EEVblog Electronics Community Forum
General => General Technical Chat => Topic started by: FlevasGR on September 04, 2015, 04:17:31 pm
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The other day i was making a BT controlled 50W flashlight and i wanted a way to change the voltage from the stepup converter but in order to change it i wanted a digitally controlled potentiometer. Unfortunately i couldn't wait for shipping so i had to improvise.
A cheap servo will do the job. 8)
(http://i.imgur.com/5rnUoJZ.jpg)
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Lol, make sure to patent this
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Facepalm + high five! :clap:
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You don't know for how long I've been looking for a digital variable resistor! This is genious! When will you start selling these things? :clap:
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You do know that there are many i2c and SPI digital pots on the market right?
One that I have used before....
http://www.digikey.com/product-search/en?KeyWords=MCP4661T-503E%2FMLCT-ND&WT.z_header=search_go (http://www.digikey.com/product-search/en?KeyWords=MCP4661T-503E%2FMLCT-ND&WT.z_header=search_go)
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You do know that there are many i2c and SPI digital pots on the market right?
One that I have used before....
http://www.digikey.com/product-search/en?KeyWords=MCP4661T-503E%2FMLCT-ND&WT.z_header=search_go (http://www.digikey.com/product-search/en?KeyWords=MCP4661T-503E%2FMLCT-ND&WT.z_header=search_go)
Yeas, but as i said i couldn't wait. The only place i can buy ICs is from ebay and it takes 20-30 days :/
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In that case, BRAVO! Great improvisation in the face of challenge. You would be a good Astronaut.
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You do know that there are many i2c and SPI digital pots on the market right?
One that I have used before....
http://www.digikey.com/product-search/en?KeyWords=MCP4661T-503E%2FMLCT-ND&WT.z_header=search_go (http://www.digikey.com/product-search/en?KeyWords=MCP4661T-503E%2FMLCT-ND&WT.z_header=search_go)
Yep and even motorized pots are widely available. Right now I'm building spindle control to PCB engraver with one.
http://www.ebay.com/itm/ALPS-2x100KA-LOG-Dual-RK16812MG-Stereo-Motorized-Potentiometer-Pot-1pc-/400714743165?hash=item5d4c75bd7d (http://www.ebay.com/itm/ALPS-2x100KA-LOG-Dual-RK16812MG-Stereo-Motorized-Potentiometer-Pot-1pc-/400714743165?hash=item5d4c75bd7d)
Axel.
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A few observations:
1) Your average digital potentiometer on a chip will only work to the chip supply voltage, feed anything more than this and you let the magic smoke out.
2) Your average digital potentiometer on a chip is a low current device, this method could be used with a wirewound resistor in medium current applications.
3) Most digital potentiometers use either I2C or SPI which means a microchip somewhere, this technique could use a simple 555 chip.
4) Isolation between the control signals and the variable resistor is very good, useful for high voltage applications.
So, this particular implementation may not be too neat but the idea is sound,
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I have to hand it to Flavas that is Great.
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I think we have the next Steve Wozniak here. Can't wait for iPot 2.0 :D. The production volume world wide in the first year is forecast at 1.
Seriously, you could put in a closed loop control algorithm and you could get an accurate resistance, but mechanical hysteresis could present a problem, making it not too different to the resolution of a digital pot IC.
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Whatever it takes to move the project on is what I say. :-+
The fundamental concept is valid. For a more professional rendition for this concept one can look here:
http://jimmyauw.com/wp-datajim/audio/119_alps1.jpg (http://jimmyauw.com/wp-datajim/audio/119_alps1.jpg)
Another advantage of this kind of arrangement is that one can both manually manipulate the pot as well as electrically. That is something you couldn't do with the hobby servo drive unless you power off servo when not changing values. IC volume chips are very handy but as already stated there are real limitations that the chip solution can't meet for some applications.
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Problem with the ALPS motorised pots is that they invariably are a log law ( typically audio taper using 2 track segments) and also are limited to a few low power rated resistance values. Servo upgraded one will work and you are free to use any pot in any power rating with only needing a shaft that is adjustable via a knob. If it needs 200Nm to turn you just get a big enough servo driver to move it.
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A few observations:
1) Your average digital potentiometer on a chip will only work to the chip supply voltage, feed anything more than this and you let the magic smoke out.
2) Your average digital potentiometer on a chip is a low current device, this method could be used with a wirewound resistor in medium current applications.
3) Most digital potentiometers use either I2C or SPI which means a microchip somewhere, this technique could use a simple 555 chip.
