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Potentiometer Voltage Divider

J

John Popelish

Jan 1, 1970
0
wt said:
When using a potentiometer as a voltage divider, any potentiometer will do
since it is just the ratio i.e. whether 10k or 100k or 1Meg.
What I want to know is are there any factor we should decide on when
choosing either 10k,100k or whatever.

There are at least two concerns.

1. How much power are you willing to dump into the
potentiometer?

2. How much current must the divider supply to the load on
the voltage tap? (Or alternately, what maximum output
impedance can you tolerate?)
 
W

wt

Jan 1, 1970
0
When using a potentiometer as a voltage divider, any potentiometer will do
since it is just the ratio i.e. whether 10k or 100k or 1Meg.
What I want to know is are there any factor we should decide on when
choosing either 10k,100k or whatever.
 
T

Tom Bruhns

Jan 1, 1970
0
There are at least two concerns.

1. How much power are you willing to dump into the
potentiometer?

2. How much current must the divider supply to the load on
the voltage tap? (Or alternately, what maximum output
impedance can you tolerate?)

One more--well, actually two: certain values are more robust than
others. For example, a wirewound potentiometer that's wound with very
fine wire may wear the wire out after fewer revolutions than you'd
like. But one that's wound with coarse wire has lower resolution (if
it's otherwise the same), so that's another consideration. Also, I
think you'll find quite a bit of variability in quality according to
how one is built, but also expect very high values to not survive as
well, as a rule, as lower values in the same product line.

Cheers,
Tom
 
D

Dave Boland

Jan 1, 1970
0
wt said:
When using a potentiometer as a voltage divider, any potentiometer will do
since it is just the ratio i.e. whether 10k or 100k or 1Meg.
What I want to know is are there any factor we should decide on when
choosing either 10k,100k or whatever.
The resistance can affect a couple of things. Let's say you
are feeding an analog to digital converter and the pots are
fed from the reference voltage. The higher the resistance,
the less current drawn from the reference. Some references
won't supply a lot a current - a few dozen mA for example.
In this case a 20 K pot will be helpful because it will draw
so little current that it won't be a serious loading factor
on the reference (i.e. 4.096.v/20K = .2mA).

However, the Zout of the 20 K pot is less than 1 Ohm to
about 1/4 the max. value, or 5 K (Using a Thevin equiv.
circuit). 5 K may be too large to drive an ADC with a fast
aperture (sample) time. This is because there is an RC time
constant created by the pot and the sample capacitor in the
ADC. You would need to drive the ADC with an op-amp to use
this high of value.

Hope this helps guide you.

Dave,
 
E

Eeyore

Jan 1, 1970
0
John said:
There are at least two concerns.

1. How much power are you willing to dump into the
potentiometer?

2. How much current must the divider supply to the load on
the voltage tap? (Or alternately, what maximum output
impedance can you tolerate?)

You can add the influence of the pot resistance on noise to that as well.

Graham
 
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