Thursday, August 9, 2012

Encoders vs. Potentiometers: Which one is best for programmable power supplies?

Recently I asked the product manager of TDK-Lambda’s new Z+ Series of programmable power supplies a question, “Why use rotary encoders to adjust the output voltage and current limit in preference to potentiometers?”

“The answer is relatively simple,” he said; it’s all about digital circuitry compatibility and resolution”. The key advantage to using a rotary encoder over a potentiometer is that that the digital signal produced by the encoder eliminates the need for the analog to digital (A/D) conversion that is required when potentiometers are used. In addition, multi-turn encoders can provide more accurate and higher resolution set-points than potentiometers.

Let’s put this into context: if a single turn potentiometer were to be used to adjust the output voltage on a 20V rated model, one full turn would typically represent 20V; this resolution is relatively low and therefore not precise enough for a programmable power supply. Of course, multi-turn potentiometers could be employed but these are bulkier than and not as reliable as encoders are.

Digital rotary encoders are available with very high resolutions and can operate in dual modes, including a coarse and fine mode. In the coarse mode, the encoder operates with a lower resolution; for example, a 20V rated model would require about six turns of the encoder knob to adjust the output from zero to 20 volts. In the fine mode, one turn equates to an approximately 40 millivolts, thereby providing a much higher resolution and more precise set-point.

To avoid an inadvertent change to the voltage setting during use, a front panel locking feature is needed. With an encoder, this locking function is done via software and internal memory, whereas a potentiometer requires a less reliable mechanical locking mechanism.

The Z+ programmable power supply employs two encoders as set-point controls for the output voltage and current. The encoder’s digital output feeds the unit’s memory, which retains the last set-points prior to the AC power being turned off and on, either intentionally or due to an unplanned power outage, and keeps the output disabled until the user enables it (aka “safe-start” mode). In the “auto-restart” mode, the Z+ power supply remembers its last output set-points and when the power is turned back on, it resumes its normal operation, which is handy for unattended applications.

To see more details including videos about the encoder driven Z+ programmable power supplies, please use this link:

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