# digital potentiometer

Discussion in 'General Electronics Chat' started by pazpaz, Sep 19, 2014.

1. ### pazpaz Thread Starter New Member

Nov 14, 2013
19
0
Hello all,
In my circuit I am using digital potentiometer MCP4641.
can you tell me how output voltage is related to wiper code
Rab = 5 KOHM
wiper code is sent is 135
MCP4641 is 8 bit poentiometer
End A is connected to 5V supply
End B is connected to ground.
In this case what voltage and resistance I should expect at the wiper end?
regards

Apr 5, 2008
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3. ### #12 Expert

Nov 30, 2010
16,665
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Here's the datasheet for the pot in question:

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4. ### pazpaz Thread Starter New Member

Nov 14, 2013
19
0
Hello,
I have gone through the datasheet. but I am still confused. if I ask you that I am setting wiper code = 0x50 then what will be the Rw resistance? and what voltage it will be developed across Vwb if 'A' terminal is connected to 5V and 'B' terminal is connected to GND. 'W' terminal is output voltage generated by digital pot.

regards

5. ### #12 Expert

Nov 30, 2010
16,665
7,310
Learning as I go...an 8 bit control has 255 steps. If each one is an equal part of 5k ohms, then each step must be 19.6 ohms or 1/255 times the Vcc. Then there is the Rw (resistance of the analog switch in series with the wiper) which is rather difficult to interpret from the datasheet. For 5.00 volts, 135/255 x 5 = 2.647 volts.

Am I making sense or just randomly stabbing at the wrong answer?

6. ### pazpaz Thread Starter New Member

Nov 14, 2013
19
0
Hi,
when I am transmitting wiper code = 135 (decimal) then at the output of the wiper pin voltage is 0.023V wrt GND
regards

7. ### #12 Expert

Nov 30, 2010
16,665
7,310
Well, that seems wrong if your only load is the meter you are measuring with.
Does 135 mean you commanded the pot to go to step 135 out of 255 steps?
or is that what you're asking us?

8. ### pazpaz Thread Starter New Member

Nov 14, 2013
19
0
yes I commanded the pot to got to step 135 out of 255 steps i.e wiper position is at 135 step.

9. ### #12 Expert

Nov 30, 2010
16,665
7,310
You should expect 2.647 volts. That is not what you're getting, so we need to examine your circuit and your method of giving instructions to the pot. Please try to present the information by uploading it.

10. ### pazpaz Thread Starter New Member

Nov 14, 2013
19
0
I am using wiper command address = 0x90 and value to be sent is 135 (0x87). This is the only thing I am doing for pot and circuits is

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11. ### #12 Expert

Nov 30, 2010
16,665
7,310
I've hit my limit for tonight. Somebody else take over, please.