Have you ever needed to
power a 12-volt relay in a circuit but only had 6 or 9 volts available?
This simple circuit will solve that problem. It allows 12-volt relays to
be operated from 6 or 9 volts, or 24-volt relays from 12 volts. While
most normal relays require the manufacturer-specified coil voltage to
reliably pull the contacts together, once the contacts are together you
only need about half that rated voltage to hold them in. This circuit
works by using that principle to provide a short burst of twice the
supply voltage to move the contacts and then applies the available 6 or 9
volts to the relay to lock the contacts in place.
With reference to Figure A., when the main supply is applied to the
circuit the 220-µF capacitor, C1, charges quickly to +6 volts through
resistor R3. The circuit is now awaiting voltage on the control input.
When a control voltage (can be as little as 3 volts) is applied to the
control input, transistor T1 switches on. The other transistor, a BC558,
is also switched on. This allows connection of the relay coil to the
main supply rail while T1 shorts the positive terminal of the 220-µF
capacitor to ground. Now the negative terminal of the capacitor is at a
potential of –6 volts. This is applied to the other side of the relay
coil. The relay coil potential is then briefly 12 volts — enough to
actuate the contact(s).
However, the coil voltage drops to the supply voltage fairly
quickly. The period is determined by the R-C time constant of the relay
coil resistance and the 220-µF capacitor. While this circuit is simple
and works well in many situations, it has a few weaknesses in its
current form. The relay may remain energized for as long as one second
after the control input has fallen. Also, if the control input goes high
before the capacitor has fully recharged, it may not have enough energy
to control the relay reliably. Also, the voltage drop across the diode
limits the voltage to about 10.8 volts.
The more complex version of the circuit shown in Figure B fixes
these problems by using an extra transistor and diode. In this
arrangement, the BC558 is now isolated from the recharge current of the
capacitor. The new transistor provides fast charging for the capacitor.
Charging is completed within the mechanical response time of the relay.
When using these circuits it should be noted that the contact pressure
of the relay contacts may be al little lower than with the nominal coil
voltage. It is therefore advisable to keep contact currents well below
the maximum specified value.