Trimpot with decent rotational life?

Den tirsdag den 9. maj 2017 kl. 23.17.57 UTC+2 skrev Phil Hobbs:

can't use a regular pot?

I think long life and trim pot, don't belong in a sentence together. I'll second Lasse and suggest a regular pot. We use a lot of clarostat 392 series,

If it's a one/ few of, the price can't matter.

George H.

Something like this, with some good pushbuttons?

This is the first non-Maxim part in DigiKey's search, so there may be better choices.

We use the Bourns 3314G surface-mount pots. They are very reliable. I think the 200 turn rating is wildly pessimistic, like those 1000 hour electrolytic cap specs.

Setting them to 0.1% of rotation is dicey. 1% is easy.

Looks that way, I agree. Regular pots are normally carbon or plastic, which are easier on the wiper plating but have horrible tempcos.

I'll probably just have to burn enough current in the pot that its tempco d oesn't matter so much. (The offset adjust circuit is a 200k resistor from t he summing node to a 20k pot between +15 and ground, so a 2000 ppm/K plasti c pot is a fairly serious inconvenience.) Might need another op amp, and th ere are six already. :(

This circuit has some weird component values, e.g. a 1000 uF cap bypassing a 10-ohm sense resistor in a 100-MHz bandwidth bootstrap.

Fun though.

Thanks

Phil Hobbs

Some claim a much higher rotational life but you'd have to obtain hard data first:

Not sure how they get to 10,000 for rotational life.

He has an army of hunchbacks, and can surely buy a few screwdrivers.

There are conductive-plastic-over-wirewound pots with ridicuous life but commensurate price tags.

Encoder and DAC?

Nice parts, I agree. I don't have a panel to mount them on, but since I hav e a 200k resistor between the pot and anything sensitive, I might be able t o make a big hole in the board to mount the pot.

Thanks

Phil Hobbs

On a sunny day (Tue, 9 May 2017 17:20:25 -0400) it happened Phil Hobbs wrote in :

Real 10 turns front panel pots come to mind, but also do it digital with up / down buttons and a PIC EEPROM memory? Perhaps driving a DAC, or some MOSFETS?

Thanks. I'm not a fan of pushbutton interfaces for tuning stuff, though, and just dumping in another op amp would be cheaper anyway. I'll probably go with a conductive-plastic pot and an OPA197.

Cheers

Phil Hobbs

Nonvolatile DPOTS are cool, but you need to provide an SPI or I2C connector, and software. It's pretty easy to grab a screwdriver or a Swiss army knife and frob a trimpot.

DPOTS are slow, too. Trimpots are usable at a GHz or so.

A dipswitch is sometimes an alternate to a trimpot, but the pot will be smaller and cheaper and have more resolution.

Eh, others were already going down that road -- I thought I'd suggest this one for completeness.

I agree with you about buttons, BTW.

Trimpot and good twiddle life are contradictory. They're corner cutting devices for when they'll seldom need adjusting. Maybe use a pot.

NT

ave a 200k resistor between the pot and anything sensitive, I might be able to make a big hole in the board to mount the pot. We use a lot of those too. 1" diameter hole for pot (trim off plastic tabs on pot) and ~1/4" hole centered (somewhere) on the 1" circumference for the solder tabs.

George H.

No specified rotational life, but much more settable than cheaper parts.

I remember specifying a similar part for the Cambridge Instruments electron beam microfabricator. It was more expensive than the part it replaced, but the fact that you could set it precisely meant that final test saved about half an hour on getting the board set up - everybody ended up pleased abou t that.

Next time get slots! Way stronger and easier to solder.

--sp