I have an issue trying operate larger PWM fans using my Wave Form Generator. I have two different models of radiator fans from Mercedes Benz vehicles. The smaller one is 600W fan which I am successfully able to operate using my cheep $65. two channel wave form generator. The larger one is 850W and I am unable to get that one to run. Both fans use the same logic.
0% on/off ratio or duty cycle is fail safe mode where the fan operates at 100% speed
10% on/off is fan stopped normally
20% to 90% on/off ratio is the normal operating range
The fans are both 4 wire units meaning 2 large 8 or 6 ga wires are directly connected to the battery at all times. The 3rd 18ga wire is connected to the ignition key to turn the fan on. The forth 18ga wire is the PWM signal wire.
From what I have read both require a 3.5V 10Hz. PWM signal.
For the smaller fan I was able to successfully run it by setting the function generator to 5V at 10Hz square wave. But it took 4V to get 3.7V while it was running and being measured with the oscilloscope. The larger fan I was not able to run even when setting the output to the 15V max it would drag down to 1.2V when I turned on the fan and consequently the fan would run in safe mode to max speed.
The two screen shots show the output with the wave form generator set to 4V at 10Hz. and 20% duty cycle.
Picture 1 shows the fan running with voltage output reading 3.7V also note the floor of the wave moves above zero compered to the second picture.
Picture 2 shows the fan disconnected and only reading the wave form generator output at 4V with the floor just above zero where it was set.
My theory is that the cheep wave form generator just cannot put out the current necessary to drive the PWM signal and while it was marginal for the smaller fan I was able to overcome it by turning up the voltage a little. But for the big fan I couldent get the voltage high enough while it was running.
So that being the case I thought I would look up the specs and find a waveform generator with more power capability. Except I couldn't find any power specifications for the one I have or any of the other ones available.
So my question is, does my theory sound reasonable and if so How do I specify a better wave form generator? And don't forget the larger fan may just be broken?
0% on/off ratio or duty cycle is fail safe mode where the fan operates at 100% speed
10% on/off is fan stopped normally
20% to 90% on/off ratio is the normal operating range
The fans are both 4 wire units meaning 2 large 8 or 6 ga wires are directly connected to the battery at all times. The 3rd 18ga wire is connected to the ignition key to turn the fan on. The forth 18ga wire is the PWM signal wire.
From what I have read both require a 3.5V 10Hz. PWM signal.
For the smaller fan I was able to successfully run it by setting the function generator to 5V at 10Hz square wave. But it took 4V to get 3.7V while it was running and being measured with the oscilloscope. The larger fan I was not able to run even when setting the output to the 15V max it would drag down to 1.2V when I turned on the fan and consequently the fan would run in safe mode to max speed.
The two screen shots show the output with the wave form generator set to 4V at 10Hz. and 20% duty cycle.
Picture 1 shows the fan running with voltage output reading 3.7V also note the floor of the wave moves above zero compered to the second picture.
Picture 2 shows the fan disconnected and only reading the wave form generator output at 4V with the floor just above zero where it was set.
My theory is that the cheep wave form generator just cannot put out the current necessary to drive the PWM signal and while it was marginal for the smaller fan I was able to overcome it by turning up the voltage a little. But for the big fan I couldent get the voltage high enough while it was running.
So that being the case I thought I would look up the specs and find a waveform generator with more power capability. Except I couldn't find any power specifications for the one I have or any of the other ones available.
So my question is, does my theory sound reasonable and if so How do I specify a better wave form generator? And don't forget the larger fan may just be broken?