Principle of Operation: continued
where R is the effective resistance of the tank. The resis-
tance component of the inductor consists primarily of core
losses and Òskin effectÓ or AC resistance.
Note that the above is only a comparison among different
metals and no attempt was made to achieve the greatest
detection distance.
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.
A different type of application involves, for example,
detecting the teeth of a rotating gear. For these applica-
tions the capacitor on DEMOD should not be selected too
small (not below 1000pF) where the ripple becomes too
large and not too large (not greater than 0.01µF) that the
response time is too slow. Figure 1 for example shows the
capacitor ripple only and Figure 2 shows the entire capaci-
tor voltage and the output pulses for an 8-tooth gear rotat-
ing at about 2400 rpm using a 2200pF capacitor on the
DEMOD pin.
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
¥ Carbon steel
¥ Copper
¥ Aluminum
¥ Brass
0.101"
0.125"
0.044"
0.053"
0.052"
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 3µs
(see Output Switching Delay vs. Temperature graph). If
more information is required about output switching
characteristics please consult the factory.
Coins:
¥ US Quarter
0.055"
0.113"
0.090"
0.080"
0.093"
0.133"
0.087"
¥ Canadian Quarter
¥ 1 German Mark
¥ 1 Pound Sterling
¥ 100 Japanese Yen
¥ 100 Italian Lira
12 oz. soda can:
VOUT
1
VTANK
VDEMOD
VDEMOD
Figure 1. Capacitor ripple.
Figure 2. Output pulse for an 8 tooth gear.
4
CHERRY [ CHERRY SEMICONDUCTOR CORPORATION ]
