In Potentiometers: Mechanical & Electronic, Part 1, I described the use of pots generically and hinted at some of the more common configurations with circuits. I now hope to cover the comparative advantages and disadvantages of both mechanical and electronic potentiometers.
There are many types of mechanical potentiometers varying in the size, technology, power dissipation, resolution, and thermal stability, but they all follow the same principle. There is a fixed resistor between two pins and a mechanical wiper that moves along the resistor element providing the variation in the resistance between a resistor end and the point of contact of the wiper. Gearing the wiper mechanism (resulting in multi-turn adjustments) allows for greater resolution in the adjustment. However, the point of contact of the wiper is a weak spot in the design. Wear and dirt can result in the contact going open circuit or introducing additional resistance. Older readers will remember the crackling sounds that could be heard on radios when there was a dirty volume control. I would be remiss if I didn’t add the aside that contact cleaner aerosol is often an effective remedy for this problem.
Because of their physical size mechanical pots typically have relatively large wattage ratings, although of course the higher the wattage dissipated, the worse the accuracy of the device because of the self-heating. Adjusting most mechanical pots is done either with a small screwdriver or fingertip control, but when you get to higher power devices like the Ohmite 210 series. The wiper consists of a clamp that fits over a tubular resistor and adjustment can be cumbersome and even dangerous if there is high voltage or the resistor is hot. Also pay attention to the wattage rating — to see what I mean, take a look at the restriction the datasheet link above. It stipulates:
- The stated wattage rating applies only when the entire resistance is in the circuit. Setting the lug at an intermediate point reduces the wattage rating by approximately the same proportion. Example: If the lug is set at half resistance, the wattage is reduced by approx. one-half.
I suspect that this is may be true for all mechanical pots.
The advantage of a mechanical pot is that it is non-volatile (poor joke, I know), but really, its setting does not change with power cycling. Setup may be intricate, but anyone intuitively knows how to adjust it. On the other hand, mechanized adjustment as part of a calibration process can be expensive and difficult.
The electronic (or digital) pot is not direct replacement for the mechanical pot. There are times that one can substitute, but there are times that it just won’t work. The electronic pots are electronic devices- they need a power supply and the signal/voltage that you are conditioning must fit within the power supply restrictions (although there may be some that allow signal operation beyond the supplies). Obviously the maximum power dissipation is much lower than the mechanical parts. Also the operating temperature range is much reduced compared to their mechanical brethren. Beware the gotcha on the wiper current. Often it must be limited to a very low value as per the specifications — forewarned is forearmed! On the plus side, the resolution is fixed and any setting is repeatable and even more importantly it allows for automation during calibration, although there will almost certainly have to be additional firmware to achieve this.
Being monolithic it means that you can have multiple pots on the same chip (although there is a possibility of interaction or even breakdown between them) and you can have additional circuitry to provide boosted functionality. The addition of an EEPROM allows the pot to be non-volatile and drivers allow for different interface protocols. Application of electronic pots can be extended well beyond the trimming capability of mechanical pots and start overlapping Digital-to-Analog Converter territory, depending on the designer’s imagination.
The wide selection also precludes me from trying to provide some kind of generic table to cover all eventualities, but as a guide here is a list of some manufacturers of electronic pots: Maxim electronic pots, Intersil electronic pots, Analog Devices electronic pots, Microchip electronic pots, On Semiconductor electronic pots, and Texas Instruments electronic pots.
Digital pots are made up of a series of resistors with analog switches that allow a particular tap setting to be selected. This allows a potentiometer configuration, but is restrictive since the current must flow through the resistor string and the wiper current (as mentioned above) is constrained by the limitations of the analog switch. I should mention that there is a way to make a true rheostat using a MOSFET with the gate being adjusted to regulate the on-resistance of the device. A more detailed discussion is beyond my self-imposed scope of this blog. However as far as I can tell, Advanced Linear Devices has extended this idea to a concept called EPAD (Electrically Programmable Analog Devices) and it claims it can be used as a trimmer substitute. You can find the data here and AN1108 covers the applications.
There are thousands of different potentiometers out there, but I know of one case where our subcontractor actually made their own high wattage potentiometer. You can see a description in my blog PCB Test Jigs & My China Connection. I hope you never have to resort to the same approach.
For a bit more information on the mechanical/digital pot comparison, see “Understanding and Applying Digital Potentiometers” from Analog Devices. Do you use pots? Have you ever replaced a mechanical pot with an electronic one?