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In the book Practical Electronics for Inventors, 3rd Ed., the authors recommend against using half-wave rectifiers because they're inefficient and cause "...the core to become polarized and to saturate in one direction." (Page 395.) Is this a valid concern and what are the risks for a long running half-wave rectifier power supply?

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I had a disastrously failing transformer once probably caused by the single wave rectification. It was used for a halogen lamp, with a dimmed and a full brightness mode. Disastrous as in a blue flash from the 12V halogen lamp when connecting it to th 230V mains. I suspect primary and secondary shorted out. – jippie Feb 12 at 20:35
    
Many illuminated doorbells (a.k.a. "ambient" doorbells) have a diode in the front door button to provide continuous power to the chime. I suspect the amount of power is low in this application and it may even be unfiltered if the lights are incandescent. This is a real example very long running half-wave rectification. Perhaps because of the low draw of these circuits the impact on the transformer is negligible? – Phil Feb 12 at 21:12
up vote 7 down vote accepted

Hammond recommends an output DC current of 0.28 times the RMS current rating of the transformer for half wave rectification and 0.62 times the RMS current rating for full wave bridge rectified current.

enter image description here

enter image description here

So if you don't mind using an AC transformer that is 2.2 times bigger (and a filter capacitor that is twice the size) you can save some diodes.

Since the smallest common size of a mains transformer is a couple of watts, it might be a reasonable choice if the current requirements are modest. Also, you save a diode drop so you get a bit more voltage.

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Yes. A half wave rectifier only draws uni-directional current. This causes the magnetisation in the core to get a DC bias, which shifts the mid point of the magnetisation curve away from zero.

The effect of this is a high saturation current pulse is drawn from the supply, as well as the normal load current. Depending on the details of the transformer winding and core, and how big the load is, this may or may not overheat the transformer.

How this happens is quite subtle. Andy_aka and Dave Tweed (and many others) insist that a transformer 'should not' exhibit this effect, secondary current should not affect the flux in the core. And certainly for an ideal transformer, with a superconducting primary, they would be correct, the load current does not influence the core flux directly.

However, when you connect an oscilloscope to a real transformer, as documented in my post here in another forum, you see a significant shift in saturation behaviour. So what's going on?

The uni-directional secondary current causes a uni-directional primary current to be drawn. Because the primary has resistance, this causes a uni-directional voltage drop in the resistance, which causes an offset DC voltage on the primary. This voltage causes a current to build in the primary inductance, causing a steady flux to build in the core.

How far does that flux build up? Without core saturation, it would build indefinitely. With core saturation, the transformer begins to take heavy pulses of current as the core goes into saturation. These large current pulses generate large voltage pulses in the primary winding resistance, and eventually, when a steady state is reached, the voltage drop due to the uni-directional load is balanced by the voltage drop due to the saturation pulses.

The flux in the transformer has moved, so that although the output current is uni-directional, the input primary current is bi-directional, zero mean again.

Quick key to my diagrams.

Blue trace - mains input voltage
Purple trace - load voltage and current
Yellow trace - mains input current

Top scope shot - transformer with no load
Middle scope shot - with normal resistive load
Bottom scope shot - with rectified resistive load

Looking at the yellow current trace, it is clear that the effect has been to return the primary current to an AC current, so that the voltage it develops in Rp is overall zero.

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The field in the core is independent of the load current. – Dave Tweed Feb 12 at 18:03
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You have measurements to back that up? – Neil_UK Feb 12 at 18:05
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No, just basic electromagnetic field theory. Do you? – Dave Tweed Feb 12 at 18:06
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This in another forum. First light on my new 4 channel Rigol. Perhaps you'd explain all the curves. This particular core is quite soft, conservatively designed, so isn't saturating hard, but it shows the effect. Other cores are harder. – Neil_UK Feb 12 at 18:09
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That forum post can be quite easily explained by a voltage source on the primary that wasn't too low in impedance. In other words the half wave rectifier current actually causes an asymmetry in the driving source waveform. Also, being as magnetization current is due to the primary unloaded inductance you will see saturation occuring as the voltage crosses thru zero (90 degrees offset) - this is EXACTLY what is seen in that post therefore proving it is mag current and not load current that causes saturation. – Andy aka Feb 12 at 19:17

Any saturation in the core of a transformer is due to the magnetization current and has nothing to do with the currents that might flow due to any load. The reason is because the ampere turns in the secondary produced by the load exactly cancel the ampere turns in the primary that are caused the load.

The book is wrong and here's why: -

enter image description here

  • Scenario 1 is a single turn primary - it acts like an inductor and current Im flows.
  • In scenario 2 the primary is converted to two parallel turns. Im/2 flows in each winding.
  • Scenario 3 is a basic transformer. The voltage seen at the output is the same phase as that at the input. It has to be else in scenario 2 there would be an unholy flow of current around the windings.
  • Scenario 4 has a load on the secondary and the current in the secondary must flow in the opposite direction to the load current in the primary.

Hence, loading a transformer secondary does not increase saturation.

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This answer does not consider the effect of the transformer winding resistance or leakage inductance. In the case of higher loads, there will be a voltage drop across this R and L during the portion of the waveform where the rectifier diode is conducting into the load. This drop will reduce the voltage seen by the core, causing the magnetization current to be decreased in one half of the cycle compared to the other half of the cycle. This may cause the transformer to gradually "walk" into saturation. – ConduitForSale Feb 12 at 19:20
    
@ConduitForSale the peak of the magnetization current is seen at the zero cross of the voltage therefore where the resistive load current peaks is of no consequence for mag current (90 degrees away). – Andy aka Feb 12 at 19:23
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This is why many countries implicitly (or sometimes explicitly) forbid half-wave rectifiers via limits on the amount of even harmonics in a device's mains current. It can cause distribution transformers to saturate. – ConduitForSale Feb 12 at 19:24
    
Pretty arguments. However, I'd like to see your measurements of a real core, with non-linear permeability leading to saturation. – Neil_UK Feb 12 at 21:07

A transformer's coil currents cause the H field, and -d/dt B causes the induced voltages, including the voltage counteracting the primary coil voltage and causing primary coil inductance. -d/dt B is the only thing actually having an effect on the external circuits, so any DC bias of the secondary current does not transfer itself to the primary current except by moving to a biased position in the B(H) curve. Since transformer saturation tends to set in rather rapidly, there is a point where -d/dt B just breaks down while current rushes in. Once you reach that point, the transformer will only offer DC resistance instead of inductance for almost half the time.

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No. "Hard on the transformer" is determined by the power applied to it. Look at the VA rating.

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