Polarity Reverser


There are systems in
which it is imperative that the supply voltage of, say, a motor, always
has the correct polarity. It is, of course, possible to use a bridge
rectifier for this, but if large currents are involved, this is not
always possible. This may be because large voltage drops across diodes
result in appreciable heat dissipation, or that the peak current exceeds
the current rating of a diode. Fortunately, a good, inexpensive
mechanical rectifier may be constructed with the aid of a relay. In the
diagram, the supply voltage is applied to K1, while the motor that needs
a supply with correct polarity is linked to K2. Provided fuse F1 is
intact, a positive potential at terminal a of K1 will be applied to the
positive terminal of K2. Diode D2 prevents the relay being energized.

Circuit diagram:

Polarity Reverser Circuit

Polarity Reverser Circuit Diagram

When the polarity at K1 is reversed, the relay will be energized via
D2. The relay contacts then interchange the connections to the
terminals of K2 to ensure that the previous polarity of the supply to
the load is retained. Diode D1 is a freewheeling diode for the relay
coil. The type of relay to be used depends on the requisite operating
voltage and the current through its contacts. Other parts of the circuit
are not critical. It stands to reason that the circuit is not suitable
for use with a small battery, since the relay coil draws a fairly large
current.
Author: K. Lorenz
Copyright: Elektor Electronics


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