Control Circuit Question


Old 10-23-07, 01:49 PM
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Control Circuit Question

I am building a control circuit and having difficulty with a voltage drop.

The application is going to control a fireplace fan via (two or three speeds) via a home automation system.

I have obtained a motor controler that takes a VDC input between 0 and 10VDC and controls the motor speed (10VDC = 100%, 5VDC = approx 50%, etc.).

The control circuit I built consists of the following:

A tranformer with an unregulated output rated at 12VDC and 1000 ma. (measured output is 15DC.

The tranformer is connected to a 12VDC voltage regulator.

The voltage regulator is connected to 6 2.2K ohm resisters connected in series.

I then tap off different points in the series network to get the desired voltage.

What is happening if I put the meter on the series network at I get the correct voltate (e.g. 10VDC). When I then connect that same point to the motor controller and measure the voltage accross the control signal input terminals I get about 5.7VDC (they share a common ground).

What might be causing this voltage drop and how might I eliminate it. I do not suspect the controller signal inputs put any load on the circuit.

Thanks for your help,
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Old 10-23-07, 02:22 PM
Join Date: Oct 2005
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this is a little confusing

Is the idea to do a continuously variable fan, or a 3 speed fan? In any case, I DO suspect input loading is causing your voltage drop. What else COULD cause it? Using smaller resistor values will lower the source impedance, and that will result in a lower voltage drop. A better way is to use a regulator for each step that you need, but more $.
Old 10-23-07, 02:34 PM
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The objective is a 3 speed fan

Thanks for the reply. The objective is a three speed fan not a continuously variable fan.

With the motor controller I have I am going to pick three voltage inputs 6, 8 and 10VDC to trigger the three speeds.

The reason why I cannot use an off the shelf switch is this is integrated into a home automation system. So I will be using relays to turn on/off the input voltage.

I have to admit this was quite a bit of brain damage for speed control but I am down this path now so I might as well finish.

I think your suggestion of three regulators makes since. Since this is a "one of" I don't think the extra cost will be prohibitive.


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