Trimpot potentiometer rotational life Vs load life

Question

I'm dealing with an application which makes use of an adjustable linear voltage regulator (LT3080). A trimpot potentiometer is used in place of the set resistance Rset to regulate the output voltage. A constant set current of 10 uA flows into the Rset and gives the output voltage following the equation Vout = Rset*Iset The linear regulator is powered by a 12V DC input source. I would like the output voltage to vary between 5V DV and 12V DC (minus the voltage dropout). Therefore I would split Rset in the series of two resistors: a 500 KOhm fixed resistor and a variable trimpot of 1 MOhm. In this way the minimum Rset will be 500 KOhm and therefore the minimum output voltage will be 5V. The maximum Rset will be 1.5 MOhm and the output voltage will go at maximum allowed. I was keen in choosing a high accuracy (0.1%) fixed Rset resistor and a good accuracy (10%) trimpot. I'm now concerned about the life of the trimpot. The datasheet of the trimpot I chose (BOURNS 3362P-1-105LF) states that rotational life is 200 cycles. I'm aware that cermet potentiometers like this one have limited rotational life. On the other hand, there is one value I am most concerned about: load life. Here again, the datasheet states 1,000 hours 0.5 watt @ 70 °C. Since I am burning a meaningless amount of power in the potentiometer, can I expect longer life? And the most important question here: What will happen after these values have been passed? Will the potentiometer be like a shortcut or and open circuit? Or there will just be derating from the set resistor value? The system is expected to work in standard room conditions, at an ambient temperature of +25C, non condensing.

^{simulate this circuit – Schematic created using CircuitLab}

Answer 1

The two main failure modes of a trimpot are ...

a) the track wearing away, from excess mechanical use, leading to a high resistance and noisy connection track to wiper

b) contaminants interrupting the contact between track and wiper, leading to a high resistance and noisy connection track to wiper

Most use, yours included, can probably ignore the first problem. The second problem has two main components, Large particles, like dust, and reactive contaminant gases, like sulphur and nitrogen compounds. These will generally react faster when hot. As you are going to be running your trimpot cool, you might like to hope that you would get more than 1000 hours out of it.

The connection you've shown is particularly nasty in adjustable voltage regulators, as the loss of track contact gives high resistance and therefore high output voltage (the nastier type of error). You can reduce the amount of damage such a fault can do by connecting the wiper to the unconnected top of the track. That way, if the wiper loses contact, the resistance only goes to 1.5M, not to infinity.

A capacitor ADJ-> GND (up to 1uF permitted by the regulator data sheet) will reduce the size of the positive excursion that any momentary losses of wiper to track contact cause as you adjust the setting.

It's worth bearing in mind that a trimpot is going to be orders of magnitude less reliable than a soldered connection to a resistor, and possibly an order of magnitude less reliable than a large potentiometer, so try to avoid them if you can.

Ideally, in this sort of application, you would not use a trimpot. If you want to set the voltage carefully to a setting and leave it there, then it's worth soldering in a resistor, measuring the voltage, calculating a new value, rinse and repeat. If you want to adjust the value often, then get a 'real' 1/4" shaft potentiometer, or an IC 'digipot', that's programmable from a microcontroller.