I need a Digital Potentiometer

Bobb Fwed

I am looking for a digital potentiometer that can handle at least 0.25W of power, 0.5W would be better. It doesn't need to be a prefabricated IC, any ideas on how I can design my own (simpler the better). If it is any easier, I will be using it as a variable resistor (wiper and one terminal).
I could manage with 8-positions, but 16 (or more) would be better. I want it to work with my propeller setup, but it can't take more than 16 I/Os (seems like a lot, but I just added a second Propeller to the mix). Can someone point me in the right direction?

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April, 2008: when I discovered the answers to all my micro-computational-botherations!

Some of my objects:
MCP3X0X ADC Driver - Programmable Schmitt inputs, frequency reading, and more!
Simple Propeller-based Database - Making life easier and more readable for all your EEPROM storage needs.
String Manipulation Library - Don't allow strings to be the bane of the Propeller, bend them to your will!
Fast Inter-Propeller Comm - Fast communication between two propellers (1.37MB/s @100MHz)!

mctrivia

Search digikey for digital pots would be my first sugestion.
What is the max resistance you need?

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Free shipping on orders over $100 and a Free LCD screen for orders over $200. Today Only(April 28th, 2010)

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Bobb Fwed

I tried that. The problem is they don't readily list the power dissipation which is usually listed in mA and is usually 1mA or so. I also tried google, but there is just a million things that come up, and 100% of the things I have looked at do not match my requirements. That's why I was hoping someone here had some ideas.
I can work with anything between 5K-ohm and 20K-ohm max resistance.

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April, 2008: when I discovered the answers to all my micro-computational-botherations!

Some of my objects:
MCP3X0X ADC Driver - Programmable Schmitt inputs, frequency reading, and more!
Simple Propeller-based Database - Making life easier and more readable for all your EEPROM storage needs.
String Manipulation Library - Don't allow strings to be the bane of the Propeller, bend them to your will!
Fast Inter-Propeller Comm - Fast communication between two propellers (1.37MB/s @100MHz)!

Rayman

I've used the AD5206/AD5204 series. Don't remember their power level though...

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My Prop Products:· http://www.rayslogic.com/Propeller/Products/Products.htm

mctrivia

If power levels are not high enough you can use several higher values in parallel .

Or if your purpose is to control current or voltage level a transistor, jfet or current mirror could be used

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Lots of propeller based products in stock at affordable prices.

Free shipping on orders over $100 and a Free LCD screen for orders over $200. Today Only(April 28th, 2010)

propmodule.com will be closed May 1-10th. Orders made in that time will be shipped on the 11th.

Bobb Fwed

Rayman said...
I've used the AD5206/AD5204 series. Don't remember their power level though...

If I am reading the datasheet correctly (please someone verify) it can do 11mA continuously. That is a little lower than I want, but it may work.

mctrivia said...
If power levels are not high enough you can use several higher values in parallel .

The problem is I have to do this eight times over on a single PCB, so I was hoping for a single component, but this may be an option.

mctrivia said...
Or if your purpose is to control current or voltage level a transistor, jfet or current mirror could be used

I have tried this in the past in this application, and it doesn't work.

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April, 2008: when I discovered the answers to all my micro-computational-botherations!

Some of my objects:
MCP3X0X ADC Driver - Programmable Schmitt inputs, frequency reading, and more!
Simple Propeller-based Database - Making life easier and more readable for all your EEPROM storage needs.
String Manipulation Library - Don't allow strings to be the bane of the Propeller, bend them to your will!
Fast Inter-Propeller Comm - Fast communication between two propellers (1.37MB/s @100MHz)!

mctrivia

what exactly is the application?

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Lots of propeller based products in stock at affordable prices.

Free shipping on orders over $100 and a Free LCD screen for orders over $200. Today Only(April 28th, 2010)

propmodule.com will be closed May 1-10th. Orders made in that time will be shipped on the 11th.

Bobb Fwed

It is difficult to say exactly. I have an electrical device that can be adjusted via 0.5W potentiometer. The internal circuitry is sealed up so what is exactly going on in there is a bit of a mystery. What I am wanting to do is mimic the traditional potentiometer with a digital one so I can automated the adjustments.

As I said before I have tried the current mirror method, and the adjustability became very limited and seems to slip (adjustment moves without the current changing).
I have also tried a 1mA digital potentiometer which promptly fried (luckily I have quite a few laying around). I lowered the voltage going to about half and then the digipot work exactly as I expected for about 2 minutes before it, too, fried. You could tell almost immediately that the digipot was getting too hot.
So a higher power digitpot seems it should do the trick.

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April, 2008: when I discovered the answers to all my micro-computational-botherations!

Some of my objects:
MCP3X0X ADC Driver - Programmable Schmitt inputs, frequency reading, and more!
Simple Propeller-based Database - Making life easier and more readable for all your EEPROM storage needs.
String Manipulation Library - Don't allow strings to be the bane of the Propeller, bend them to your will!
Fast Inter-Propeller Comm - Fast communication between two propellers (1.37MB/s @100MHz)!

BTX

Hi Bobb
I don't know exactly your application for that pot, but...
What about a 4 bit outputs from the pchip, each connected to a mosfet gate, who drives to GND each common resistor (previously calculated) to get a 16 differents values of R.
Or 5 bit, etc, etc to get more resolution.
Do you understand my idea? but this could be great, if you've to tie to GND one potentiometer terminals.

