How to implement a 5v variable rheostat overide.

Thread Starter

Qsilverrdc

Joined Aug 3, 2014
61
I have a fan drive that has a rheostat pot to control speed.

2020-12-05 13.21.44.png
I want the ability to go to max speed with a push button I would add, in blue. A little problem I noticed is if the rheostat is near 0v then pushing the button is a short.

Any thoughts how to achieve this, without a short?
 

Delta prime

Joined Nov 15, 2019
610
Hello there :) welcome to AAC
Could you possibly take a photo of your fan drive specifically the circuit board if you could open it up or have access to it thank you
 

AnalogKid

Joined Aug 1, 2013
8,804
You are correct to be concerned. Depending on how the drive board circuit works, you might be creating an over-current condition for something.

One way to solve this without carving on the fan board is to break the connection between the drive board and the pot wiper, and use a SPDT switch to switch the wiper input between the actual wiper and the high side of the pot.

ak
 

bertus

Joined Apr 5, 2008
21,024
Hello,

I do not know the input impedance of the controller.
To protect the potentiometer, put a resistor between the wiper and the shown switch.
That way there is always a resistor between the input and the lower side.

Fan controller extra resistor.png

Bertus
 

Tonyr1084

Joined Sep 24, 2015
5,491
The way I read the schematic, the pushbutton will only operate the fan at full speed while the PB is pressed. Since you show a momentary push button switch, I suspect you may want a different option; that of using a SPDT switch. Added in with blue, in the down position the fan is controlled normally. In the up position the fan will operate at full speed. There is no chance of short circuit. IF you went with the PB and the rheostat were set to near zero and you push the button, you will send massive amounts of current (whatever the supply is capable of) from the top down to the bottom. The switch eliminates the possibility of ANY input from the rheostat by taking it out of circuit. Unless both sides of the input is needed. Without knowing the drive board circuitry we can't give a solid answer. But the switch will eliminate any chance of harming something should it not work as anticipated.

1607199523011.png
 

MisterBill2

Joined Jan 23, 2018
7,687
For starters now, we have no information about what the actual voltages at the speed setting potentiometer are. AND, it is not a rheostat, they only have two connections. It is indeed a potentiometer.
The fan is clearly not a simple small fan, it is driven by a variable speed brushless motor. The 5 volts label may be the power supply for a sensor on he motor. And while the picture shows things it gives us no clue at all about what the speed control adjustment does or what it is connected to.

And, given the complexity of the circuit board I suggest not messing with the speed control at all. THat board would be very expensive to replace and impossible for most folks to repair.
 

Thread Starter

Qsilverrdc

Joined Aug 3, 2014
61
For starters now, we have no information about what the actual voltages at the speed setting potentiometer are. AND, it is not a rheostat, they only have two connections. It is indeed a potentiometer.
The fan is clearly not a simple small fan, it is driven by a variable speed brushless motor.
My bad for getting the terminology wrong. Rheostat, pot, variable resistor, I kind of think of these as the same. Yes this is indeed a 3 wire potentiometer. From the circuit board label it is +5v, Signal (connected to the wiper), and ground. The connectors shown in the image are, left to right. Speed control, 12v - 5A DC, and the brushless fan motor. On the back board the labels for the speed control are
+5, VSR, GND, EN, FR, FG. VSR=Voltage Signal Reference? Here is a small document about the board connectors.

2020-12-06 09.38.39.jpg
 

MisterBill2

Joined Jan 23, 2018
7,687
OK, so presently there is a single pot of some resistance providing the speed setpoint voltage input.
Once again, opening the connection from the "bottom" of the speed set pot will raise the input voltage to a level closer to full speed. At thatpoint the actual control voltage will be determined by a voltage divider consisting of the resistance above the wiper on the pot and the 5 volt supply on the top side, and the input resistance of the control circuit on the low side. my prediction is that the resulting speed will be close to the maximum..And it will certainly be simple to check and see if that is close enough.
 

MisterBill2

Joined Jan 23, 2018
7,687
I was quite certain that it would work.
The other good news is that the IC with the lumpy top is evisenly not fried, but rather a custom device with some transistors epoxyed to the top. I have seen that before and while it looks really strange it is a way to improve assembly and quality. Presently it is much less common, though.
 
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