Quad 100k Digital Rheostat - Can it be wired for 300k?

Thread Starter

mriksman

Joined Aug 31, 2010
113
Hi,

I have a Fairchild FL51x0 which has a control pin where the voltage at the pin is input to an 8 bit ADC with 2.5V reference. The pin sources 10uA current, so that an external resistor can be used to set the voltage (0-250k).
1642018543841.png

I'd like to use a Digital Potentiometer/Rheostat. However, the 250k ones are expensive and not in stock.

I was wondering if I could use a quad rheostat like the MCP4432 and wire in series with the DIM Control pin?
1642018772118.png
I can then control each channel to give me 0-300k and 129*3 taps.

Would that work? Because the DIM Control pin sources 10uA? 10uA through the 0-300k series digital rheostat would generate 0-3V, right?
 

MrChips

Joined Oct 2, 2009
27,165
Why not use the 100kΩ pot as a voltage divider fed with a 2.5V source?

Edit: How linear do you want the control to be? If you want close to linear, buffer the 0-2.5V source with an op-amp.
 

panic mode

Joined Oct 10, 2011
2,303
it should work but it may be an unnecessary complication. you could also add external 20uA source in parallel with internal 10uA source. then you only need 0-100k resistance.
also the Dim control pin is simply an analog input. if you can set analog voltage 0-3V there (any way you like) you will be controlling the dimmer. all your analog output driving DIM control pin need to do is absorb 10uA current and frankly analog output that cannot do that is not much of an output.
 

Thread Starter

mriksman

Joined Aug 31, 2010
113
Thanks for your replies.

@MrChips... Hmmmm. That makes me wonder... Will this work...?
1642056339787.png
I feel like that 10uA current source is going to cause an issue with the above...?

It doesn't have to be linear - I can linearise it in my microcontroller code.


@panic mode
I have considered DACs and op-amps, but there are a few issues.

The 'GND' for the FL5150 is ...constant. So when I have two FL5150's (dual dimmer), they must be separate. Which means they cannot share GND with the microcontroller. So how would a DAC work (the ICs I have seen seem to suggest sharing a GND/Ref with the microcontroller)?

Also, my microcontroller gets a separate 3.3V supply. Plenty of juice. However, the FL5150 gets it's power from a cleverly designed bridge rectify and an internal 17V zener. It then creates a 5V Vdd pin - but this is ONLY used for setting the 'MODE' pin. If I try and draw any current from Vdd, it causes a POR. So I can't add any extra components that draw power to from the 5V Vdd.

Hence why the digiPOT would serve me well. I only need to sink the 10uA current to create a voltage on the DIM Control pin.

I originally had this;
1642055764558.png
I take a PWM, smooth it out, and then use the dual-opto (one acting as a feedback) to control the amount of current flowing through the other. And it kind of works. It's just a little too noisy/unstable, resulting in a slight flicker.
 

MrChips

Joined Oct 2, 2009
27,165
If the only power source you have is 5V, put a fixed 100kΩ resistor from 5V to the 100kΩ to reduce the voltage to 2.5V.
The combined 200kΩ voltage divider is drawing 25μA and will be affected by the 10μA current source at the ADC input.
Use a non-inverting buffer to drive the ADC input using a rail-to-rail single supply opamp.

1642087900350.png
 

Thread Starter

mriksman

Joined Aug 31, 2010
113
I just did an LTSpice. I put a 150k in series with the 100k potentiometer. Linearity isn't that bad!
1642093417508.png
It gives me a 0-2.6V signal.
 
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