[Amps] Rectifier diode strings

[Tom Rauch]

> Unless I misunderstood Tom's point about mains transformers saturating
> at 1.3-1.4 times normal primary voltage, surely that would only apply
> at full load? The main risk to the rectifiers is more likely to be
> during RX periods, when the load is at a minimum but the DC voltage is
> at a maximum. I don't believe the saturation argument would apply
> then, would it?

Yes, saturation applies. Remember what it is. The core can only 
obtain a certain flux density, and that flux density relates only to 
the volt-turn of the magnetizing winding. Flux density is the same at 
full load or no load, ignoring small differences caused by resistance 
in the primary and power mains.

Secondary voltage depends only on the flux density and number of 
turns, again ignoring small losses.

To find the saturation point all you need do is raise the primary 
voltage while watching peak current draw, with all secondary windings 
disconnected. You'll see peak current increases in direct 
proportional to voltage increase until the core saturates. At that 
voltage, peak current will increase greatly with very small increases 
in primary voltage. If you look with a scope, you will see the 
waveform flatten-off on peaks.

That is the point where the core is loaded with the maximum flux 
density it can carry. 

Once that voltage is reached, you just can't obtain significantly 
more voltage from the secondary. It has nothing to do with current 
load.   

When a transient comes along, it must increase flux density in the 
core to increase secondary voltage. That's true even if you are not 
pulling ANY current from the transformer. The transient must also 
overcome any eddy currents generated because the transient frequency 
is high, and overcome distributed capacitance.

By far the worse problems are in transformers with interleaved or 
bifilar windings (like audio transformers), and transformers that 
step voltage DOWN. That's why we hear whining about transients today, 
and never did back in the earlier years when things operated at 
higher voltages. When a transformer steps down, direct capacitive 
coupling can allow the several hundred volt transient to spike the 
secondary with a high-voltage pulse somewhat less than the line 
transient value but still much more than the low voltage lightly 
loaded secondary. Since the transient is NOT in a push-pull mode or 
mode normal to the rectifiers, it CAN find it's way into a reverse 
biased rectifier and break the junction down. The normal voltage 
being applied can help supply current to the broken-down junction 
before the rectifier turns back off (remember it is reverse biased in 
this example), and the longer duration high current trough a junction 
trying to turn off cooks the rectifier.

If a 1000 volt transient capacitance couples to the secondary of a 
4000 volt transformer, who cares? It is insignificant.

By a very large margin the supplies that are worrisome for transients 
are low-voltage low-current supplies, usually having half-wave 
rectifiers and generally frail components. The insulation is poor, 
and the winding close together to save space.

HV supplies are not only much less of a risk, they generally protect 
themselves so well adding an MOV is like throwing money into a forge. 
   
 > However, that argument cannot hold up forever as the spikes get
> longer, and turn into voltage "surges" that last for more than one
> mains cycle. It only takes a couple of half-cycles at a new, higher
> primary voltage to pump the capacitor up to the new rectified DC
> voltage (within a few percent)

The MOV won't do much good when it explodes from the dissipation. 
Fortunately the transformer of any amplifier beyond sissy-class will 
generally be large enough to protect the thin-lead frail MOV from 
damage. 
 
> These longer surges can come from things like braking a large spinning
> motor while it's still connected to the mains, as Paul mentioned.
 
When I brake a spinning motor, I disconnect it from the mains and 
either short it or apply DC bias. It is generally very unwise and of 
limited value to throw a motor into reverse to brake it. 

> Another common source is the Field Day generator, when all the
> stations just happen to stop transmitting at the same time; for a few
> moments the motor is left running at full throttle

Motor speed is constant, unless you have a variable frequency 
generator. Voltage is the controlled by changing flux density, not 
speed. As a matter of the fact, the generator impedance is almost 
perfectly matched to the load impedance by the feedback system 
that control flux in the field, resulting in a near-conjugate match 
at any load impedance.

The surge comes because the core in the generator retains magnetic 
flux from higher demand loads, and when load is removed the 
resistance of the winding no longer drops voltages. Since flux is 
high, voltage rises. As a matter of fact we used to measure ESR of 
generators by loading them with a load and removing the load abruptly 
at or just before the sine-wave peak. The voltage difference 
between loaded and unloaded voltage allowed us to calculate generator 
ESR and power loss.

If you think a MOV will handle that transient, better think again. 
Any large transformers will generally saturate long before the MOV 
clamps. Thankfully, the MOV will be protected by any large 
transformers.

Small whimpy transformers are another matter, they might need the 
protection of a MOV.73, Tom W8JI
W8JI@contesting.com