Potentiometer sizing question - Page 2
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  1. #21
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    Quote Originally Posted by JST View Post
    Just about any of them will have a buffer on the input, but some MAY not. Those may have a voltage divider setup to get the 0-10V reduced to what the A/D can use. So the input of the divider is what your pot will "drive".

    You should figure that the input resistance MAY be in the 25k ohm range. What that does is to drag down the output of the pot.

    At half rotation, the output impedance of the pot is 1/4 the total resistance. So, with a 1k pot, it is 250 ohms. That is approximately 1% of the 25k considered as a divider. So your worst error from that source is about 1%. Most likely that is what the designer figured on.

    If you use a 10k ohm pot, the source resistance is 2500 ohms. That is 10%, so the voltage at half rotation will be lower. The control will be non-linear, and will, as the pot resistance goes up, start to look more like a log taper pot.

    Maybe that is of no consequence to you, and if so, a 10K pot is just as good. But maybe it IS important, and if so, you need to take the input resistance into account. The spec for that should be in the manual.

    For Rons:

    1) It sill be an A/D input, definitely analog, so digital will not be an issue.

    2) Chip level (internal chip design) is not the issue here, all you need are the specs of the device. If you mean design at the level of assembling ICs and parts to make a functional circuit, well, yes, that is what affects the specs of the device. But you still only need the device specs.
    Analog of course, I was thinking along the lines that the input pin on the processor is a digital/analog input assigned by micro-controller pin programming during initialization.

  2. #22
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    Lets cut to the chase. I have 6 different brands of VFDs in service. Most spec different resistance values, and some spec maximum current loading. In nearly every case I have used the specified value. I have had ZERO problem with any install. For most 1K is to low and for others 10K may be high. When you need an industrial STYLE potentiometer it is often difficult to buy the exact one (at a reasonable price) so I use what is available. All of my machines are wired using NEMA (AB) or IEC (22mm) control devices with Shielded cable (Belden 83604 or 83606). I have ZERO problems. Do what you will with your own stuff, mine just works.

  3. #23
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    Quote Originally Posted by JST View Post
    Yes.

    But do not count on the high impedance, depending on what you call "high". Many are in the 25k ohm region, and then will significantly load a 10k control, while retaining very good linearity with a 1k control.
    OP COULD RTFM if he wished? Page 8 shows the analog input Z is 10k, not 5k, not 25k. It shows it then goes to a cheap 10bit ADC.

    So the only real issues touched on are 1) 10k pot will be a bit unlinear in center 1/2 section (for manual pot who cares?), and 2) it WILL be 10 times more noise susceptible since power thru it will be down by 10x. Sure shielded wires help but why do that to yourself when you can buy a 50 cent 1k pot instead?

    http://www.hitachi-america.us/suppor...NT341BX%20.pdf

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    Quote Originally Posted by MattiJ View Post
    1k pot burning out sounds very strange to me. Are you absolutely sure that the connection is correct?
    0.1 watts is rather tiny amount of power even for smaller resistors and pot track has typically plenty of real estate to dissipate the power.
    If you accidentally connect the 10v supply between the pot wiper and one of the ends it's destined to fry..
    The ONLY way he burned out his .5 or 1 watt pot, if indeed he burned it out (unresolved/questionable with the NES1 specs).

    The NES1 has 20ma available so only capable of outputting only .2watt; IIRC, short the 10v and MAYBE get 30ma; so still less than any pot rating likely used.

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    Quote Originally Posted by mike_kilroy View Post
    when you can buy a 50 cent 1k pot instead?
    Please, where are you getting a reliable pot, delivered, for $0.50? I'm definitely interested in knowing of these sources because the commons ones are $5 for a single + S&H.
    Quote Originally Posted by mike_kilroy View Post
    The ONLY way he burned out his .5 or 1 watt pot, if indeed he burned it out (unresolved/questionable with the NES1 specs).
    As I said previously, super cheapo pot, likely listed power is .2 watts but the vendor cannot verify power rating. Pot was verified functional prior to install, now open-circuit on the slider but 1k from end-to-end. I don't know that I truly burned the pot, but it's clearly NOT a reliable pot.

    The reason I'm asking about higher values is that I have Allen Bradley Type J (some of the best pots made in the US) on the shelf in vales other than spec'd in the manual.

    As a matter of fact, I hooked one up with jumpers last night just to see and it worked ok. I think I need to adjust a setting that somehow got goofed up on the VFD though. I have my min freq set to 40 hz, and at one time I had the pot controls set so that minimum value on the pot was 40 hz. For some reason that function isn't working (it didn't work with the 1k in there before it gave up the ghost either). So I need to check that parameter and try again. As it stands, with a 12k AB Type J pot in there I get nothing for the first ~60% of pot turn, then it jumps to 40 hz and goes up to the max of 70 hz from there. This indicates something happened to my A011/A013 parameter settings; I cannot recall if I changed those or not. I may have done so when testing out a larger 3-phase motor that I don't have the proper VFD for (just as a test). Oh, and just for grins, I put a 25k AB Type J in and it also had the same problem, but functioned fine from 60-100% pot value.

    Lastly, I'm using shielded, high-EMI comms cable for the communications lines so I don't expect too many issues with increasing interference problems by using an other-than-spec'd pot.

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    Then noise is not an issue with you, and you show by your reply the slight unlinearity is not an issue with you. So you should be good to go.

    You can tweak the A parameters to have pot start at the MIN rather than 0rpm to eliminate the deadband at the bottom... I dont have time right this second or I would show you the parameter nos and how I would set them... Sounds like you have a decent handle on that tweaking.

