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Author Topic: Digital Potentiometer  (Read 2145 times)
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max
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« on: March 21, 2014, 06:13:26 18:13 »

Hi,

I have an analog circuit board uses normal linear potentiometer for parameter settings,
the circuit is using the single 12v power supply, please suggest a low cost solution to replace
the linear pot with digital one, the available 12v digital pot ics are expensive and are out of reach.

Regards
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solutions
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« Reply #1 on: March 22, 2014, 03:54:14 03:54 »

Sorry, can't help you. not enough info:

What is controlling the pot?

What power supply does the controller use...a 12V microcontroller will cost a lot more than a pot

What accuracy does it need?

What compliance range?

Temperature range?

Noise performance?

Rheostat or pot?

etc

etc

etc
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CocaCola
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« Reply #2 on: March 22, 2014, 04:52:15 04:52 »

Yeah solutions pretty much nailed it, need a lot more info...

What is the reason you want to go to digital over the existing and presumably working analog?  And how is this digital pot going to be adjusted?
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Gallymimu
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« Reply #3 on: March 22, 2014, 06:28:34 06:28 »

Solutions, I am surprised you didn't provide a stock photo of a stepper motor connected to a rheostat for him since he didn't provide any specs.

 Cheesy
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solutions
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« Reply #4 on: March 24, 2014, 02:41:06 02:41 »

Solutions, I am surprised you didn't provide a stock photo of a stepper motor connected to a rheostat for him since he didn't provide any specs.

 Cheesy

Yeah,my bad.

Sorry guys.

I should have included at least that, so here it is, better late than never:



Edit: I was actually looking for (and have actually used) one of these v when I found that ^ one

« Last Edit: March 24, 2014, 02:45:35 02:45 by solutions » Logged
Gallymimu
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« Reply #5 on: March 24, 2014, 05:31:27 05:31 »

Yeah,my bad.

Sorry guys.

I should have included at least that, so here it is, better late than never:


OH HO HO!

That is spectacular!  You never fail to come through man!  I'm busting a gut right now.
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« Reply #6 on: March 24, 2014, 08:23:39 08:23 »

Even though not enough details are provided, but once I came across the same problem. Digital Pot that tolerate higher voltages are very expensive and rare.
I solved my problem by placing three resistors in series with different values, connected to an optocoupler i could short out any combination I wanted to get the needed resistance. This solution is not perfect but it did the job for me and my circuit still working as expected for few years now.
Check the attached image.
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« Reply #7 on: March 24, 2014, 09:32:50 21:32 »

@max

your main parameter to consider is the type of digital pot required: is it volatile or Non-volatile? it works as it sounds , the non-volatile is once setted will keep its value when power is OFF and the volite will resst with the power reset. you can add this parameter to the rest of reqiurments that solution noted.

the non-voltile MICROCHIP MCP4021 5V 64 steps may cost as low as $0.8 US while the voltile is  MICROCHIP MCP40D18T 128 Steps 5V will cost as low as $0.5 US. the solution is within your design more than your components.


 
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max
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« Reply #8 on: March 24, 2014, 11:41:15 23:41 »

@LzEn

Thanks for the circuit, I was thinking about the same idea but instead of using the opto to use
the 4066 analog switches.

Regards
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« Reply #9 on: March 25, 2014, 05:05:18 05:05 »

@LzEn

Thanks for the circuit, I was thinking about the same idea but instead of using the opto to use
the 4066 analog switches.

Regards


All teasing aside you can get very good results with analog switches and a network of discrete resistors.  It's costly and takes a lot of space, but there isn't really a down side from a functionality standpoint.  I've done this before and it worked very well, no serious gotchas
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Sideshow Bob
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« Reply #10 on: March 25, 2014, 10:34:51 10:34 »

In some cases a DA converter may replace a potentiometer. Howver as said by others as long as we do not know anything about your circuit. This thread will only go around in circles. You must provide some circuit diagram in order for us to give proper answers
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« Reply #11 on: March 25, 2014, 02:15:59 14:15 »

@LzEn

Thanks for the circuit, I was thinking about the same idea but instead of using the opto to use
the 4066 analog switches.