If you want more steps, use a HC595 to control the mosfets (eight), you'll get until 256 steps, with only three pchip pins used.

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Regards.

Alberto.

James Newman

Like BTX said, use some transistors to either bypass resistors or pull certain points to ground. You would have full control over how many bits of resolution you want, power ratings, and should be doable with some cheapo transistors.

Bobb Fwed

Neither pin on the pot is connected to ground. One terminal fluctuates between 0 and 1V though. The other goes up to 7V or so. Both were measured with respect to incoming ground.

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April, 2008: when I discovered the answers to all my micro-computational-botherations!

Some of my objects:
MCP3X0X ADC Driver - Programmable Schmitt inputs, frequency reading, and more!
Simple Propeller-based Database - Making life easier and more readable for all your EEPROM storage needs.
String Manipulation Library - Don't allow strings to be the bane of the Propeller, bend them to your will!
Fast Inter-Propeller Comm - Fast communication between two propellers (1.37MB/s @100MHz)!

T Chap

I use an opto jfet for a 'digital resistor' application, using PWM to set the resistance. Modify the PWMasm object to go from 0 - 1000 duty for higher res.

Mike Green

You may be better off using a small servo to turn a standard pot of the value and rating you need. You may be able to find one with the sort of current rating you need, but they're really not made for that sort of use. They're also usually not designed for voltages on the order of 7V or higher although there are some on the market that will handle as much as 30V if you power it with a slightly higher voltage than the maximum pot voltage.

ErNa

Is it really a potentiometer circuit or a variable resistor? If you apply 10 V to 10k, I calculate 100 mW. "One terminal fluctuates between 0 and 1V though. The other goes up to 7V or so. " ?? the tap should show a voltage between 0V and 7V. Is 7V stable? Is the mechanical pot still ok? Is there AC or DC applied?

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Install a propeller and blow them away

Bobb Fwed

It is a variable resistor as I understand it. Only the wiper and one of the two other terminals is being used.
It is DC, but the high(er) and low(er) voltages on the potentiometer change more or less frequent depending on the setting of the potentiometer. The 7V side at times is 0V (at the same time the lower voltage side is 0V). So it is pulsed power.
"Is the mechanical pot still ok?" why do you ask? -- yes it is. We have had these parts running for years on the traditional pot setup without any issues. 100mW is not nearing their 500mW rating. And even at that, the power is pulsed so it is likely even less. That is why I wanted 250mW digipot, but more than likely a lesser one would be sufficient. For our long term equipment, I like using things at about half (or less) of their maximum rating.

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April, 2008: when I discovered the answers to all my micro-computational-botherations!

Some of my objects:
MCP3X0X ADC Driver - Programmable Schmitt inputs, frequency reading, and more!
Simple Propeller-based Database - Making life easier and more readable for all your EEPROM storage needs.
String Manipulation Library - Don't allow strings to be the bane of the Propeller, bend them to your will!
Fast Inter-Propeller Comm - Fast communication between two propellers (1.37MB/s @100MHz)!

ErNa

If one of the terminals is connected to 7V and the other one shows 0-1 volt, then, turning the pot to (close to) zero resistance will lead to a quit high current, that could destroy him. A normal pot will always show some residual resistance, so he can stand the current. A silicon device will melt more likely. Supposing, the 7V and the 0-1 V are more or less constant, then, changing the resistance will just change the current flowing. Again, supposing that 7V is just the supply and the function is controlled by the current flowing into the low voltage terminal, than a current source might be the right solution. Did you measure the current range through the variable resistor? (It is not a potentiometer, because a potentiometer is characterized by having two end terminals and a tap.)

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Hello Debris
Install a propeller and blow them away

Tubular

Here's an alternate approach - use a couple of opto FETS such as the H11F1/2/3. This gives you isolation and gets around the floating pot issue.
www.fairchildsemi.com/ds/H1/H11F2M.pdf

I'd start by leaving you existing pot in there, and parallel up 2 OptoFets - one from wiper to "high" end of the pot, and one from wiper to "low" end of the pot. Thats on the "FET" side of the opto fet. On the Opto Side, connect each LED to your prop circuit ground. And connect say 4 prop pins through different resistors to the anode of each the opto LEDs to give you different drive currents (and therefore different "remote resistances").

That way you can start at a known operating point, and deviate/experiment with moving either direction from the operating point. Leaving the original pot in there gives you a fail safe in case the prop is off (both fets will go very high resistance). You can always replace the original pot with two resistors to give a permanent operating point

tubular

ErNa

This optofet is surely the right way, but, the circuit is a variable resistor, not a potentiometer! Therefor, only one is needed and there has to be some current limitation, so an additional resistor in series

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cmapspublic3.ihmc.us:80/servlet/SBReadResourceServlet?rid=1181572927203_421963583_5511&partName=htmltext
Hello Rest Of The World
Hello Debris
Install a propeller and blow them away

Bobb Fwed

Thank you all. I will have to give that a try.

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April, 2008: when I discovered the answers to all my micro-computational-botherations!

Some of my objects:
MCP3X0X ADC Driver - Programmable Schmitt inputs, frequency reading, and more!
Simple Propeller-based Database - Making life easier and more readable for all your EEPROM storage needs.
String Manipulation Library - Don't allow strings to be the bane of the Propeller, bend them to your will!
Fast Inter-Propeller Comm - Fast communication between two propellers (1.37MB/s @100MHz)!