This blog investigates control and speed monitoring of a 4 wire Comair Rotron fan using the Program input and Tacho output. The fan speed was controlled using a resistor then separately tested with an optocoupler driven by an external PWM signal.
Comair Fan Control Article
Tim Shafer, of Comair Roton, wrote a brief but helpful article relating to methods of fan speed control. This article is available on the Comair Rotron website or from Digi-Key as a PDF Document. The details from Tim were a start although did not clearly define methods of interfacing with the 'Program' input connection used by Comair Rotron fans. Even after reviewing the data sheet for the fan (CD24R7X) neither a suggested resistor range or recommended PWM frequency was suggested for the fan. Time for some bench testing and fan characterisation.
The Comair fan tacho output connection (open collector) was pulled up to the 24VDC supply via a 220K resistor and then monitored by the oscilloscope.
The fan program input was connected to the optocoupler collector and emitter to 0VDC. To change the speed of the fan the Program input must be connected to 0VDC. The input LED of the opto was driven directly by the function generator.
The duty cycle against the derived fan RPM was measured next. To derive the fan RPM, the frequency of the Tacho output was divided by two, since the fan produces 2 PPR - Pulses Per Revolution, then multiplied by sixty to convert into Pulses Per Minute aka RPM.
From tests performed on this fan, the speed changed from 1488 RPM at full speed down to 702 RPM at the lowest speed. This range of change in RPM equated to a change in duty cycle from 99% to 1%.
One possible method to check the rotational speed of the fan, based on the duty cycle provided to the Program input, is the formula below which serves as a guide with a + 10% variation in the Tacho reading. A line of best fit was applied on test data from a single fan.
Where the duty cycle is the ON duration of the period as a percentage of the total period.
For the tests conducted with the fan the operational voltage range was found to be 16V to 28VDC with a stall voltage of approximately 6VDC.
Additional Measurements
The PWM frequency was varied in the range from 100Hz to 10kHz with some minor difference in the current drawn by the fan. A difference of 2% was noted between some of the duty cycle readings which may be attributed to instrumentation and or user measurement error.
As a matter of completeness the Tacho output was captured when the Program input was shorted to 0VDC, as shown below. This equated to approximately 717 RPM.
Operating Specifications for the Comair Fan (Suggested)
For the fan under test in this blog, the Comair Rotron CD24R7X, the suggested operating parameters are as detailed by the manufacturer with some additional information below.
Supply Voltage: 16 - 28VDC, Nominal 24VDC
Supply Current: Max 1.22A at 24VDC (Full Speed)
Fan Speed: 700-1490RPM + 5%
Program Input Resistance: 0R to 12k equating to slowest to fastest fan rotation. Estimated Fan RPM = (0.068 x Resistance in Ohms) + 720
Program Input Frequency: PWM of 1kHz, duty cycle 1% to 100%. Estimated Fan Speed (RPM) = (7.9 x Duty Cycle) + 720
Tach Output: 2 PPR (Non-Isolated Open Collector) 28VDC at 20mA
Connections: Red - 24VDC
Black - 0VDC
Blue/White - Tacho Output
Yellow - Program Input
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| Comair Rotron CD24R7X |
A test unit was purchased from Digi-Key and arrived in the Comair Rotron packaging. There was plenty of cardboard to secure the fan inside the box for transport which was a pleasant find in these times of bubble wrap packaging. The test unit was manufactured Dec 30th, 2015 with a serial number of 362.
Comair Fan Testing
Since the fan was earmarked as a test device to regulate temperature and air quality of a room, the range of fan speeds against changes in the Program input needed to be known. With the article from Tim Schafer in mind, the Program input was tested with an optocoupler driven from a signal generator then separately with a variable resistor. Additionally the effect of fan speed against variations in power supply voltage was also captured.
Comair Fan Test Setup
To perform the required tests, the fan was connected to a Rigol DP832 PSU and Function Generator, Agilent oscilloscope and vanilla multi-meter for current measurement verification.The Comair fan tacho output connection (open collector) was pulled up to the 24VDC supply via a 220K resistor and then monitored by the oscilloscope.
