Belimo TRANE TAF Series, Technical Documentation Manual

Page: 146 / 168

146
I20683 - Subject to change.
I-H. Series Connection of Resistors
Resistors that are connected in series have a total resistance value that is equal to the sum of all the resistance values of the resis-
tors.
Example: R
1
= 200
Ω
R
2
= 250
Ω
R
3
= 1.0k
Ω
R
Total
= R
1
+ R
2
+ R
3
= 200
Ω
+ 250
Ω
+ 1.0K
Ω
= 1.45k
Ω
I-I. Parallel Connection of Resistors
If all the resistors have the same resistance value, the total resistance will be equal to the resistance value of one resistor divided
by the number of resistors.
Example: Five equal resistors R = 100k are con-
nected in parallel.
The total resistance R
Total
= R/5 = 100/5 = 20k
If the resistors that are connected in parallel have different values, the following formula must be used:
Example: R1 = 200 R2 = 250 R3 = 1.0k
I-J. Impedance
The expression “impedance” is used in the literature in the following way:
• Input impedance: The input circuit of a control device, based on its circuitry, has a certain electrical resistance. The value of
this resistance determines how much current the device will draw from the controller. This value must be taken into consider-
ation when connecting any device to a controller output. Example: “Input impedance 100 k
Ω
.” This means that the DC resist-
ance between the input (Y or Y1) and common (COM) is 100 k
Ω
(100,000 ohm). When the signal is 10 VDC, using Ohm’s
Law (I=E/R), the current draw on the output of the controller will be (10V/100,000
Ω
) = .0001A = .1 mA for each actuator that is
connected to the signal. The combined input impedance must be higher than the controller output impedance.
• Output impedance: The output of a controller has a limited amount of current capacity to supply to the devices it is control-
ling. The capacity can be given in one of 2 ways. One way is by stating it as “Maximum output current .2 mA.” The other is
by giving its output impedance. The output impedance must always be lower than the combined input impedance of the devices
being controlled.
Example I: “Output impedance 1000
Ω
minimum.” This means that the combined input impedance of the devices being con-
trolled must be greater than 1000
Ω
.
Example II: “Maximum output current .2 mA.” Based on a 0 to 10 VDC control signal, the output impedance would be equal
to R=E/I or (10V)/(.0002A) = 50k
Ω
In general, the higher the input impedance, the lower the current draw, therefore less strain on the controller output. The lower
the output impedance, the more current available; the more current available, the more devices can be controlled.
I-K. Power Consumption (W) / Volt Amperes (VA)
When a device is powered with direct current (DC), or alternating current (AC) into a pure resistive load (bulb, heater, etc.), the rated
power consumption is watts (W) and is the product of the current (I) and voltage (E), (W = E x I).
R1=200
Ω
R
TOTAL
= 1.45k
Ω
R
2
=250
Ω
R
3
=1.0k
Ω
The total resistance is always larger than the largest single resistor!
100k 100k 100k 100k 100k R
TOTAL
= 20k
R
1
R
TOTAL
R
2
R
3
1 1 1 1
_______ = ____
+
____
+
____
R
1
R
2
R
3
R
TOTAL
1 1 1 1
_______ = ____
+
____
+
____
200 250 1000
R
TOTAL
R
TOTAL
R
TOTAL
= 100
Ω
= 100
Ω
.005 + .004 + .001 = .01
1
.01
=
=
The total resistance is always smaller than
the smallest single resistor!
Basic Electricity