Universal Relay Board
Voltage Cut Out/In Setup
Table 2. Thermistor Resistance/Temperature Resistances.
Here, a zener diode and resistor take the
place of R5 and R6. To use the relay as a low-voltage cut out/in for a 12-volt-car, NiCad,
NmHi, or other battery, you will need a zener
diode and a resistor — one to 10 kilohm — as
the value isn’t critical. The zener needs to be
at least a volt or so lower than the cut out/in
voltage you want to use.
10kΩ Thermistor was used here.
69KΩ = 14ºF or -10ºC
37KΩ = 32ºF or 0.0ºC — Freezing point of water.
21KΩ = 50ºF or 10ºC
13KΩ = 68ºF or 20ºC
10KΩ = 77ºF or 25ºC — Rating (10KΩ NTC) is taken from here at 25-degrees C.
7.8KΩ = 86ºF or 30ºC
5.0KΩ = 104ºF or 40ºC — Try to match the opposing resistor to the sensors
3.3KΩ = 122ºF or 50ºC resistance value at the temperature or light level.
2.2KΩ = 140ºF or 60ºC This gives a better ± pot rotation.
590Ω = 212ºF or 100ºC — Boiling point of water = 20 mA current flow at 12 V.
Low voltage cut out
Setting: zener to positive, reversed biased,
of course, resistor to negative. Drop the total
supply to the voltage needed and set VR1 just OFF. Restore
full supply voltage, when the supply is reduced to the cut-out
voltage, the relay will go OFF.
High voltage cut off
Setting: zener to negative, still reversed biased, resistor
to positive. Raise the total supply to the voltage needed; set
VR1 just OFF. Lower the supply voltage. When supply is
raised to the cut-out voltage, the relay will go OFF. Here,
C2 is a link and the dotted line below it, R8, is a link. R7 is
omitted, and the input terminals are not used here.
Note this configuration will only work for voltages around
± 3 volts at 12 volts and around ±2 volts at six volts, where you
will need a six-volt relay. The circuit has to have enough voltage to activate the correct voltage relay, but not so much as
to cook the relay. Try bypassing R1 if you’re under the relay
voltage or leaving it in place if just over the relay voltage.
Now, because the supply voltage is not constant in
both the above instances and the zener is, the zener
becomes the only constant in the circuit. The circuit is virtually flipped upside down with the zener becoming the reference and the rest of the circuit becoming the varying
Circle #47 on the Reader Service Card.