CS209A Datasheet, PDF (6/8 Pages) Cherry Semiconductor Corporation – Proximity Detector

CS209A
For this application it is recommended to use a core which
concentrates the magnetic field in only one direction. This
is accomplished very well with a pot core half. The next step
is to select a core material with low loss factor (inverse of Q).
The loss factor can be represented by a resistance in series
with the inductor which arises from core losses and is a
function of frequency.
The final step in obtaining a high Q inductor is the selection
of wire size. The higher the frequency the faster the decrease
in current density towards the center of the wire. Thus most
of the current flow is concentrated on the surface of the wire
resulting in a high AC resistance. LITZ wire is recommended
for this application. Considering the many factors involved,
it is also recommended to operate at a resonant frequency
between 200 and 700 kHz. The formula commonly used to
determine the Q for parallel resonant circuits is:
QP
^
R
2pfRL
where R is the effective resistance of the tank. The resistance
component of the inductor consists primarily of core losses
and “skin effect” or AC resistance.
The resonant capacitor should be selected to resonate with
the inductor within the frequency range recommended in
order to yield the highest Q. The capacitor type should be
selected to have low ESR: multilayer ceramic for example.
Detection distances vary for different metals. Following
are different detection distances for some selected metals
and metal objects relative to one particular circuit set−up:
Commonly Encountered Metals
Stainless Steel . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0.101″
Carbon Steel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0.125″
Copper . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0.044″
Aluminum . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0.053″
Brass . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0.052″
Coins
US Quarter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0.055″
Canadian Quarter . . . . . . . . . . . . . . . . . . . . . . . . . . 0.113″
1 German Mark . . . . . . . . . . . . . . . . . . . . . . . . . . 0.090″
1 Pound Sterling . . . . . . . . . . . . . . . . . . . . . . . . . . 0.080″
100 Japanese Yen . . . . . . . . . . . . . . . . . . . . . . . . . 0.093″
100 Italian Lira . . . . . . . . . . . . . . . . . . . . . . . . . . . 0.133″
Other
12 oz. soda can . . . . . . . . . . . . . . . . . . . . . . . . . . . 0.087″
Note that the above is only a comparison among different
metals and no attempt was made to achieve the greatest
detection distance.
A different type of application involves, for example,
detecting the teeth of a rotating gear. For these applications
the capacitor on DEMOD should not be selected too small
(not below 1000 pF) where the ripple becomes too large and
not too large (not greater than 0.01 μF) that the response time
is too slow. Figure 6 for example shows the capacitor ripple
only and Figure 7 shows the entire capacitor voltage and the
output pulses for an 8−tooth gear rotating at about 2400 rpm
using a 2200 pF capacitor on the DEMOD pin.
Because the output stages go into hard saturation, a time
interval is required to remove the stored base charge
resulting in both outputs being low for approximately 3.0 μs.
(See Figure 3.)
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