[Amps] Solid state relays..again
manfred at ludens.cl
Wed Feb 12 12:56:05 EST 2014
> Once in a while that thing turns on with a
> WHummpppp and a low frequency ringing that just trails off.
Let's see if I can explain this in a way everyone can understand.
When a power transformer is operating normally, the magnetic flux
density in its core fluctuates in a sine wave function. The peaks of
this sine wave are usually roughly where saturation of the core begins,
but is not yet too bad. Typical values for power transformers would be
between 1 and 1.3 tesla of flux density. Those who learned
electromagnetics at least 40 or 50 years ago, probably are more used to
the antiquated gauss unit - anyway, 10,000 gauss is 1 tesla.
The sine wave of the magnetic flux is in the same phase as the
magnetizing current in the primary, which is 90 degrees lagging the line
voltage. This means that when the line voltage crosses zero, flux
density is maximum, and when the line voltage is maximum, flux density
Whenever the line voltage is positive, flux goes up, and whenever line
voltage is negative, flux goes down (accepting some simplistic
conventions on what is positive line voltage and increasing flux). So,
at the time the voltage waveform crosses zero going from negative to
positive, the flux density is at its negative peak. With the voltage
being positive from there on, the flux density goes up - that is, it
goes from -1 tesla to -0.9 tesla to -0.5 tesla, etc, reaches zero, then
gets positive, keeps climbing, and by the end of the voltage semicycle
the flux density has become +1 tesla. Then the voltage goes negative,
making the flux density start going downwards again.
This should be clear enough, I hope. Now let's see what happens when you
switch on a transformer.
If you do that precisely at the peak of the line voltage, which is the
time when the flux density should be zero, and given that it _is_ zero
in the transformer that was off, the transformer simply starts
operating, without any sort of strange behaviour, overcurrent, spikes,
or anything. This would be the optimal situation, from the transformer's
point of view. But if you power up the transformer at any other time,
things are not as good. The worst situation is switching on the
transformer at the voltage zero crossing. In this case, the core is at
zero flux density and now gets a full semicycle of voltage in one and
the same polarity. So, instead of starting from -1 tesla to go to +1
tesla, it will start from 0 tesla and thus will try to go up to 2 tesla
in that first semicycle! And 2 tesla causes total saturation of the
core. When a core saturates, several things happen. One is that the
inductance of the windings drops drastically, producing a strong current
pulse in the primary. And another is that the core no longer
concentrates most of the flux, so that a significant amount of the total
flux leaves the core, and flows through the air around it, and specially
through anything magnetic in the vicinity. If the transformer is housed
in a steel cabinet, the panels of that cabinet will attract a huge part
of this leaked flux.
During the next several cycles, the resistance of the windings, losses
of the core, etc, play together to gradually center the magnetic flux
around zero. That is, while in the first cycle the flux moved between
zero and 2 tesla, during the second cycle it might fall between -0.2 and
+1.8 tesla, in the third cycle it might go from -0.35 and +1.65 tesla,
and so on. It can take ten or more cycles to center the flux well enough
to make the transformer work normally, without spreading flux around any
The cabinet panels will thus pick up lots of flux initially, and then
gradually less, over several cycles. And they will act like
electromagnets, while they are picking up flux. This makes them move,
proportionally to the flux they get. And since the flux is alternating,
the panels will vibrate.
And this, my dear friends, is why equipment with steel boxes and big
transformers inside can make that "WHummpppp" at the moment it's
Of course, in equipment controlled by a simple mechanical power switch,
it's impossible to decide when exactly in the cycle the switch will
close. It's this a lottery how close we will get to the peak voltage
point, or the zero crossing. That's why sometimes the "WHummpppp" is
strong, sometimes weaker, and sometimes even nonexistant.
OK. Now somebody out there might say that it would be a good idea to
make a circuit that always powers up an amplifier or power supply at the
peak voltage, to avoid that "WHummpppp". But it ain't that simple... We
aren't powering up just a transformer, but instead a combination of a
transformer and filter capacitors. The filter caps would prefer to be
powered up at the zero crossing... they don't agree with the transformer
That's why large power supplies should have at least a step start
circuit, or even better, a full soft start circuit. Full soft start can
be combined with power factor correction in the same circuit.
About solid state relays: They exist with TRIAC output, SCR output,
MOSFET output or IGBT output. Each of them can be made in versions that
switch on immediately when they get the control signal, or at the next
zero crossing of the applied voltage. All SCR and TRIAC SSR's switch off
as soon as the current through them falls to a certain level, after the
control signal has ceased. Usually that's close to the first current
zero crossing after the control signal ceases. But note that this is the
_current_ zero crossing, not voltage! MOSFET and IGBT SSR's can be made
to switch off immediately when the control signal ceases, or at the next
current zero crossing.
Handling inductive loads is more difficult than resistive ones, because
of switch-off, not switch-on. When a SSR switches off, there is still a
small current flowing in the load inductance. Even if small, it can be
enough to induce a high voltage spike at the moment the SSR switches
off, and this spike is easily strong enough to make the SSR switch on
again, which collapses the voltage, leaves a tiny current flowing, so it
switches off again, causing the next spike... leading to an oscillation,
which places great stress on the small control areas of the
semiconductor structure that forms a TRIAC or SCR. This is what can
kill SSR's or TRIACs when switching inductive loads.
The trick to safely switch inductive loads is to install a snubber in
parallel with the power device. It's typically as simple as a 0.1 µF
capacitor in series with a small 100 ohm resistor. This will absorb the
small residual current of the inductor, avoiding the creation of a
voltage spike, and will give the power device time to switch off fully.
But this network will also pass a tiny current even while the SSR is
supposedly off. That's why not all SSRs are fitted with that snubber.
Enough for now...
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