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I have a Cooper Controls SF10P-W Slide 0-10V Dimmer designed for controlling LED or Fluorescent light fixtures.

Cooper Controls SF10P-W Slide 0-10V Dimmer

I assume this is the kind of dimmer described in IES TM-23-11 as "4-Wire Low Voltage 0–10VDC (Class 2) for Fluorescent Dimming":

In this method, the ballast is the current source for the DC signal. Since the dimmer is the point of reference, this Back End method is sometimes referred to as current sinking. In the 0-10 VDC 4 wire method (also known as 1-10 VDC 4 wire since 1 volt is the minimum level signal), the 2 low voltage wires (violet & gray wires) must be kept separate from the power wiring in the system except at the ballast itself. The control wires are polarity sensitive so accuracy is important in wiring. If the ballast senses a loss of the control signal, the lamps go to full light output. This is described in NEMA ANSI C82.11-2002 High Frequency Fluorescent Lamp Ballasts - Annex A Low Voltage Control Interface for Controllable Ballasts. This protocol definition is also in IEC 60929 Annex E.2. Note that the switch in the dimmer for this application is required to turn off the ballast. [...] This must not be confused with 0-10V current source applications, found in Section 7.1.

The "Physical Layer" is described as:

A topology free, two wire, polarity sensitive, parallel bus is connected to all the controllable ballasts and to the controller. Typically, the ballast is capable of withstanding a voltage range of -15V to +15V. 10VDC produces maximum light output, while 1VDC produces minimum light output. Ballasts are protected from reverse polarity. If the control terminals of the ballast experience reverse polarity, the ballast produces minimum light output. If the control unit is not connected, as in an open circuit or control equipment failure, the ballast produces maximum light output. Ballasts are the source of the control current. Controllers for this method are variable resistors or an electronic equivalent capable of handling a range from 0.01ma - 2ma and are hardwired to a ballast or group of ballasts. Typical applications include architectural dimming and daylight harvesting.

Wiring Diagram for 0-10VDC Fluorescent Dimming

This seems straightforward enough. The physical toggle switch at the bottom of the control is a standard SPST switch for switching the ballast power source and can be ignored. The top slide portion of the control is just a rheostat/two terminal potentiometer. The ballast/light fixture uses the 0-10v "DIM+" and "DIM-" control wires to provide a constant current source. When the slide is at the top of the control, resistance is at maximum so the constant current source voltage is driven to maximum (nominally 10v). When the slide is at the bottom of the control, resistance is at minimum so the constant current source voltage is driven to minimum (nominally 0v).

Now for the confusing part. When I measure the resistance of the 0-10v wires with my multimeter, I get 207kΩ resistance at maximum and 29kΩ at minimum. This is likely supposed to be 200kΩ to 20kΩ with some room for trimming (the SF10P-W has a trim knob). The current on a constant current source at 10v and 200kΩ would have to be 10v/200kΩ = 50µA. Then at the low end we have 50µA * 20kΩ = 1v which matches up with "1VDC produces minimum light output".

However, there are a few problems with that 50µA current:

  1. What kind of constant current source has a range of 1-10v and is stable with only a 50µA load? That doesn't seem to match the range of any of the standard TI voltage references: TI voltage references chart
  2. A lot of less authoritative websites claim that the nominal impedance of the dimmer should be 100kΩ, not 200kΩ. Besides the discrepancy, a ballast designed for a 50µA constant current source paired with a dimmer with a maximum resistance of 100kΩ would only be able to reach half brightness.
  3. If two ballasts/light fixtures were wired to the same control (as is very commonly done), the math here would also fall apart and again we'd have lights that could only reach 50% brightness.
  4. The standard said "Controllers for this method are variable resistors or an electronic equivalent capable of handling a range from 0.01ma - 2ma". Again many less authoritative websites cite that 2ma number as the nominal current of the 0-10v dimming circuit. 50µA fits in the 0.01mA - 2mA range but how does that range make sense? 2 mA * 200kΩ = 400v. A rheostat/potentiometer couldn't handle a range of currents.

How does this dimmer actually work? Is it really just a variable resistor or is there something more complex going on inside? I cited the IES TM-23-11 document as my source for all this as it's the most authoritative and consistent reference I could find but is it wrong or describing something else?

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1 Answer 1

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  1. The ballast can contain a very simple current source, not necessarily a constant current source IC. It could be a 600R resistor from a 12V rail, so that when the dimming pair is shorted out you get 2mA, but when the variable resistance is 200kohms it is 60 microamps. The ballast sets its dimming level based on the voltage formed by this divider.

  2. It doesn't make any difference whether it is 100k or 200k at maximum since compared to 600 ohms you will get the full 10V, but it would affect the linearity of control, eg 50% would be where the resistance is 600 ohms.

  3. Each one sources up to 2mA maximum. That dimmer switch states a maximum load of 200mA so 10 ballasts.

  4. See 1.

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  • \$\begingroup\$ A non-constant current source from the ballast makes a lot of sense but I don't think the math works out for the voltage divider. Consider maximum brightness: A 600Ω resistor in the ballast and a 200kΩ resistor in the dimmer with 12V rail gives: 12*200000/(200000+600) = 11.964V Then at minimum brightness (20kΩ resistor in the dimmer): 12*20000/(20000+600) = 11.650 I can mess around with the ballast resistor value to get some dynamic range out of it but no value gives anything close to 1-10v of range and it would be specifically tuned to this particular dimmer I have in my hand. \$\endgroup\$
    – gmbuell
    Commented Nov 10, 2023 at 16:51
  • \$\begingroup\$ I suspect that this is not just a variable resistor directly connected to the terminals, but an active device controlled by a variable resistor. The power failure memory, ESD protection and preset function are the clues. I agree that if you used just a variable resistor it would be better as 1200 ohms so that 50% is half way. \$\endgroup\$
    – Martin
    Commented Nov 10, 2023 at 17:17
  • \$\begingroup\$ I suspect it's an active device as well. Though for what it's worth, I have a bit more of a skeptical read on those specs. "Power failure memory" could just mean that if the power goes out, the physical dimmer slide doesn't move and it's easy for a resistor to be ESD resistant... \$\endgroup\$
    – gmbuell
    Commented Nov 10, 2023 at 17:42

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