Series 80 Control Circuits; Low-Limit Control; Application; Equipment - Honeywell AUTOMATIC CONTROL SI Edition Engineering Manual

ELECTRIC CONTROL FUNDAMENTALS

LOW-LIMIT CONTROL

A low-limit controller is connected in parallel with the
thermostat as shown in Figure 9. The low-limit controller can
complete the circuit to the valve actuator even though the
thermostat has opened the circuit. The actuator remains
energized when the contacts on either controller are closed.

SERIES 80 CONTROL CIRCUITS

APPLICATION

Series 80 is the low-voltage equivalent of Series 40. It is
suited for applications requiring low-voltage, two-position
control of two-wire circuits by single-pole, single-throw

controllers.

Series 80 has two advantages over Series 40:
1. Since contacts of Series 80 controllers are required to
carry less current, the controller mechanism can be
physically smaller than a Series 40. The controller there-
fore has less lag and the capability to have a narrower
differential.
2. Low-voltage wiring (when not in conduit or armor) costs
less than line-voltage wiring.

EQUIPMENT

CONTROLLERS
1. Temperature controllers.
2. Humidity controllers.
3. Any low-voltage device with a minimum of one
normally-open contact such as a low-voltage relay.
A Series 80 controller cannot switch line voltage directly. A
relay must be used between the controller and a line-voltage
load. However, Series 40 or Series 80 controllers can be used
with Series 80 actuators.
A Series 80 controller has snap-acting contacts or a mercury
switch. Some Series 80 controllers are three-wire, double-throw
devices as in heating-cooling thermostats.
THERMOSTAT
LINE VOLTAGE
Fig. 9. Series 40 Control System with Low Limit.
Many Series 80 heating thermostats have internal adjustable
heaters in series with the load. This provides enough artificial
heat during the on cycle to cause the sensing element to open
the switch on a smaller rise in space temperature. Some Series
80 cooling thermostats have a fixed heater which energizes when
the cooling circuit is deenergized. This provides artificial heat
to the sensing element to close the switch on a small rise in
space temperature. This cooling and heating anticipation (also
called time proportioning) provides close space-temperature
control. See the Control Fundamentals section.

RELAYS AND ACTUATORS

Relays and actuators are similar to those of Series 40 except
that they operate on low voltage.

OPERATION

Figure 10 illustrates a simple Series 80 circuit. This is the
same as a Series 40 circuit with the added transformer.
THERMOSTAT
T1 T2
ACTUATOR
Fig. 10. Series 80 Actuator Application.
ENGINEERING MANUAL OF AUTOMATIC CONTROL
102
LOW-LIMIT
CONTROLLER
ACTUATOR
L1
L2
HOT WATER
SUPPLY COIL
VALVE
C2510
TRANSFORMER
LINE
VOLTAGE
C2511
TO