will have a specific positive or negative polarity, but it is always followed by a much smaller and longer-duration pulse of the opposite polarity. There is net zero direct current from the pulse transformer secondary! Or stated another way: transformer secondaries do not produce direct current, no matter how you excite the primary.

@Jodie25: "Hope this will help." is NOT a strategy. And your post is just wrong. Transformers DO NOT pass direct current from primary to secondary. If you believe differently, please post a circuit schematic with fully specified components and an oscilloscope waveform showing the DC output.

 

[crutschow]

In summary, if you want 700Adc, then you will need to rectify the 700Aac to get the DC.

I have a circuit using MOSFETs in a bridge to reduce the dissipation as compared to Schottky rectifiers, but it would require several MOSFETs in each leg of the bridge to handle that much current.

If you double the number of secondary turns and add a center-tap, then you could use a 2-diode, full-wave rectifier configuration and cut the diode losses in half.

 

[batdetector]

Hi. I see... you are trying to use a low current rectifier on the primary to avoid using a big one on the secondary. Sorry it won't work. The transformer core will just saturate and stop being an inductor and pass loads of current. If you get any secondary current, it will be AC.

Why not charge up some car batteries and use them for the high current supply?

 

[hevans1944]

I've been busy with an electrolysis project for quite some time now with some great results...

I sincerely hope this isn't another HHO electrolysis attempt to get "free" energy to operate an automobile internal combustion engine using just water, an alternator to generate electrolysis, and hand-waving explanations... hmmm. To better get electrolysis, pure water needs an assist, a catalyst perhaps. Common baking soda OR white vinegar (NEVER BOTH AT THE SAME TIME!) can be added to the water, stirred vigorously, and electrodes energized to begin the electrolysis process. It gets complicated really quickly once the electrical current is established.

The electroplating industry is quite well-developed, and if large areas are to be electroplated, very large currents, thousands of amperes, are required. Electroplating can also be very dangerous because of poisonous chemicals that are often required for plating. So... what is really going on here?

 

[danadak]

Update: I'm not a very patient person so I went ahead and tried it anyway...

Just a word of caution, this can get you killed in this business, careful thought, preparation, and safety are
intertwined at the most basic level. Hopefully you care about coworkers, random children passing thur
your lab, loved ones......not in the least yourself.

Some good practice ref material : http://www.arrl.org/electrical-safety