Digital Potentiometer - Page 1
Forum section: electronics / beginners

author: szymonm2
author_rank: Contributor
timestamp: February 13, 2016, 06:47:55 pm
content: Hi Guys,Im trying to make a digital potentiometer (500k ohm) that is attached to a dc-dc buck converter (ltc3780 to be exact)I plan on making a very cheap programmable power supply.Where do I start? What materials do I need?I already have an arduino, breadboard, 16x2 lcd.Thanks

author: DivineChaos
author_rank: Regular Contributor
timestamp: February 13, 2016, 09:18:47 pm
content: https://www.arduino.cc/en/Tutorial/DigitalPotentiometerhttps://www.arduino.cc/en/Tutorial/DigitalPotControl]https://www.arduino.cc/en/Tutorial/DigitalPotControlhttp://lmgtfy.com/?q=digital+potentiometer+arduinoO0

author: uncle_bob
author_rank: Unknown
timestamp: February 14, 2016, 03:31:19 am
content: Quote from: szymonm2 on February 13, 2016, 06:47:55 pmHiConsider carefully the fact that stability on a buck converter is dependent on the output voltage, output current, the inductor, and the input / output caps. As you change the voltage over a significant range, you will need to change the inductor, the caps, or something else in the circuit. Things may work fine at low current and go nuts at full load.Most commercial supplies get around this by using the switcher as a pre-regulator. It has a couple of fixed operating points. The rest of the regulation is done with a linear output stage.I caution because sometimes the failure involves smoke and high output voltage that takes your load with it. I have seen designs that ran for quite a while and then freaked out.Bob

author: gautamdamodar
author_rank: Contributor
timestamp: February 14, 2016, 08:04:48 pm
content: Quote from: szymonm2 on February 13, 2016, 06:47:55 pmAren't they already available as ICs?Sent from my ASUS_T00I using Tapatalk

author: szymonm2
author_rank: Contributor
timestamp: February 14, 2016, 09:04:44 pm
content: Quote from: gautamdamodar on February 14, 2016, 08:04:48 pm

author: uncle_bob
author_rank: Unknown
timestamp: February 14, 2016, 09:10:49 pm
content: Quote from: szymonm2 on February 14, 2016, 09:04:44 pmHiIt's just the ratio of the two resistors (the one to ground and the one to the supply). If you have a 50K pot, use a 750 ohm resistor to ground rather than 7.5K.Bob

author: MarkS
author_rank: Unknown
timestamp: February 15, 2016, 01:01:39 am
content: How is a digital pot made? This has me curious...

author: uncle_bob
author_rank: Unknown
timestamp: February 15, 2016, 01:07:59 am
content: Quote from: MarkS on February 15, 2016, 01:01:39 amHiIt's a silicon IC. They diffuse resistors into the substrate and then switch to a tap with MOSFET switches. Like any IC there are voltage limits.Bob

author: MarkS
author_rank: Unknown
timestamp: February 15, 2016, 01:10:46 am
content: Quote from: uncle_bob on February 15, 2016, 01:07:59 amThat's what I was thinking. I am wondering if something similar could be done with a voltage divider tapped to an I/O expander via MOSFETs/transistors?

author: uncle_bob
author_rank: Unknown
timestamp: February 15, 2016, 01:22:08 am
content: Quote from: MarkS on February 15, 2016, 01:10:46 amHiIf you want 1024 taps that's going to be a bit of a struggle.It's far easier to just use a DAC.Bob

author: MarkS
author_rank: Unknown
timestamp: February 15, 2016, 01:25:17 am
content: I haven't seen a digital pot with 1024 positions!I was looking at this one and it has 64:http://www.analog.com/media/en/technical-documentation/data-sheets/AD5171.pdfStill quite a bit, but doable.

author: uncle_bob
author_rank: Unknown
timestamp: February 15, 2016, 01:27:33 am
content: Quote from: MarkS on February 15, 2016, 01:25:17 amHiThe next issue would be voltage range if we are trying to run a 0-30V power supply with one end of the pot hooked to B+. We haven't quite made it that far into the circuit yet.Bob

author: michaeliv
author_rank: Frequent Contributor
timestamp: February 15, 2016, 01:32:41 am
content: What is the schematic for a switch inside a digital pot ? It can't just be a single MOSFET since they need to handle AC.

author: uncle_bob
author_rank: Unknown
timestamp: February 15, 2016, 01:42:55 am
content: Quote from: michaeliv on February 15, 2016, 01:32:41 amHiMany of them are essentially DC parts. The signals on both ends need to be inside the rails of the IC. Others are floating gate (like a flash memory). There are limits and benefits to each. You (unfortunately) do not get to randomly mix and match technologies. They all are silicon so voltage and reverse bias comes in one way or the other.To directly answer the question:For a 5V p-p ground referenced AC signal capable part. You generally see a +/-6V DC supply attached to the pot.Bob