The fan program input was connected to the optocoupler collector and emitter to 0VDC. To change the speed of the fan the Program input must be connected to 0VDC. The input LED of the opto was driven directly by the function generator.
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| Test Setup: Duty Cycle Measurements |
Test 1: PWM Control
With a PWM frequency of 1kHz, the duty cycle was changed in fixed steps and plotted against the average fan current. |
| Duty Cycle against Fan DC Current |
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| Duty Cycle against Fan RPM (Derived) |
From tests performed on this fan, the speed changed from 1488 RPM at full speed down to 702 RPM at the lowest speed. This range of change in RPM equated to a change in duty cycle from 99% to 1%.
One possible method to check the rotational speed of the fan, based on the duty cycle provided to the Program input, is the formula below which serves as a guide with a + 10% variation in the Tacho reading. A line of best fit was applied on test data from a single fan.
Estimated Fan Speed (RPM) = (7.9 x Duty Cycle) + 720
Where the duty cycle is the ON duration of the period as a percentage of the total period.
Test 2: Resistive Control
With a 100k variable resistor connected between the fan Program input and 0VDC, change in the fan tech output was recorded against changes in the resistance. |
| Test Setup: Resistance Measurements |
Again the fan speed in RPM was derived from the 2 PPR Tacho output.
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| Program Resistance against Fan RPM (Derived) |
The fan RPM varied from 1485 RPM down to 714 RPM which equated to approximately a resistance in the range of 12k to 0R. Tests were conducted with resistances over 20k with no change in the fan speed noted.
One possible method to check the rotational speed of the fan, based on a resistance between the Program input and 0VDC, is the formula below which serves as a guide with a + 6% variation in the Tacho reading.
A line of best fit was applied on the measured data between 0 and 12k for the test fan giving the estimation below.
One possible method to check the rotational speed of the fan, based on a resistance between the Program input and 0VDC, is the formula below which serves as a guide with a + 6% variation in the Tacho reading.
A line of best fit was applied on the measured data between 0 and 12k for the test fan giving the estimation below.
Estimated Fan Speed (RPM) = (0.068 x Resistance in Ohms) + 720
Test 3: Power Supply Fluctuations
Lastly the DC supply to the fan was varied with PWM to determine the stall speed of the fan for a 1% duty cycle. It should be noted that when the supply was raised above 29VDC the fan speed became erratic. The recommended 28VDC limit listed on the unit by the manufacturer should not be exceeded.
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| DC supply Voltage against Fan DC Current |
For the tests conducted with the fan the operational voltage range was found to be 16V to 28VDC with a stall voltage of approximately 6VDC.
Additional Measurements
The PWM frequency was varied in the range from 100Hz to 10kHz with some minor difference in the current drawn by the fan. A difference of 2% was noted between some of the duty cycle readings which may be attributed to instrumentation and or user measurement error.
As a matter of completeness the Tacho output was captured when the Program input was shorted to 0VDC, as shown below. This equated to approximately 717 RPM.
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| Tacho Output: Program Input Shorted to 0V |
The Tacho output shown below was captured when the Program input was connected to a 100k potentiometer to 0VDC. The pot setting was at the highest resistance. This setting equated to 1428 RPM.
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| Tach Output: Program Input connected to 100k Potentiometer |
Operating Specifications for the Comair Fan (Suggested)
For the fan under test in this blog, the Comair Rotron CD24R7X, the suggested operating parameters are as detailed by the manufacturer with some additional information below.
Supply Voltage: 16 - 28VDC, Nominal 24VDC
Supply Current: Max 1.22A at 24VDC (Full Speed)
Fan Speed: 700-1490RPM + 5%
Program Input Resistance: 0R to 12k equating to slowest to fastest fan rotation. Estimated Fan RPM = (0.068 x Resistance in Ohms) + 720
Program Input Frequency: PWM of 1kHz, duty cycle 1% to 100%. Estimated Fan Speed (RPM) = (7.9 x Duty Cycle) + 720
Tach Output: 2 PPR (Non-Isolated Open Collector) 28VDC at 20mA
Connections: Red - 24VDC
Black - 0VDC
Blue/White - Tacho Output
Yellow - Program Input
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