4) Isolation between the control signals and the variable resistor is very good, useful for high voltage applications.
So, this particular implementation may not be too neat but the idea is sound,
I used an optocoupler for that in the past.
All potentiometers I saw are used as variable resistor or voltage divider. A simple schematic with some resistors can do that.
It isn't digitally controlled, an additional D/A or reconfiguring an output pin can do that.
also from me:
Facepalm + high five! :clap:
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You do know that there are many i2c and SPI digital pots on the market right?
One that I have used before....
http://www.digikey.com/product-search/en?KeyWords=MCP4661T-503E%2FMLCT-ND&WT.z_header=search_go (http://www.digikey.com/product-search/en?KeyWords=MCP4661T-503E%2FMLCT-ND&WT.z_header=search_go)
Yes, but using this packages in DIY projects is not very handy...
About the motorized pots, they are a bit expensive and run it on a low power environment could be a bit tricky... I know that a servo isn't better than this, this was just a funny project.
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You do know that there are many i2c and SPI digital pots on the market right?
One that I have used before....
http://www.digikey.com/product-search/en?KeyWords=MCP4661T-503E%2FMLCT-ND&WT.z_header=search_go (http://www.digikey.com/product-search/en?KeyWords=MCP4661T-503E%2FMLCT-ND&WT.z_header=search_go)
Yes, but using this packages in DIY projects is not very handy...
About the motorized pots, they are a bit expensive and run it on a low power environment could be a bit tricky... I know that a servo isn't better than this, this was just a funny project.
They are available in soic and DIP as well. http://www.microchip.com/wwwproducts/Devices.aspx?product=MCP41010 (http://www.microchip.com/wwwproducts/Devices.aspx?product=MCP41010)
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Search "motorised rheostat" and you can find some very high-power ones, used in industrial control applications:
http://www.micronor.com/products.php?product_id=248 (http://www.micronor.com/products.php?product_id=248)
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You do know that there are many i2c and SPI digital pots on the market right?
One that I have used before....
http://www.digikey.com/product-search/en?KeyWords=MCP4661T-503E%2FMLCT-ND&WT.z_header=search_go (http://www.digikey.com/product-search/en?KeyWords=MCP4661T-503E%2FMLCT-ND&WT.z_header=search_go)
Yes, but using this packages in DIY projects is not very handy...
About the motorized pots, they are a bit expensive and run it on a low power environment could be a bit tricky... I know that a servo isn't better than this, this was just a funny project.
They are available in soic and DIP as well. http://www.microchip.com/wwwproducts/Devices.aspx?product=MCP41010 (http://www.microchip.com/wwwproducts/Devices.aspx?product=MCP41010)
OMG how could I never see this? :o |O I've asked so many times for an electronic variable resistor and the only replys I got were recommending that other IC rx8pilot posted.
Thank you very much!!!
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Search "motorised rheostat" and you can find some very high-power ones, used in industrial control applications:
http://www.micronor.com/products.php?product_id=248 (http://www.micronor.com/products.php?product_id=248)
The word makes me think of the "actuator" that controls the throttle of a Diesel engine in a 1000kVA UPS/GenSet.
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True there are digital potentiometer chips...but this one also features several kV isolation :)
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It would probably have been much simpler to vary the boost converter output voltage by injecting current at its controller's feedback pin to offset the feedback voltage. Filtered PWM from your MCU fed via a resistor to that pin could sweep the output from a little above that set by the feedback divider down to the input voltage as you increase the duty cycle.
OTOH that's boring and wouldn't have provided much needed amusement on a bank holiday!
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Search "motorised rheostat" and you can find some very high-power ones, used in industrial control applications:
http://www.micronor.com/products.php?product_id=248 (http://www.micronor.com/products.php?product_id=248)
The word makes me think of the "actuator" that controls the throttle of a Diesel engine in a 1000kVA UPS/GenSet.
You also find them in old (and not quite) xray power supplies. At least I did when servicing crystallography equipment. Many of them were all vacuum tube and the servo arrangement was very clever.
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When I saw OP's avatar and the top part of the image, I thought I was on a different forum and was going to see "a digitally controlled potentiometer landed on the Mun" or something.
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Motorized linear actuated pots were a thing in professional audio mixing gear back then, weren't they ?
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Cunning solution.similar to the old rc car,no complicate electronics, speed controllers
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Nice! this is a well known thing, as previously stated. I first encoutered it some audio amplifers, that utilized this to control the volume (=gain) with a remote controller.
https://www.ebay.co.uk/itm/351605935402 (https://www.ebay.co.uk/itm/351605935402)