Re: 10 turn Pots

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From: Ed Pacyna (pacyna@auratek.com)
Date: Wed May 28 1997 - 11:21:07 EDT


At 09:13 PM 5/27/97 -0700, Norm Melick wrote:
>My 38S works great. That is until I attempted to change the 100K tuning
>pot to a 10 turn 100k pot. Now I keep ruining the 10 turn pots.
>
>So the question of the day for us is, When I buy the next (and hopefully
>last) 10 turn pot, I have a choice between one rated at 2W and one rated
>at 5W. Which one?

A 2W 10T pot is fine (only needs to dissipate E^2 / R or 64/100K or 00064
watts!!).

>From ohms law the current through the pot is minuscule (80uA). Perhaps the
wiper arm is not making solid contact with the wire wound resistive core
(e.g. poor quality pot, oxidation etc.). Measure the voltage as you turn
the pot. If it doesn't change (from 0 to 8V) it's either a defective pot,
wired incorrectly (correct wiper terminal?) or the bottom leg is not
returned to ground.

There is nothing sacred about using a 100K pot for P1. In fact the toplogy
and values, 100K tuning pot (P1) and 10K series resistor (R4), are rather
poor choices for a varactor tuning circuit.

The purpose of the pot is to provide a variable voltage (i.e. reverse bias)
to the varactor which will change it's capacitance. The minimum bias level
should always be greater than the RF potential in the tuned circuit (to
keep the varactor diode from being forward biased). This is usually done by
including a resistor in the voltage divider circuit between the bottom of
the pot and ground. The primary purpose of the series resistor is to
provide isolation. The use of a lower value pot (e.g. 5K to 50K) for P1
would promote reliability. A higher value series resistor (100K+) would
provide far more isolation.

73

Ed, W1AAZ


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