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[Moved] Will a 5k digital pot be fine to replace a 4k7 analog pot?

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SadamHu

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Hi there, I'm wondering if I can use a 5k ohm digital pot where a 4k7 ohm analog pot used to be?

There is .3k ohms of difference of resistance, so would that mean that I would have to crop that off in my arduino code maybe using the map function perhaps?

This is where I'm wanting to buy the 5k ohm digital potentiometers from.
https://nz.element14.com/microchip/mcp4251-502e-p/ic-dgtl-pot-5k-2ch-14dip/dp/1840759

I'm wanting to digitally control a 24vdc speed controller. Unfortunately for me the speed controller uses a 4k7 ohm analog potentiometer, and I want to replace that and use a digital potentiometer and control the speed using an arduino. Buuutttt, the element14 website where I get my electronic components from looks like they only sell 5k digital pots...

Thanks very much in advance!!
 

jiripolivka

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Hi there, I'm wondering if I can use a 5k ohm digital pot where a 4k7 ohm analog pot used to be?

There is .3k ohms of difference of resistance, so would that mean that I would have to crop that off in my arduino code maybe using the map function perhaps?

This is where I'm wanting to buy the 5k ohm digital potentiometers from.
https://nz.element14.com/microchip/mcp4251-502e-p/ic-dgtl-pot-5k-2ch-14dip/dp/1840759

I'm wanting to digitally control a 24vdc speed controller. Unfortunately for me the speed controller uses a 4k7 ohm analog potentiometer, and I want to replace that and use a digital potentiometer and control the speed using an arduino. Buuutttt, the element14 website where I get my electronic components from looks like they only sell 5k digital pots...

Thanks very much in advance!!

The only difference between a digital and analog pot is the way it is adjusted. Instead of a knob, the digi pot is driven through a microcontroller by a digital code.
So I think the difference between 4k7 and 5k is not important, both will work.
 

bigdogguru

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I'm wondering if I can use a 5k ohm digital pot where a 4k7 ohm analog pot used to be?

There is .3k ohms of difference of resistance, so would that mean that I would have to crop that off in my arduino code maybe using the map function perhaps?
....
....
I'm wanting to digitally control a 24vdc speed controller. Unfortunately for me the speed controller uses a 4k7 ohm analog potentiometer, and I want to replace that and use a digital potentiometer and control the speed using an arduino.

The difference in maximum resistance of the device is not an issue, however most digital pots have a relatively limited maximum input voltage and current handling capabilities.

In the case of the Microchip family of devices for which you've posted the datasheet,

Reference: Microchip MCP413X/415X/423X/425X Datasheet

Operating Voltage Range of VDD = 1.8V to 5.5V or 2.7V to 5.5V, depending on the specific device.

Resistor Terminal Input Voltage Range (Terminals A, B and W) of VA, VW, VB = VSS to VDD

Maximum current through A, W or B of Iw = 2.5mA

Therefore, most likely you will not be able to control a 24VDC motor directly, you will need to implement additional control circuitry to directly drive the motor.

BigDog
 

Genovator

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If your 4.7kΩ pot is used as a voltage divider, and the output analog signal is feeded to arduino via an ADC, then 4.7kΩ & 5kΩ pot are same (as long as both are analog pots)..
 
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SunnySkyguy

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If you use Linear Voltage control for speed, at half speed the regulator will dissipate more than the motor due to extra drop on LDO.

In this example PWM is used with low side driver, which works, but suffers speed control with variable load since switch is off 1-D% of time. thus half bridge using MOSFETs which are both off for 0.5us or so during transition will provide low impedance source , RdsOn, while Vavg is duty cycle, D * Vin. You want RdsOn<< Rs of motor coil.
https://www.circuitstoday.com/pwm-generation-and-control-using-arduino

this will be one direction only if using half bridge modified on above instead of just low side switch. Full bridge is used for bi-dir. Speed control. Inertia of load affects current duration (acceleration time), while Imax is Vavg/Rs for coil, Rs of motor.

so rethink need for pot.
 

SadamHu

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I think you guys might have misread what I said. I'm using a speed controller, I'm wanting to replace the 4k7 pot on thats on there, for a DIGITAL 5k pot. Will it matter that the resistance is .3 ohms more than a 4k7 pot? I don't think there is a problem, but I thought I would ask you guys first...

Look in the picture, I have a motor, a car battery, a speed controller. On the speed controller there is a switch that goes from forwards, off, reverse. Then there is a 4k7 pot...

7118222200_1436671869[1].jpg
 

bigdogguru

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As I clearly stated in my previously reply, the additional 0.3kΩ is negligible as it is inline with analog pot tolerances.

However, regardless of the manner the digital pot is utilized, the maximum allowed voltage across any the digital pot's terminals A, B and W must not exceed VDD of the device, presumably 5V if controlled by an Arduino, or likewise the maximum current through any the terminals A, B and W must not exceed 2.5mA.

Therefore, unless you have a datasheet for the motor controller which clearly provides these specifications for the potentiometer connections or a full schematic for the motor controller, you will need to derive these maximum values empirically.

Do you have these required specifications available or the schematics of the motor controller?

I suspect the analog pot forms one of the legs of a resistive voltage divider, which maybe why the recommended value for the analog pot is 4.7kΩ, to match an internal fixed resistor forming the other leg of the resistive voltage divider and provide the required voltage ratio range.

In any event you can determine these maximum values empirically with a multimeter, using the current configuration:

1. Using the multimeter measure the maximum voltage applied across the analog pot while slowly adjusting the pot from its minimum position to its maximum position.

2. Likewise measure the maximum current flowing through the analog pot by breaking one of the connection to the analog pot and inserting the multimeter in current mode, then like above, slowly adjusting the pot from its minimum position to its maximum position.

If either the maximum voltage or maximum current observed exceeds the 5V or 2.5mA respectfully, you will not be able to directly replace the analog pot with the digital pot, additional changes to the circuit will be required.



BigDog
 

kripacharya

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The short & direct answer to your specific question is : NO YOU CANNOT replace the 4.7K pot directly with a 5K digital.

The reasons for this is, however, different from what you think ( 5K - 4.7K), and has been explained in detail by BigDogguru and others.
 

KlausST

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Hi,

Just a comment ot the total resistance of pots:

Most 4k7 pot usually have a total resitance tolerance of +/-10% or more.
Therfore their resistance may vary form 4.23k up to 5.17. Here the 5k is included.

Klaus
 

Audioguru

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Without seeing a schematic of the motor speed controller, we do not know how much voltage is fed to the pot. The pot in the photo has only 2 wires (not 3) so it is a rheostat, not a voltage divider.
 

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