Substitude an Potentiometer - Automation

[Freiheitpt]

Hello,

How are you guys?

I am building something like a "central control unit".
I'm trying to interface something simple.
I have a device, which activity is controled by a normal 236 KOhm (measured) 240V potentiometer.

I want to be able to control this device with a pic.

I have thought abou an digital potentiomenter, but I cant find anything suiting my needs.

Is there any other way, or even a better way?

Thanks a lot!
Freiheitpt

 

[alec_t]

We don't have enough information. Is the pot wired as a simple variable resistor (i.e.only 2 of its 3 terminals are actually connected to something), or as a potential divider (all 3 terminals used)? Can you post a schematic of your controlled device?

 

[crutschow]

It's not likely you will find an electronic potentiometer that will work at 240V. Is that the actual voltage on the pot?

 

[Freiheitpt]

I made the schematic of the little pcb here.
This little controller, controlls the air flow of the membran air pump.

I hope i didnt made a mistake making the schematic.

Anyways, thanks a lot for your fast answer alec_t!

Scanned Schematic: **broken link removed**

 

[carbonzit]

Just so you know, your variable resistor is connected as a rheostat (2 terminals) instead of a potentiometer (= voltage divider, 3 terminals). Not that it really matters, just for your own knowledge.

 

[Freiheitpt]

Thanks for the info. I'm really only a newbie with electronics. Its a hobby for me. And I'm trying to learn some stuff here in foruns and googling around. so every info is welcome! Thanks!

 

[Reloadron]

Actually what you have there is pretty much a dimmer circuit, and where you show a diode it is likely a diac. Here nor there, but if you look at what you have and then take a look at this you will see the similarity. A Google of Dimmer Circuits will bring up a dozen similar circuits.

So what you want to do is use an AO (Analog Output) from a pic to control the voltage out of a dimmer as in 0 to some voltage DC like 0 to 5 volts gets you 0 to 240 VAC correct. Take a look at the circuiit found in this data sheet as I believe that is what you are after. The downside is it uses 0 to 10 VDC so you would need to amplify your PIC AO X2. The kit is available here. or you can choose to roll your own.

To answer your initial question I don't know of any way to go about what you want to do other than the method presented in the link(s).

A Google of DC Controlled Dimmer should yield more results.

Ron

 

[Freiheitpt]

Actually what you have there is pretty much a dimmer circuit, and where you show a diode it is likely a diac. Here nor there, but if you look at what you have and then take a look at this you will see the similarity. A Google of Dimmer Circuits will bring up a dozen similar circuits.

So what you want to do is use an AO (Analog Output) from a pic to control the voltage out of a dimmer as in 0 to some voltage DC like 0 to 5 volts gets you 0 to 240 VAC correct. Take a look at the circuiit found in this data sheet as I believe that is what you are after. The downside is it uses 0 to 10 VDC so you would need to amplify your PIC AO X2. The kit is available here. or you can choose to roll your own.

To answer your initial question I don't know of any way to go about what you want to do other than the method presented in the link(s).

A Google of DC Controlled Dimmer should yield more results.

Ron

Hi Ron,

Thanks for the infos!
I was thinking of something like that. But as I'm a little newbie in this bussiness, I didnt want to invent xD
My first thought was using a DAC.
What is the max voltage that the gate of the TRIAC gets?
If possible, could you tell me how to calculate/see this?
So next time I wouldnt have to ask =)

Thanks a again!

 

[Freiheitpt]

Actually what you have there is pretty much a dimmer circuit, and where you show a diode it is likely a diac. Here nor there, but if you look at what you have and then take a look at this you will see the similarity. A Google of Dimmer Circuits will bring up a dozen similar circuits.

So what you want to do is use an AO (Analog Output) from a pic to control the voltage out of a dimmer as in 0 to some voltage DC like 0 to 5 volts gets you 0 to 240 VAC correct. Take a look at the circuiit found in this data sheet as I believe that is what you are after. The downside is it uses 0 to 10 VDC so you would need to amplify your PIC AO X2. The kit is available here. or you can choose to roll your own.

