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### Topic: Help with digital potentiometer (Read 491 times)previous topic - next topic

#### YaaqobhPT

##### Apr 18, 2019, 07:45 pmLast Edit: Apr 18, 2019, 07:50 pm by YaaqobhPT
Hello Guys!
I'm on a project right now and I'm facing a problem that I cant solve....

I need with an arduino to do a digital potenciometer, so with the arduino I can increase and decrease the resistance so that I can filter the resistance in (0%->100%).
How I can do that since the corrent and the Voltage is so high?
(In "RESISTOR" the voltage must be always the same(60-65V), but the resistence will variate project to project)

#### DVDdoug

#1
##### Apr 18, 2019, 08:04 pm
Right...   A digital pot won't handle the voltage or current.  That looks like a difficult project!    I have a lot of electronics experience and education but to build something like that I'd have to do a lot of study and experimentation (and possibly blow some parts along the way ).

What's the purpose?   i.e. Can you use voltage or current control rather than trying to control/set the resistance?

Is that an actual transformer (with AC output) or is it DC power supply?  Is the output voltage 65V, or is that voltage across the fixed/load resistor?

You might be able to do something with MOSFETs and optical isolators, but opto-isolators are not linear.   And if it's AC you'd need complimentary MOSFETs/circuits to handle voltage & current in both directions.

#### YaaqobhPT

#2
##### Apr 18, 2019, 08:29 pmLast Edit: Apr 18, 2019, 08:31 pm by YaaqobhPT
So the "Resistor" its a material that changes the color of a glass with current and to have the full effect must have ~65V. The thing is that each project will be diferent because each project will have a diferent area, so diferents areas(diferents resistances) will have diferents transformers so that we can provide enough corrent to have ~65V.
The transformer will be AC to DC.

#### DrAzzy

#3
##### Apr 18, 2019, 09:59 pmLast Edit: Apr 18, 2019, 10:07 pm by DrAzzy
The highest voltage digital pot I'm aware of is 36v, and the digital power and ground must be between the high and low rails (and you need to supply V+ and V-, which must be within 36v of eachother and no terminal on the digipot can be outside of that range).

Digital pots are also limited to a few milliamps of current - you need about 100x the current that a digipot is capable of handling.

Hence, I conclude that your proposed use of a digital pot is not a viable approach.

I think what you need is a power supply that allows you to adjust the output voltage electronically... I suspect there are options for this available, I just am not familiar with them.

Also, a word of terminology: A "transformer" has AC as input and output. A transformer cannot be powered by nor output DC. The "old school" way of making a DC power supply was with a bridge rectifier, filter caps, and regulator on the output of a transformer - this method fell out of favor as the more modern switch mode power supplies (SMPS) became readily available, as they are smaller, cheaper, and more efficient. But regardless of the topology, a device that takes in AC mains power, and outputs a DC voltage is not a "transformer" - it's probably a "power supply"
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#### YaaqobhPT

#4
##### Apr 18, 2019, 10:25 pm
Yes, power supply sry

#### DVDdoug

#5
##### Apr 18, 2019, 10:50 pmLast Edit: Apr 18, 2019, 10:59 pm by DVDdoug
Quote
So the "Resistor" its a material that changes the color of a glass with current and to have the full effect must have ~65V.
So... Would a 0-65V variable voltage work?   It's easier to make a variable voltage (or a variable current) than to make a variable resistance.

That's easier,,  but not necessarily easy at these voltages...

You can get constant current (or variable controlled  current) power supplies designed to power high-power LEDs.   The industry standard for these "dimmable" LED power supplies is  0-10VDC or 10V PWM to control the dim-level (current).    You can't get 10V directly from the Arduino but with a ~10V power supply and a transistor or MOSFET, you can "boost" the Arduino's 5V PWM.

#### YaaqobhPT

#6
##### Apr 20, 2019, 01:31 amLast Edit: Apr 20, 2019, 01:38 am by YaaqobhPT
I will use something like this, Power Supply , and I thought to use a potentiometer, control it with a small motor and and arduino.
But yes I think decreasing the voltage we can do the same effect.
The thing is to do this in the easier, safest and cheaper way

#### YaaqobhPT

#7
##### Apr 27, 2019, 01:39 amLast Edit: Apr 27, 2019, 01:46 am by YaaqobhPT
Hello guys!
I'm doing a project with the next scheme:

But I dont know the type of transformer should I pick....
It needs to have:
- 220V input
- 65V output
- Power: 50W->600W

(I need to make sure that in the resistor haves 65V.)

I found this types :
- EI Transformer
- BK Transformer
- Power Transformer
- High Frequency Transformer

#### dougp

#8
##### Apr 27, 2019, 01:45 am
Putting aside the transformer for the moment, have you found a digital potentiometer which will handle .8a?

What's the purpose of the resistor?
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#### jremington

#9
##### Apr 27, 2019, 05:41 am
Quote
- Power: 50W->600W
What does this mean?

#10
##### Apr 27, 2019, 01:39 pm
Putting aside the transformer for the moment, have you found a digital potentiometer which will handle .8a?

Or will handle 65V?

#### Grumpy_Mike

#11
##### Apr 27, 2019, 11:53 pm
Quote
I'm doing a project with the next scheme:
Sorry you are not.

#### MarkT

#12
##### Apr 28, 2019, 03:01 amLast Edit: Apr 28, 2019, 03:02 am by MarkT
Digital pots only handle signal levels, not power.  The only high power potentiometers I've seen look like this:

You won't be using one of those.

So what are you actually trying to do with this resistor?
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#### MorganS

#13
##### Apr 28, 2019, 08:54 am
This appears to be a continuation of https://forum.arduino.cc/index.php?topic=610914.0