Verify some general assumptions about a potentiometer?

That is a tricky one, I can guess, but your guess is probably as good as mine.  What does the pot control?  Is there any way you can take it out and add resistor values in to determine the limits of a desired device behavior?  

I agree that the 5K probably doesn't belong, but that must be pretty corroded to get 5k of resistance between there.  There is always that chance that they did some creative engineering to save a penny somewhere and added some resistance in there.  

you can always post a pic of it...

"Am I safe in assuming that this extra 5K of resistance is a result of corrosion between the center tap/strip and the underlying strip?"
NO it is not safe.

Especially if there is no unexpected resistance change as the center tap is moved. Corrosion raerly provides a CONSTANT extra resistance.
Contact cleaner was a good suggestion.

I am still unsure as to the physical and electrical aspects of your circuit.

That is a good point, corrosion would not cause a constant resistance.  Is there any way you can solder test leads on to help reduce the corrosion effect of the contact?  

You are not safe to assume that it is 750 ohms.  That is not a standard value.

From your description, you are measuring the surrounding circuit, not the pot itself.

Depending on the position of the pot, it sounds like a 5k, since that is what you are measuring to the center.

You should remove the pot from the board and measure it.  I would not be surprised if it was either 5k or 10k ohms.  These are more common values.


Good Luck!

You can see an image of the board here:

http://trollingmotorparts.com/index.cfm/act/Catalog.cfm/CFID/30473028/CFTOKEN/99112311/catalogid/8598/Subcategory/Pedals%20and%20Steering/category/Minnkota/browse/null/MenuGroup/Home/desc/Minnkota%20PC%20Board%20for%20MXT.htm

That should also answer some questions about what I'm doing...;-)

I went ahead and tried using the 750 Ohm 6-position stepping switch that I built in the circuit, but the motor speed tops out at the fourth position, rather than the sixth.  Wondering what this means...

>Corrosion rarely provides a CONSTANT extra resistance

True - 5k is a *very* approximate value - sometime the readings were a lot greater, but I couldn't get it much below 5k when measuring across the center tap.

I still don't see how it could possibly be a value greater than 750 Ohms, given that a reading across the end taps never reads more than 750 Ohms...  Am I missing something?

Cheers,
-Jon

If you can't tell from the image, the circuit is connected to all three leads of the alleged potentiometer...

** Edited and added ** Oh - and I have isolated it from every other connection except ground (which is obviously common across all the other switches in the circuit - but that's all they are - just switches - no silicon in there...;-)

Cheers,
-Jon

It doesn't make sense that you would read 750 ohms from end to end and 5k to the center.

Can you measure from the center to one end, and make the value change?

>Can you measure from the center to one end, and make the value change?

Yes - the value from the center to one end can be made to vary approximately 750 Ohms, but the total is always about 5K or above - due to the wide range of values I get from the almost certainly extremely dirty center tap, it's hard to verify the range - it might be as little as 500 Ohm, but once again the wide range of values I get makes it hard to tell.

Any ideas what it means with respect to the value of potentiometer I'm using (750 Ohms) when the speed control tops out at around 2/3 full?

Thanks,
-Jon

It could be that the end to end value is too low, and causing a reduction in the current available to the center tap.

Captain,


"Any ideas what it means with respect to the value of potentiometer I'm using (750 Ohms) when the speed control tops out at around 2/3 full?"

Two things come to my mind:

1. A faulty pot center tap with that extra resistance resulting in only being able to tap-off 2/3 of the max.
2. A faulty pot end-to-end resistance is presenting an improper load to the circuit causing the circuit (and motor) to operate at less than its optimum.


"I went ahead and tried using the 750 Ohm 6-position stepping switch that I built in the circuit, but the motor speed tops out at the fourth position, rather than the sixth.  Wondering what this means..."

I suspect the 750 Ohm is less than the circuit wants to "see," and you might be loading it down.


I'd start experimenting by disconnecting the current pot, and then...

1.  If you have a mix of resistors in the 200 to 2K range connect them in series for a range of end-to-end resistance of 1K to 5k with various spots in-between where you can tap-off.

2.  Buy a 1k pot, a 5k pot.  They are a few bucks from Radio Shack, and a 5k resistor.
Try each pot as a replacement. Also try the 5k pot with the 5k resistor across it two ends - making a 2.5k pot.

Whatever pot (or resistance) you settle on, you should be sure the wattage rating is not less that what you replaced.  You can estimate the wattage by measuring the current through the pot and the voltage drop (including through the tap).  Watts = current times voltage.  Then get a pot that has a larger wattage rating.  The others here might have a suggestion about this.  One test would be let the whole thing run with the motor at full and then much minimal speed and see it the pot gets very warm.  Then you would need a higher wattage

If all else fails, you can contact Minn Kota at the number below for technical help and ask the correct size of the pot to replace.  As a last resort, you can pay them $60 or so for a replacement board.

"How can I get technical information about my Minn Kota product?  We are happy to answer any technical questions you might have.   In the U.S. and Canada please call 800-227-6433."
http://www.minnkotamotors.com/support/customer_service.asp

good luck

>It could be that the end to end value is too low, and causing a reduction in the
>current available to the center tap.

Yes, but can you explain how it could be a larger pot when the readings end-to-end are no higher than 750 Ohm?

Thanks,
-Jon

1.  >It could be that the end to end value is too low, and causing a reduction in the
>current available to the center tap.

uh! lower end-to-end would cause more current.

Current might not be the issue anyway.  The center tap might provide a bias voltage that sets the adjacent circuit at the appropriate operating point of its performance.


