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Digital Potentiometer and Power Supply

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cmore082

New Member
Hi, and thanks for helping me :)

I purchased Tusotek Auto DC-DC Constant Current Boost Buck Converter Voltage Regulator 6-35V to 1-35. I am trying to find digitally control the boost buck converter by replacing its 100K potentiometer with a 100K MCP41100 digital potentiometer. I tried using the digital potentiometer with Arduino and the DC-DC booster and it worked, but the booster does not go as high as with the original mechanical potentiometer. Would you know why I the digital potentiometer is no working as intended? What are the factors I must consider if I want to control boost buck converters with a digital potentiometer? Specs of the mechanical potentiometer can be found in the following website (scroll down): http://www.aliexpress.com/item/Trim...ohm-Variable-Resistors-100pcs/1596795967.html
 

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ronsimpson

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What are the factors I must consider
I wish I has a schematic of your boost power supply. That will help.

The digital pot is not like a mechanical pot!
A digital pot must not have a voltage applies to any pin that is outside of the power supply range. (example power =5V & 0V. so the pot must also be in the 0 to 5 volt range)
 

cmore082

New Member
How could I digitally control the booster? My aim was to control the digital potentiometer with an Arduino. Thanks a lot for your help, this is for a school project.
 

schmitt trigger

Well-Known Member
A schematic of the power supply is required.
Or, at the very least, what is the applied voltage end-to-end on the mechanical pot. Measure it at both lowest and highest voltage settings.

As Ron has mentioned, if the voltage goes beyond the digipot supply, it will no longer behave correctly and may be even be damaged.
 

cmore082

New Member
Hi, sorry for the late response, I was in class. Schmitt trigger could you please explain what you meant
if the voltage goes beyond the digipot supply, it will no longer behave correctly and may be even be damaged.
Would you mean the voltage from terminal A to B? I am sorry Ebay has not provided me with the schematic.
Ronsimpson would you like to know the IC of the booster? I thought it would have been simple to digitally control a booster with a micro controller.
Tomizett, I would say it would reach about 1/4 of original performance.

Thank you all for taking your time in trying to help
 

ronsimpson

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Ronsimpson would you like to know the IC of the booster? I thought it would have been simple to digitally control a booster with a micro controller.
I would think it would be easy to read the part number.

So I pick a part. The feed back voltage is 1.23 volts. (this part) R1 and R2 divide 24V down to 1.23 volts. If R1/R2 is a pot then 24 volts is across the pot. BUT Your digital pot can only handle 5 or 3.3V across it.
upload_2016-7-5_18-19-59.png
What is the maximum output voltage?
What is the minimum output voltage? (can not be lower then the max input voltage)
What is the feedback voltage?
I really need to know what parts are connected to the feedback pin. (above R1 & R2) In your case 10k pot & fixed resistor or two. What values?
After that it is just math.
----edited-----------
Important questions:
When power is first turned on and the micro has not addresses the digital pot yet, what do you want to happen? It is very likely that the power supply will output the maximum voltage and brake something. So do you want Vin=Vout which is the minimum output voltage? You can not have an output of zero volts. (or if that is very important then we need to make big changes)
 
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cmore082

New Member
Ronsimpson, I will look up for the information that you asked and will let you know. Thank you for the help!
 

cmore082

New Member
Important questions:
Hi Ronsimpson, I figure it's best to describe the aim of my school project. I am looking towards building a heating jacket for winter (it gets really cold in Ottawa, Canada). I am doing this for my electronics class. Currently I am looking on having 3 heating elements that have roughly a maximum load of 10 V and 2 A each. I would like to use a dc-dc buck booster as a voltage regulator and a voltage booster as well. It would impress more my professor if I am able to digitally control the voltage with a micro controller. This is where I am struggling the most. As stated in the thread, I am hoping to digitally control a buck booster in order to change temperature.

I am open to any suggestions and advises you have. You are more knowledgeable than me and not that many people have tried to help me like you have. I greatly your time and help.
 

dougy83

Well-Known Member
Ref feedback resistors R1 and R2 in Ron's schematic above (Digital Potentiometer and Power Supply), if you use your pot in place of R2 (just connect the wiper and one leg), then you won't have any issues with over-voltage, as the voltage across the pot will always be the feedback voltage of 1.23V. The relationship between the pot value and the output voltage won't be linear with this set up, but you may be able to get the desired setting (I haven't done the calculations).

EDIT: If you use a 100k pot in series with a 10k resistor for R2, and a 100k resistor for R1, the relationship between the pot value (step) and the output voltage should be as shown in the following chart (provided that the feedback voltage is 1.23V).

upload_2016-7-6_15-10-17.png
 
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spec

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Ref feedback resistors R1 and R2 in Ron's schematic above (Digital Potentiometer and Power Supply), if you use your pot in place of R2 (just connect the wiper and one leg), then you won't have any issues with over-voltage, as the voltage across the pot will always be the feedback voltage of 1.23V. The relationship between the pot value and the output voltage won't be linear with this set up, but you may be able to get the desired setting (I haven't done the calculations).

EDIT: If you use a 100k pot in series with a 10k resistor for R2, and a 100k resistor for R1, the relationship between the pot value (step) and the output voltage should be as shown in the following chart (provided that the feedback voltage is 1.23V).

