30 November 1998
l. Multiple actuators connected to the same control damper. When the operating torque requirement
for an HVAC system damper exceeds the output torque of a single actuator, additional actuators are
connected together to operate in parallel to control the damper. The designer is not required to show
multiple actuators connected to the same damper on the schematic. The vendor has the information
necessary in the contract specification to apply multiple actuators when required.
m. Design of modulating control circuits within a 600 ohm circuit impedance limitation.
(1) The output of an HVAC system controller is connected in series to actuators external to the
HVAC control panel, and also to other devices in the HVAC control panel in a direct current series circuit.
The number of devices varies with the complexity of the control sequence, and the impedance of each
connected device is additive as a resistance in the circuit. The amount of output circuit impedance that a
controller will tolerate is product specific. The limitation of 600 ohms in the output circuit design is needed
to permit the controllers of several manufacturers to function in the same circuit. The limitation permits
standardization in the design and permits substitution of one manufacturer's controller for that of another
during maintenance of the system.
(2) If a modulating control circuit is designed to use electric or electronic actuators, the impedance
can exceed the 600 ohm limitation if:
(a) Multiple actuators are required for the same damper.
(b) More than one damper is modulated from the same control signal, such as in the case of
modulating outside air, return air, and relief air dampers.
(c) Multiple control system devices located within the HVAC control panel are necessary to
achieve the sequence of control.
(3) Individual control system devices typically add 250 ohms impedance to a series circuit. This
250 ohm impedance value comes from a dropping resistor in the device that is used to convert the 4 - 20
milliampere current signal at 24 volts dc to a 1 - 5 volt signal used by the device's internal circuitry.
(4) Figure 2-5 shows methods for designing circuits within the 600 ohm limitation. The figure
shows the following examples:
(a) The limitation exceeded by connecting 750 ohms in series.
(b) Limiting the control circuit connection to a single actuator.
(c) The circuit designed within the limitation by the use of an additional control circuit device.
Figure 2-5. Modulating control circuits impedance limitation.
(5) In the first example, shown in the upper part of the figure 2-5, control devices 1, 2 and 3 can be
actuators or devices in the HVAC control panel. In the second example, the controller is connected to
actuator 1, and actuator 2 is operated by an auxiliary actuator driver (AAD) circuit on actuator 1; actuator 2
can operate another actuator by its AAD. (Note that the functionality provided by the AAD is usually a built-
in feature of the actuator and the AAD is not necessarily a separate device.) In the third example, the same
control devices are connected to a loop driver (LD). Control device 1 and the loop driver are connected to
the controller. The modulating circuit from the controller is limited to 500 ohms, consisting of 250 ohms for
control device 1 and 250 ohms for the input to LD. Control devices 2 and 3 add a total of 500 ohms to the
output circuit of LD. The output signal of LD varies in a 1:1 ratio with its input signal.