Digital Potentiometer

[szymonm2]

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 

[DivineChaos]

https://www.arduino.cc/en/Tutorial/DigitalPotentiometer
https://www.arduino.cc/en/Tutorial/DigitalPotControl]
[url]https://www.arduino.cc/en/Tutorial/DigitalPotControl[/url]

http://lmgtfy.com/?q=digital+potentiometer+arduino

 O0

[uncle_bob]

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 

Hi

Consider 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

[gautamdamodar]

Hi Guys,
Im trying to make a digital potentiometer (500k ohm)

Aren't they already available as ICs?



Sent from my ASUS_T00I using Tapatalk

[szymonm2]

Hi Guys,
Im trying to make a digital potentiometer (500k ohm)

Aren't they already available as ICs?

I actually couldnt find any 500k ohm digital potentiometers. Can you give me any links I could buy it from preferably from europe or china?

Sent from my ASUS_T00I using Tapatalk

[uncle_bob]

Hi Guys,
Im trying to make a digital potentiometer (500k ohm)

Aren't they already available as ICs?

I actually couldnt find any 500k ohm digital potentiometers. Can you give me any links I could buy it from preferably from europe or china?

Sent from my ASUS_T00I using Tapatalk

Hi

It'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

[MarkS]

How is a digital pot made? This has me curious...

[uncle_bob]

How is a digital pot made? This has me curious...

Hi

It'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

[MarkS]

How is a digital pot made? This has me curious...

Hi

It'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

That'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?

[uncle_bob]

How is a digital pot made? This has me curious...

Hi

It'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

That'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?

Hi

If you want 1024 taps that's going to be a bit of a struggle.

It's far easier to just use a DAC.

Bob

[MarkS]

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.pdf Still quite a bit, but doable.

[uncle_bob]

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.pdf Still quite a bit, but doable.

Hi

The 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

[michaeliv]

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.

[uncle_bob]

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.

Hi

Many 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