View attachment 100314
Brilliant :cool:

I knew there was a way but just couldn't see it.

spec
 

Wade_Hassler

New Member
An array of open-collector/open-drain drivers could parallel resistors with lower member of the feedback network('R2' in post #8 above) to achieve several output levels.
This would make the power-supply's internal voltage-levels more or less irrelevant
 

cmore082

New Member
Hi All,

Thanks a lot for the useful input. You guys are awesome and of great help!

I took new measurements of each heating element. Each heating element (120 mm length 15 mm wide) at its max load of 3V and 1.5A generates 60 celsius. Say I want to have three of these heating elements on the chest (Circuit 1), three on the back (Circuit 2), and two on the stomach area (Circuit 3). I am wondering what is the best set up for the design. Looking at one circuit individually, say circuit 1, would it be best to connect the heating elements in series or in parallel?

Once we find what is the best set up for each circuit, how should we connect all circuits (parallel or series)?

It would be awesome to have a portable battery source that can supply power to the jacket. That's why I was wondering if I would have to use a dc booster.

I greatly appreciate your expert advice. Thank you!
 
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spec

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Hi All,

Thanks a lot for the useful input. You guys are awesome and of great help!

I took new measurements of each heating element. Each heating element (120 mm length 15 mm wide) at its max load of 3V and 1.5A generates 60 celsius. Say I want to have three of these heating elements on the chest (Circuit 1), three on the back (Circuit 2), and two on the stomach area (Circuit 3). I am wondering what is the best set up for the design. Looking at one circuit individually, say circuit 1, would it be best to connect the heating elements in series or in parallel?

Once we find what is the best set up for each circuit, how should we connect all circuits (parallel or series)?

It would be awesome to have a portable battery source that can supply power to the jacket. That's why I was wondering if I would have to use a dc booster.

I greatly appreciate your expert advice. Thank you!
All that you say is doable.

What duration would you like for the battery?

The best implementation would be to have temperature sensors on each heater group and pulse width modulation (PWM) power controllers for each heater group. The temperature could then be set accurately by a micro-controller unit (MCU). I would think this approach would really impress your professor, especially if you do a nice write up on the rational of your approach. :D

spec
 
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ronsimpson

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Heating element is 3V and 1.5A and 2 ohms and 4.5 watts.
Using three elements:
Parallel: 3V, 4.5A
Series:9V, 1.5A, 6 ohms, 13.5 watt

So the real question is what battery?

Option:
Do not use the PWM power supply. Let the Arduino turn on/off the current to the element at some rate.
The PWM runs at 100khz but your Arduino could turn on/off the current at some much slower rate. Maybe 100hz or 1khz.
By simply adding a MOSFET you can control the current.
Say you have a 12 volt battery. Applying power will give you 12V, 2A, 24 watts. Which is too much. But if you turn on the power for 50% of the time you will get 12 watts. 56%=13.5 watts. Pretty simple math and all you need is a simple "logic level MOSFET + one resistor".
--------------------------
DMG3420U-7 20V 5.5A 0.021 ohms mosfet.
The MOSFET sits from the bottom end of the load to ground.
 
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spec

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My first thought are connect the heaters in parallel in 3 groups. The current through each group would then be switched by a MOSFET controlled by an MCU. Each group would also have a temperature sensor in physical contact with one of the heaters. The output from the heaters would be read by the MCU which would control the average current through the heaters using pulse width modulation (PWM).

One high current 18650 sized LiIon battery would provide 18 minutes battery duration with all eight heaters running flat out. You could increase the duration by simply adding batteries in parallel. Each additional battery would add another 18 minutes. There are three battery types commonly used for high current applications like this.

spec

PS: crossed threads Ron :)
 
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ronsimpson

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One 18650 sized LiIon battery
I like the idea of using about 3V battery on a 3V load. If the micro dies or the software crashes either the power will be zero or about 100%. (120%?)
You will need a 10A MOSFET.
I don't like discharging a battery in 15 minutes. Too much power is lost in the battery.
Another thought is to series three batteries and three loads. So we have 9V battery (9.9V) and a 9V load. If what Spec said is right then this will last 45 minutes at full power.

We agree that the boost/buck power supply is not needed.
 

spec

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Most Helpful Member
I like the idea of using about 3V battery on a 3V load. If the micro dies or the software crashes either the power will be zero or about 100%. (120%?)
Are woried about human safety?
You will need a 10A MOSFET.
Yeah at least, but the most important parameter is a low RDSS to minimise power loss. The problem with a single battery is turning a MOSFET on with just 3V. But there are ways around this.
I don't like discharging a battery in 15 minutes. Too much power is lost in the battery.
High discharge batteries are pretty efficient with high currents. But the single battery was only to give a yard-stick. Depending on what the OP says there may be N batteries in parallel. Also, I suspect that all three heaters banks will not be running flat out all the time.
Another thought is to series three batteries and three loads. So we have 9V battery (9.9V) and a 9V load. If what Spec said is right then this will last 45 minutes at full power.
or put three batteries in parallel. Don't forget that there are three banks of heaters 3+3+2
We agree that the boost/buck power supply is not needed.
:)

spec
 
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