Hi,
I'm always afraid of using small trimmers where a significant current will flow through the wiper. Will the following approach (U1 section) be fine for biasing a CCS using a pot? The extra resulting resistor in the current path is small enough to not affect the CCS, I believe. Any drawback of this approach?
Thank you!
Jose
I'm always afraid of using small trimmers where a significant current will flow through the wiper. Will the following approach (U1 section) be fine for biasing a CCS using a pot? The extra resulting resistor in the current path is small enough to not affect the CCS, I believe. Any drawback of this approach?
Thank you!
Jose
Yes, lower wiper current is better.
If the 2nd works the way you want it to, why not?
Check the dynamic impedance of the two cases against an AC on the supply, that'll give you the answer immediately, don't just guess.
I always make a point of having no trimmers in my designs.
May take a bit more effort, but worth it in my book.
Jan
If the 2nd works the way you want it to, why not?
Check the dynamic impedance of the two cases against an AC on the supply, that'll give you the answer immediately, don't just guess.
I always make a point of having no trimmers in my designs.
May take a bit more effort, but worth it in my book.
Jan
As long as you don't exceed the max power dissipation (AC + DC) of the potentiometer/trimmers, it's fine.
This is mostly stated in the datasheet of your trimmer.
I think for most THT trimpots, that's around 250-500mW.
(SMD ones are often 100mW)
In this case the max dissipation would be P=I²*R = 9mA * 300 = 24mW
So nothing to be worried about.
Lower current does cause little less drift in theory.
It totally depends on your application how significant that is, but we are talking about pretty small amounts here.
This is mostly stated in the datasheet of your trimmer.
I think for most THT trimpots, that's around 250-500mW.
(SMD ones are often 100mW)
In this case the max dissipation would be P=I²*R = 9mA * 300 = 24mW
So nothing to be worried about.
Lower current does cause little less drift in theory.
It totally depends on your application how significant that is, but we are talking about pretty small amounts here.
I understand, but isn't that situation for the left and right circuit very similar?
Anyway, in that case I would always use a limit resistor and just a smaller trimpot to sneak up to the right value.
Whatever floats your boat 🙂
I read the start post (as well as your response) more in sense of power dissipation. 🙂
I use CCS (Constant Current Source, and Constant Current Sink).
But my circuits do not have to have an exact amount of current.
So, I do not use any potentiometer.
If you can design your circuits to work with an approximate CCS current, then you can use fixed resistors.
CCS may need heat sinking, and may be used in different ambient temperatures, but the precision can go away, according to the drift versus temperature, when the ambient temperature changes (is your house temperature tightly controlled?)
If not, then design the rest of your circuitry to deal with variable currents.
My latest amplifiers do not use any CCS.
But my circuits do not have to have an exact amount of current.
So, I do not use any potentiometer.
If you can design your circuits to work with an approximate CCS current, then you can use fixed resistors.
CCS may need heat sinking, and may be used in different ambient temperatures, but the precision can go away, according to the drift versus temperature, when the ambient temperature changes (is your house temperature tightly controlled?)
If not, then design the rest of your circuitry to deal with variable currents.
My latest amplifiers do not use any CCS.
According to an old Panasonic application note I once read, the wiper of a carbon track potmeter or carbon track trimming potmeter slowly gets anodized and loses contact when there is a DC current flowing out of the wiper. Countermeasures: let the current flow into the wiper, design the circuit such that no damage is done when the wiper does lose contact, use something else than carbon.
See the attachment of https://www.diyaudio.com/community/...-makes-high-pitched-noise.371917/post-6660161
See the attachment of https://www.diyaudio.com/community/...-makes-high-pitched-noise.371917/post-6660161
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Everything you always wanted to know about potentiometers and trimmers:
https://www.bourns.com/pdfs/OnlinePotentiometerHandbook.pdf
https://www.bourns.com/docs/default-document-library/bourns_trimmer_primer.pdf?sfvrsn=ebd892f1_6
https://www.bourns.com/docs/technic...hnical-articles/trmrpmr.pdf?sfvrsn=5c163e40_0
https://www.bourns.com/pdfs/OnlinePotentiometerHandbook.pdf
https://www.bourns.com/docs/default-document-library/bourns_trimmer_primer.pdf?sfvrsn=ebd892f1_6
https://www.bourns.com/docs/technic...hnical-articles/trmrpmr.pdf?sfvrsn=5c163e40_0
This was the reason I opted for the attached design. By selecting R2 and R3 to be 'pretty close' to the expected resistance, R4 and P1 are merely for that last bit of tweaking. That way you could have R2 and R3 something like 220 ohms, and get away with R4=470 and P1=1k. Very little current will flow through the wiper, and if it fails, the circuit keeps working (albeit at a slightly lower CCS current).
Interesting that in your third link (and presumably also the second), they actually recommend a nonzero wiper current, at least 25 uA, preferably 100 uA, with no indication about the required polarity. It contradicts the Panasonic application note and everything I ever learned about potmeters, but maybe that's because Bourns uses cermet rather than carbon.
Polarity?
There are no polarizing elements in trimpots or potentiometers.
Only difference in inductance at best, maybe that's what you mean.
That is only the misuse of the word polarity.
Polarity is a property of how a device reacts (or even works at all) on the positive or negative potential difference.
The only thing that current can do, is heat up the device, which could have a (slight) effect on drifting (those PPM's).
Although, as long as that temperature is stable again, there isn't much going on anymore.
b_force, I would like to disagree. Current polarity has a big impact on chemical reactions, especially corrosion and anodization. Material sets used in the device are very frequently subject to such effects, especially in the presence of moisture. Moisture is almost always present. Once the surface is of the wiper and resistive track experience these effects, the contacts can and often do form rectifying junctions.
Current polarity affects both the onset of contact failure and creates polarity dependant resistance.
Current polarity affects both the onset of contact failure and creates polarity dependant resistance.
A bit cryptic.
For those who also wonder, it's about the anodizing effect.
Which is a little vague on itself without giving any numbers or values whatsoever.
Let's suppose the polarity on a
potentiometer is important.
In that case, you also have to pay attention to the polarity on a Rheostat.
Or, be a Genius and design the circuit so it does not require a potentiometer, and does not require a rheostat.
potentiometer is important.
In that case, you also have to pay attention to the polarity on a Rheostat.
Or, be a Genius and design the circuit so it does not require a potentiometer, and does not require a rheostat.
Lets be precise in discussion, its not polarity of the POT that's important, its the direction and magnitude of the current flowing in its wiper.
Its not a crime to allow some current flow, even in the wrong direction ( heaven forbid ), just be aware there are effects that will reduce reliability and may create hazards in some circuits. Some current flow in the wiper is always present.
Its not a crime to allow some current flow, even in the wrong direction ( heaven forbid ), just be aware there are effects that will reduce reliability and may create hazards in some circuits. Some current flow in the wiper is always present.