on February 16th, 2011

Dear Colleagues

The ‘pot’ or potentiometer as it is called, is one of the workhorses of electrical and electronic engineering. I believe we can all still remember working with a pot on some sideline design or in a work related application. They have been around a long time. There are more sold than any other form of position sensor. Simple, cheap and small with no real obsolescence problems. So as Mark Howard asks: Why today does every designer look for a non-contact alternative?

Going potty about pots

For those that don't remember clearly - a potentiometer or 'pot' is a three-terminal resistor with a sliding contact that forms an adjustable voltage divider (Thanks Wikipedia). Commonly used to control electrical devices such as volume controls and can be used in a control stick or level indicator or position transducer. As evidenced when my student buddy in an engineering lab tried to control a large electric motor with a potentiometer drawing full the current and it disappeared in a gigantic smouldering bang; they are not used to control significant amounts of power (more likely a watt or less). A potentiometer could be used to control the switching of a Triac to indirectly control the brightness of a lamp.

The Nemesis of pots

Because of perceived problems with reliability (and indeed wear and tear); these days we typically are always on the lookout for non-contact alternatives. Thus pots are not considered uber-sexy any longer. Why the unreliable image ? Pots can be rated for 500,000 cycles and can thus be good for 5 years or more (with linear displacement changes every 5 minutes). There are three main causes of pot failures: vibration, ingress of foreign matter and extreme climatic conditions. With vibration, the pots wiper is normally at the same place most of the time and significant wear occurs producing a flat spot with no electrical response. Extreme environments can produce condensation on the wiper and corrosion.

So in harsh environments one needs to seriously consider alternatives. But for the average application, one should realize that pots are still extraordinarily reliable and will work perfectly.

Be Wary

But if you do need to change to a non-contact approach - be wary (and afraid) ! Besides costing more, non-contact alternatives often produce a digital electrical output. So your entire associated electrical system may need to be redesigned from analog to digital. And then retested and approved. Quite a considerable additional expense! In addition, pots are normally very compact and you may find the non-contact alternative is considerably larger, causing a mechanical re-jig of your system.

A few take away lessons on the 'pot':

• Don't despise older technology - it often gets the task done and is effective
• Marketing often relies on high tech so-called improvements which aren't better than the old and tested approach – often considerably worse
• When you do make changes to your engineering design with a new component; the biggest impact may be on all the associated interfacing equipment.
• Ensure your components are matched to the entire range of the engineering environment.

I hope with my weekly commentary, the following comment from Gertrude Stein is not true ? Everybody gets so much information all day long that they lose their common sense.
Thanks to Mark Howard of Zettlex Ltd for an interesting article., Wikipedia for your information and Rod Elliot for a great informative web site on pots.

Yours in engineering learning

Steve


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