[time-nuts] digital pots Part 2

[ed breya]

Helipots are wonderful, old-school parts that are hard to beat - except 
for being quite expensive (but, they always were, relatively). As others 
have mentioned, there are many advantages of these over digital ones, 
for manually-adjusted functions.

I would never use anything but a wirewound pot for manual fine tuning of 
LF/DC high precision circuits. For me, it's a no-brainer though, since I 
have lots and lots of helipots and WW trimmers, salvaged over many years.

They are expensive new, but it seems to me there should be plenty of 
salvaged or NOS for much less. If you just need an adjustment, it's 
pretty straightforward, with a spinning knob. If you want some sort of 
indicator, then also consider available dials, which can show show 
relative rotation to ten to twenty turns or so, with drum or clock-type 
readout, or even mechanical-digital (odometer style), and other 
variations - some dials may cost more than the pot.

I think the typical, most common and useful resistance value for 
helipots is around 5-50 k. As you go down in resistance, the mechanical 
resolution gets worse, because bigger manganin wire is needed to form 
the resistor. As you go up in resistance (I think around 100-500 k is 
typically the highest available), the resolution gets better, but the 
net source resistance of the wiper and noise go up.

Another option may be to use WW multi-turn trimmer pots, if the 
adjustment needed is more like an occasional tweak with a screwdriver 
rather than a knob or dial. These are available up to twenty turns or 
so, and can be in board- or panel-mount styles. They likely are a lot 
cheaper than full-sized pots, but with lesser performance TC and power 
rating.

Ed