Adjuts Signal Voltage on an EC Motor Controller

[ploppyboom]

Hi, I have an EC Motor controller which monitors ambient temperature and adjusts the EC motor speed via the DC10V signal wire. After measuring, this wire will send anything from DC0V (OFF) - DC7V (100%) to control the fan speed.

My issue is that the controller switches OFF (0V on the signal wire) when a particular temperature is reached, whereas I want it to remain running at a particular speed.
So basically I want to hack this circuit and insert another circuit on the signal wire, which will ensure voltage never drops below a particular threshold, ie 2v.
This would in effect make the fan continue running at about 20% speed even when the original controller is sending 0V down the wire. The controller is powered by DC10V coming from the motor so I have this as the power source.

Having scoured the internet I am unable to name such a component/circuit. Is it an inverting voltage comparator I need perhaps?
Bonus cookie points if I could adjust the threshold point/output voltage via a potentiometer or similar.
Extra bonus cookie points if such a circuit already exists and I can buy it without having to solder a single component apart from power and signal wires.

 

[rjenkinsgb]

If the control voltage source gives a fairly low impedance output (eg. it can drive a moderate load). then just a potentiometer between the controlling sensor and +10V, with the pot wiper to the speed control, would do that.

At one end of the rotation the output would be as it is, at the other end permanently maxed out, and mid positions proportionally scaled up; eg. at 20%, sensor zero would give 20% speed with max being unchanged.

A 10K linear pot would likely be OK.

 

[ploppyboom]

Thanks so much Rjenkingsb, that sounds to do exactly what I want. I just want to confirm,
there's no way of disturbing the original wire voltage and the controller attached? I just want to ensure what I build in will be safe.
Here's a diagram (note I added the 420ohm resistor to knock down the max voltage to 7v as that is what I measured on the signal wire, would this be okay?)

 

[rjenkinsgb]

Almost; that would short the controller output though.
The resistor also probably needs to be nearer 3k9 to give the 7V limit.

Try this:

 

[ploppyboom]

Oh thanks yes your diagram makes more sense. I used some online simulator and was manually adjusting the resistor values to give me 7V drop from 10v input, so a 3k instead, great.

 

[gophert]

All may work well but many industrial sensors are current based outputs and voltage will depend on the resistor values used. 4 to 20mA is typical. Multiplying a 20mA current by 500 ohms gets you to 10v. So resistance above 500 will give you full fan speed if the controller is a 4-20mA style device.