    PM is having a major issue on replying not working today...

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    Not a burned-out pot, more likely just a "bad" one, if the slider is open but ends show resistance OK.

    Pots of various qualities available from Digi-key and Mouser electronics.

    A 10V source and 1k pot will be 10 mA. Not going to burn out any reasonable pot.

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    Quote Originally Posted by mike_kilroy View Post
    Then noise is not an issue with you, and you show by your reply the slight unlinearity is not an issue with you. So you should be good to go.

    You can tweak the A parameters to have pot start at the MIN rather than 0rpm to eliminate the deadband at the bottom... I dont have time right this second or I would show you the parameter nos and how I would set them... Sounds like you have a decent handle on that tweaking.

    PM is having a major issue on replying not working today...
    Thanks!!! I was having the same problems with the replies. I do have a good handle on the parameter numbers (I went back and refreshed on them from the manual), just haven't had a chance to verify that they are correct.

    Quote Originally Posted by JST View Post
    Not a burned-out pot, more likely just a "bad" one, if the slider is open but ends show resistance OK.

    Pots of various qualities available from Digi-key and Mouser electronics.

    A 10V source and 1k pot will be 10 mA. Not going to burn out any reasonable pot.
    The "cheap" pots from Digi-Key, Mouser, Jameco, etc are all the same cheapo pots, that's what I have. I have the 16mm format, .2 watt, not the 22 mm format .5 watt. I had assumed that because the pot functioned before installation, then when installed demonstrated some functionality that quickly faded to symptoms of a burned or dying pot, the pot wasn't truly a .2 watt. Thus my desire to use a higher quality pot. I do have a couple more of the cheapo pots but if the 1st one died that quick I wasn't sure that I would benefit by trying another one. Maybe it was just a bad pot. Like I said, I've done simple circuits etc but comprehensive circuitry knowledge is outside of my expertise, I know just enough to fumble my way through it. I'll try another cheapo 1k pot, if it dies too then I'll know it's either a bad batch of pots, or they aren't truly to power spec.

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    Quote Originally Posted by CountryBoy19 View Post
    Please, where are you getting a reliable pot, delivered, for $0.50? I'm definitely interested in knowing of these sources because the commons ones are $5 for a single + S&H.
    You can likely find them for LESS than 50 cents if you search long and hard... surplus places, etc. But for kicks I spent 5 minutes searching and found:

    20 cents: 1K 2K 5K 1K OHM Linear Taper Rotary Potentiometer Pot 3 Sets/(12pcs) Metal | eBay


    99 cents: 1K OHM Linear Taper Rotary Potentiometer B1K Pot | eBay

    72 cents: http://www.mouser.com/ProductDetail/...KY3u9HDBCqs%3d

    46 cents: http://www.mouser.com/ProductDetail/...FQlgGkswLsw%3d

    50 cents: PDB12-G2251-12BF Bourns Inc. | Potentiometers, Variable Resistors | DigiKey

    76 cents: https://www.digikey.com/product-deta...309-ND/2408886

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    Quote Originally Posted by mike_kilroy View Post
    Was about to complain that those are probably less-than-spectacularly reliable carbon track pots but quite many were marked as a conductive plastic
    Last edited by MattiJ; 09-21-2017 at 05:59 AM.

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    I would go with the manufacturer’s recommendation. The pot has to be high enough wattage to take the maximum current to the analog input via the wiper. And it has to be low enough resistance to not be affected too much by the analog inputs resistance (so that it stays linear) But, it has to be high enough resistance so that it doesn’t overload the supply voltage (current rating). I suspect that the overrated pot wattage rating recommended by the manufacturers is for durability, not heat.

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    Quote Originally Posted by mike_kilroy View Post
    You can likely find them for LESS than 50 cents if you search long and hard... surplus places, etc. But for kicks I spent 5 minutes searching and found:
    You haven't found anything I haven't already found. Outside of the ebay links, all of those places charge shipping that is relatively unreasonable compared to the cost of the pot... Which makes the pot a $12 pot for a $0.50 part (which was my point). If I'm going to pay $12 shipping, I'm going to buy a $5 pot that I KNOW will last vs a dubious Chinese pot that may or may not die on the first use.

    Assuming that all these Chinese sellers using the same stock photo/specs are selling the exact same pots, this is exactly what I have that didn't last.


    Quote Originally Posted by mike_kilroy View Post
    .08 watt isn't big enough... 1k is putting at least .1 watt through the pot...

    Another .08 watt pot

    [/quote]
    Once again, same cheap pot as listed on ebay with the .2 watt power rating. Tried and failed. I do have some more of these so I'll try the others to see if they fail as well.

    My point was, you implied that this was a non-issue because I could get reliable pots for $0.50 each and that simply isn't the case. The ONLY pots I can get that cheap are the ones I already have (not reliable), or require bulk purchases to get the price-point per pot down close to that price.

    That being said, I have Allen Bradley Made in USA pots that will work, even if they distort it a little bit. For grins I put a 1k 10-turn heli-pot (I have 10+ of them, old-school Made In USA) on it and didn't notice much difference in linearity between that and the 12k A-B pot.

  14. #33
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    You only know that ONE pot failed, in a way obviously not related to power in the pot (by your own statements).

    That is not equal to "these pots are not reliable".

    I have used the Digikey 16mm controls in various applications and have not had them fail. I've tossed plenty of bad US-made Allen-Bradley over the years.

    Since you seem to have more of them, if I were you, I would solder in another one and use the thing. Be more careful soldering this time.


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