Regards

I only used the optouplers because it was required by UL that the output should be isolated. You should be fine using analog switches.
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« Reply #12 on: March 25, 2014, 11:03:03 23:03 »

There is no circuit diagram of the card, the card is used to control an
spot welding machine, the card is using the 12v supply 7812 on the card,
cmos 4000 series and 555 ics on the card, there are four variables

300K -> three variables naming squeez, weld and hold
50K -> variable naming current

The user is only interested to replace these variables with digital control.
I think 64 steps are enough, three 4066 can be used to control 2 variables.
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« Reply #13 on: March 26, 2014, 06:04:58 06:04 »

What are the wipers attached to?
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« Reply #14 on: March 26, 2014, 06:51:30 18:51 »

All potentiometers are connected as a variable resistor, wipers are shorted to other end.

Regards
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« Reply #15 on: March 26, 2014, 09:20:12 21:20 »

All potentiometers are connected as a variable resistor, wipers are shorted to other end.

Regards
See if you can do a TAD more thorough reverse engineering job regarding the pots. You must post information that have some information for us who are not sitting beside you at your work bench. Start with making a drawing showing the posts signal chain. The wipers must be connected to something
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solutions
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« Reply #16 on: March 28, 2014, 04:02:14 04:02 »

They are rheostats, Bob

Now I have to wonder what 12V has to do with anything
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« Reply #17 on: March 28, 2014, 01:23:01 13:23 »

There is no circuit diagram of the card, the card is used to control an
spot welding machine, the card is using the 12v supply 7812 on the card,
cmos 4000 series and 555 ics on the card, there are four variables

300K -> three variables naming squeez, weld and hold
50K -> variable naming current

The user is only interested to replace these variables with digital control.
I think 64 steps are enough, three 4066 can be used to control 2 variables.

They are used to change the duration of Firing pulse(envelop frequency) for the Drivers(AFAIK of very high current rated SCRs) of the Spot welding machine. Carrier frequency by HEF4066 something in relation to PWM control by NE555. I worked on this type of machine which I reverse engineered way back in 1999.

In my machine I saw and used elctromechanical thumbwheel decade counter assembly creating my own rheostat with 4K7 1/4W 1% MFR resistors in series combination coming out through a 1N4148 diode. This type of combination was in 4 digits with different valued resistors. One more thing I remember is that is this type 4X thumbwheel(with different valued resistors) sets were used also for power meaning total three

1>>>>>>>>>>Firing Pulse
2>>>>>>>>>>Voltage range
3>>>>>>>>>>Current as per the thickness of material (in my case Lead plates in Automotive Battery cell) used

They are rheostats, Bob

Now I have to wonder what 12V has to do with anything
That 12vdc is for peak amplitude of driving pulse envelope........

I hope my info(old practical experience) helps

Later on my colleagues shifted to Micro-controller based system control box using Microchip devices like MCP41050
 

Posted on: 28 March 2014, 17:40:10 - Automerged

Hi,

I have an analog circuit board uses normal linear potentiometer for parameter settings,
the circuit is using the single 12v power supply, please suggest a low cost solution to replace
the linear pot with digital one, the available 12v digital pot ics are expensive and are out of reach.

Regards

As I already have explained use thumbwheel switches for accuracy in analog oscilator frequency control. For both Pulse Shaping as well as PWM control
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« Reply #18 on: April 15, 2014, 04:05:39 04:05 »

MAX5430 or AD5290
Hope this help  Smiley
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« Reply #19 on: June 30, 2014, 02:02:14 14:02 »

I think 64 steps are enough, three 4066 can be used to control 2 variables.
Carrier frequency by HEF4066 something in relation to PWM control by NE555.
I am sure here is some mismatch. While max said about binary-weighted "impedance" DAC, Unhappy had in mind more elegant technique like used in switched capacitor filters. I very like this technique and used it several times with success to modulate impedance (even using single MOSFET with floating (cap coupled) 'source' (around +-1.5V near ground). But knowing nothing about timings set for 555 output I can not estimate acceptability of resulting jitter and rely on Unhappy's decision as he had been solving the exact task.
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