In this column, Kristen answers questions about all aspects of electronics, including
computer hardware, software, circuits, electronic theory, troubleshooting, and anything
else of interest to the hobbyist. Feel free to participate with your questions, comments, or
suggestions. Send all questions and comments to: Q&A@nutsvolts.com.
; WITH KRISTEN A. McINTYRE
Neuter the Spray
QI have a kitchen mixer that comes up to full power gradually after it’s turned on to prevent ingredients from being flung around. I also have a router with the same feature to prevent
jerking when it’s turned on. Each of these tools takes about
a second to come up to full power but is much easier to
use because of this feature. I also have a hand (immersion)
blender that I use in the kitchen that doesn’t have this
capability and more often than not it sprays ingredients all
over when I turn it on.
I want to make a device to plug the hand blender into
that will bring it up to full power gradually (over a second,
more or less.) The hand blender is rated at 400 watts, 120
volts, 60 Hz. What approach would you take to implement
such a device?
Greg Cook KE7DO
AThis is a very interesting question. Since I don’t know much about the internal construction of the hand mixer (for example, the motor type), the best approach might
be to use a dimmer style circuit to
bring it up slowly. This can be done
externally without using an external
switch to turn it on, I think. If we’re
really clever, we can use the load
itself to activate the circuit.
A basic light dimmer looks
something like what we have in
Figure 1. The circuit operation
is actually fairly simple if you
understand how a TRIAC (or its
simpler cousin, the SCR) functions.
It assumes that we can control
whatever the load is by a form of
pulse-width modulating the AC line,
though it’s not precisely a pulse. Note
that this basic circuit is not augmented for some
common dimmer problems, like hysteresis and RF
An SCR (Silicon Controlled Rectifier) has the
property that once turned on by a current flowing
into the gate, current will flow from the anode to
cathode even if the gate current is removed, and
until the anode-cathode current goes away or the
voltage relationship reverses, reversing the current. So, you
turn it on and it stays stuck on (or latches) until a reversal,
which is what we care about in this case.
Figure 2 shows a transistor equivalent circuit of an
SCR. Note that this is not a practical circuit, but instead is
just a way of showing how it acts. Real world transistors
would not be happy with the amount of base-emitter
current in this configuration.
A TRIAC is a pair of SCRs that are connected back-to-back so that we can have current flow in both directions,
with the same property where one or the other SCR gets
stuck on until a voltage reversal.
Since the input waveform is a 60 Hz (or 50 Hz) sine
wave, roughly, we can decide when to turn on the TRIAC
in the cycle, and it will stay on until the next zero crossing.
If we turn it on early, we get an almost unmodified 60
Hz waveform. If we turn it on late, we get just part of the
cycle. You can see an example of the input (in blue) and
output (in red) waveforms in Figure 3. The red waveform
is near zero until the turn-on point, and then it tracks the
input blue waveform.
• Neuter the Spray
• A Clearly Better Baby Monitor
Q & A
; FIGURE 1. Basic TRIAC dimmer circuit.
; FIGURE 2. Silicon Controlled
Rectifier equivalent circuit.
6 February 2018