Book Analog Interfacing to Embedded Microprocessors
ing the time until the contacts open. You can see that the higher the flyback
voltage is allowed to rise, the faster the field dissipates and the faster the con-
tacts open.
Solenoids, Relays, and Other Analog Outputs 139
140
Analog Interfacing to Embedded Microprocessors
Figure 6.2
Using a zener clamp to speed up relay release time.
Tranzorbs can also be used to clamp a relay or solenoid. A Tranzorb is a
zener-like device that is used for clamping high-energy transients. A Tranzorb
clamps at the same voltage in both directions, so no blocking diode is needed.
Pick/Hold
The DC current drawn by a relay has to be high enough to pull the relay con-
tacts from one end of its travel to the other. However, the current needed to
hold that position is much lower—typically 50% of the pull-in (or pick) voltage.
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