What is the trick?

[cat]

Setting trimpots.

I am trying to get the drain voltage set on some FET's by adjusting the trimpots in my circuit. Now even the slightest movement raises or lowers the voltage so much and I just cant nail the voltage Im aiming for.

There has to be an easier way! What is the trick?

Cat

[Bernardduur]

Bigger or smaller (as in value) trimpot. When I build the Flipster I had a HUGE problem setting the Q4 voltage with a 100k trimpot. When I used a 10k trimpot, I had all the control I needed and it was quite easy to set the voltage

[cat]

I am using a 100k trimpot. Perhaps I need to switch the value for better control.

Cat

[kvb]

There are multi-turn trimpots too. The one's I found at a local Elec store are blue and rectangular with a small screwhead on top.

[amz-fx]

Setting trimpots.

I am trying to get the drain voltage set on some FET's by adjusting the trimpots in my circuit. Now even the slightest movement raises or lowers the voltage so much and I just cant nail the voltage Im aiming for.

There has to be an easier way! What is the trick?

I suggest going to a 25k trimpot...  10k might be too small for some fets.  The multi-turns are more expensive but worth the extra precision.

regards, Jack

[Mark Hammer]

We often run into "dialability" issues here with pots, and finding juuuuuuuuust the right pot taper to easily achieve desired settings.  I suspect, though, that such dialability issues also exist with trimpots.  I imagine that in a great many cases, the same solution that works in the one (pots) context can also work in the other (trimpots). And that is parallel resistors which change the taper of pot so that what used to be too much resistance change in too few degrees of rotation turns into an easier-to-tweak smaller amount of resistance change for X degrees of rotation.

You can read all about it in "The Secret Life of Pots" over at www.geofex.com

The nice part is that while good trimpots in the needed value may be expensive or hard to find, parallel resistors are cheap and plentiful.

[MartyMart]

I've started using 50k's, seem good for general Jfet bias and there's "twice the range"
of adjustment compared to a 100k ( across the surface )

MM

[alderbody]

50K multi-turn trimmers.

damn accurate!

[Mark Hammer]

50K multi-turn trimmers.

damn accurate!

...but sadly too damn big for the space allotted on the board.

[petemoore]

  Swapping to a different Jfet that likes the provided R Range of the trimpot, showing it by biasing nicely @ around 1/2v.
  I've gone as far as using a 5k lin. trimpot > socket for setting drain resistance. socketting resistors to get near 1/2v, then using the 5k to precisely dial it to about 1/2v...lol.
  But have used this Resistor>pot method for drain resistance setting and..
  bigger pot with a trim resistor across it...100k that works perfect isn't always what I find when using them for Jfet Drain biasing, too often, much too touchy...I end up swappingthe Jfet, or after that...still having to 'diddle' something in to attain the required Drain resistance.

[George Giblet]

I'm with Jack, use a 25k pot.  If you can only get 20k's use a 50k or add a series resistor like 4k7 to 10k.  That's all assuming you are using J201's.  (The MPF102 stuff requires lower resistor values, perhaps 2k2 to 10k range - it depends on the specifics of the circuit).

[redeffect]

50K multi-turn trimmers.

damn accurate!

...but sadly too damn big for the space allotted on the board.

There are 1/4" square multi turn trimmers in the Mouser Cat. Is this too small to fit?
red

[Noplasticrobots]

You can get a nice assortment of Multi Turns in Rat Shack's pot assortment.

[aron]

I think the smaller value trimpot will solve everything.

[stm]

Use the right tool for each job, they say.

If you look at the Revisited Fetzer Valve article, in particular to the calculator section and use the average values you'll get the following typical values:

J201: average Rd=10322 ohms, thus a 20K or 25K pot will work fine here.
2N5457: average Rd=1607 ohms, thus a 5K pot will work fine here.
MPF102: average Rd=688 ohms, thus a 2K will work fine here.

The above guidelines assume you are using a 9V supply and should cover the typical 2:1 parameter deviation usually found in FETs.  It also assumes you are biasing to the "optimal" drain voltage, which is roughly 6, 6.5 and 7 volts, respectively, however it is likely to cover biasing down to Vcc/2 or 4.5V in most cases.

[alderbody]

50K multi-turn trimmers.

damn accurate!

...but sadly too damn big for the space allotted on the board.

not necessarily, Mark!
the ones i use are quite small. They are narrower and taller compared to some "normal" trimmers, but that's OK with me.

I know there are some multi-turn ones that are huge, but i avoid them when possible.

[cat]

Guys thanks for your efforts, I really appreciate everyone jumping on board to help. Im gonna go get myself a 25k trimpot today and try that out.

Ill let you know how it turns out.

Cat

[R.G.]

The trick in setting trimpots is to restrict the range of the trimpot so that it only adjusts what you're trying to adjust by a small fraction of the total range. If you want to, for example, set a collector to 5.0V and you can figure resistors to get it to between 3 and 7V, you make the trimpot only have the 3V to 7V range, not 0V to 9V.

That does mean that you have to know how to restrict the range of whatever you're adjusting down to where it's easy to set with a trimpot.

Using one trimpot to set something over a wider than necessary range is a prescription for disappointment.

[lovric]

board space is always scarce but if there's any left try two trimpots per fet. half the values of what stm said. one to get you near and the other to nail it.

''
J201: average Rd=10322 ohms, thus a 20K or 25K pot will work fine here. (2 X 10k)
2N5457: average Rd=1607 ohms, thus a 5K pot will work fine here.          (2 X   2k)
MPF102: average Rd=688 ohms, thus a 2K will work fine here.                 (2 X   1k)
"