To answer your initial question I don't know of any way to go about what you want to do other than the method presented in the link(s).

A Google of DC Controlled Dimmer should yield more results.

Ron

I was thinking something similar.
Using a DAC.

Do you know what is the max voltage that, with that schematic, will get into the gate?
If possible please tell me how to calculate this so I don't need to ask next time =)



Thanks a lot!

 

[OlPhart]

Granting there's little current involved, 1 or more CdS cells in parallel each illuminated by an LED would be a quick and cheesy answer. The micro would PWM the LEDs by a table to set the response curve.
I did this using Clairex cells ~5/8" dia. They and the LED fit into the cable boot used on distributor caps to plug wires. It was only on/off and for opto isolation. 'Mem to put black RTV behind the LED. <<<)))

 

[panic mode]

why not check PIC application notes etc. this was done before, for example:
https://www.electro-tech-online.com/custompdfs/2011/08/91094A.pdf

 

[KeepItSimpleStupid]

These are basic building blocks in industry. One example (cream of the crop) is **broken link removed**

These contain semiconductor fuses and sometimes optional current limit. They may have different output modes: phase angle fired, short cycle triac (1/2 the waveform is passed as the minimum). Eutotherm typically used back to back SCR's. They even have true power controllers.

This is another one: https://www.electro-tech-online.com/custompdfs/2011/08/SSR-P-15to70amps.pdf

or even this one: https://scrcontroller.com/product/details/7

Typically one uses 4-20 mA or 0-20 mA as a control signal. If the system has a 0-5 volt input, then adding a resistor makes it a 0-20 mA control signal.

 

[Freiheitpt]

Hello Guys, thanks a lot for all your infos and help.
I found something interesting. It is in German (no probs for me), but would the picture in the page 3 be a good solution?
https://www.unmuth.de/pdf/doku_dimmer.pdf

Foto of the inside parts of the pump:
**broken link removed**

Right side is OUTPUT
Down side is AC Power

EDIT:

I made some measurements and It seems that the Output signal, with pump at max air flow, is the whole and normal AC Signal, and when the airflow is at minimum, the AC Signal is "moved down". Only a little bit of the signal is positive and the rest is negative.

 

[panic mode]

that german file with dimmer circuit is a classic example of AC load control. if uses zero crossing detection (nulldurchgang) and in this case has three triac outputs (Leistungteil). Everything is optoisolated, nice and clean solution.

 

[Freiheitpt]

Thanks a lot panic mode.
I will try the solution in the PDF and then I'll post the news!

 

[Freiheitpt]

Hello again,

I was doing some research and I am confused with a little thing:

The optocoupler triggers the triac. Before the input of the optocoupler, there is a 220Ohm resistor 1/4w.

So... 230V * 1A (more or less) = 230Watt

Isnt it too much for the resistor?

Thanks.

**broken link removed**

 

[alec_t]

The resistor carries significant current for a very brief period only, until the triac fires. After that most of the mains voltage is across the load, only a small voltage is across the resistor, so the resistor current is negligible. The rms power in the resistor, as distinct from peak power, is within specification.

 

[Freiheitpt]

The resistor carries significant current for a very brief period only, until the triac fires. After that most of the mains voltage is across the load, only a small voltage is across the resistor, so the resistor current is negligible. The rms power in the resistor, as distinct from peak power, is within specification.

Thanks a lot for your fast answer.
As i'm trying to do as much as possible with SMDs, do you think that an 1/8 Watt would be enough? What do you recommend?
And about the value?
230V -> 230*1.414 = 325 peak.
Gate current surge peak = 1A.

So 330ohms would be better as 220ohms , right?

Again, thanks a lot for your help and fast answer!

EDIT: Or is 230V peak voltage and 230/1.414 = Vrms?

 

[Freiheitpt]

alec_t, sorry if I'm taking too much time from you, but do you have any recommendations about my last post?

Thanks a lot

(Bump)

 

[panic mode]

did you look at the http://www.unmuth.de/pdf/doku_dimmer.pdf?
it uses 220 Ohm, 1/4 W resistors