2.  "Yes, but can you explain how it could be a larger pot when the readings end-to-end are no higher than 750 Ohm?"

(I assume you removed it from the circuit when you measured it.  I wasn't clear about that)

But, in answer to your question, corrosion, degradation, partial burn-out, or something got in there to partially short it.

>>"Any ideas what it means with respect to the value of potentiometer I'm using
>>(750 Ohms) when the speed control tops out at around 2/3 full?"

>Two things come to my mind:

>1. A faulty pot center tap with that extra resistance resulting in only being able to
>tap-off 2/3 of the max.
>2. A faulty pot end-to-end resistance is presenting an improper load to the circuit
>causing the circuit (and motor) to operate at less than its optimum.

Oops - I guess I should've been clearer - the 2/3 full I was talking about was the same as below (position 4 out of 6) - I was using the 750 Ohm stepping switch I built, and the speed topped at 2/3 (position 4).  I have verified multiple times that the stepping switch I built is not faulty - it ranges from zero to close to 750 Ohm (damn 5% variances in labeled values).

If you're still dubious because of the pot apparently being a weird value, I just found a schematic for a remote controlled trolling motor speed control - value for the speed control pot is 600 Ohm.

Sign up on one of those free patent viewing sites and feast your peepers on US patent #5171173.  R17 in Fig 20 is what you're looking for...

So, I'm thinking I'm not too far off - may replace those 150 Ohm resistors I'm currently using in the stepping switch with 100s or 125s

In any case, no one has been able yet to answer the million-dollar question - "How can the pot be of a rating greater than 750 Ohms when an end-to-end resistance tests out at 750 Ohms or lower".

(I added "or lower" because some of that 750 Ohms could be corrosion, however unlikely)

Cheers,
-Jon

OK, ignore the million dollar question - we were typing simultaneously, and I didn't see your answer before I posted ;-)

-Jon

BTW, the patent even describes the pot in the speed control as being some kind of annoying flat strip just like the original in my circuit...

-Jon

If that's the case, you'll need to watch your wattage rating of the resistors or pot.   See if the pot or resistors get too warm when the motor is at the slowest speed.  You don't have to worry about the fastest speed because the pot is effectely out of the circuit (bypassed).

WaterStreet  - I'll try calling Minn Kota tomorrow, but I'm guessing I'm going to get the phone equivalent of a blank stare, if my prior history of asking weird detailed technical questions to companies is any indicator.  Worth a try, though.

Looking at the schematic in the patent (and your own diagram), it appears that the center tap and the ungrounded end of the pot are directly connected - why would they bother?  Wouldn't that mean that one end of the pot is never even used unless the center tap completely fails.  Is it some sort of safeguard in case the pot gets partially damaged or broken?  Is this common practice in control circuits?  If you couldn't tell, I *am* not an electrical engineer.

Finally (regarding the $1M question), doesn't corrosion, etc typically increase the value of resistance (as others have mentioned)?  There does not appear to be any kind of damage (kinetic or heat) to the pot (but I can't rule out corrosion, since most of the switches on the board have corrosion damage).  Also, might I not be able to expect the measured 750 Ohm value measured across the ends to change when I use the slider if there was indeed damage to it that was lowering the resistance?

I hope Minn Kota support has the answer - I'm not looking forward additional multiple painful soldering jobs just to test whether it needs a lower or higher valued pot (soldering both ends of 5 various resistors, the associated center tap connections for each, and the end tap connections is not my idea of a really fun time...;-).  I guess I could save some time and guesswork simply by buying cheap fixed resistors to simulate various pots without having to build additional circuit boards...

Finally, I did check for warmth during the tests I already ran - no detectable increase in temperature - I guess I *could* break out my remote temperature sensor to be sure ;-)

If this is getting to be too much, I can increase pts...

Cheers,
-Jon

Oops - should have read your link - looks like my guess about the other end of the rheostat was a decent assumption.

I verified via my tester that the two ungrounded leads are not directly connected - my continuity alarms dies with a pathetic whine as resistance rises (I'm guessing maybe there's a capacitor in there somewhere).

Cheers,
-Jon

When I mentioned heat before, I meant hot enough or almost hot enough that you wouldn't want to keep your finger on it.

Hope things work well for you when you contact Minn Kota.  If they can't help you about the pot then maybe they can tell you the manufacture and part so that it can be looked-up - and maybe a replacement ordered.

If I could see a schematic of the circuit board and its connection through the cable in the picture then I could probably say a lot more.

By the way, all this talk assumes that the problem is with the pot.  Have you had any success with various resistors to get the motor going at a good or highest speed?  With everything running, try putting a 1k or 500k resistor in parallel with pot, an then in parallel between the tap and each of the two ends.  See if that makes the motor go faster.  If so then the problem would seem to be the pot.

I'm tempted to tell you to use a wire to make a short-circuit between the tap and either end of the pot, in order to see if that makes the motor run at full speed.  That only works if we assume the pot is a rheostat.  If not if might damage something.  Again, I would have to see a full schematic including the cable connections, in order to know for sure.

>I'm tempted to tell you to use a wire to make a short-circuit between the tap and
>either end of the pot, in order to see if that makes the motor run at full speed.

Hasn't that already been tried (since my home-grown stepping switch would short the tap to one end when turned all the way down, and short the center tap to the other end when turned all the way up - just like a pot would)?  

This pedal is for a Minn Kota 765mx trolling motor - maybe the easiest way to find the schematic might be to look for an associated patent.  Also, I could always crack the case and take a look inside - maybe it's not that complex and I could devise a schematic myself (yeah right)...

My buddy (and co-owner of the boat) thinks that the motor speed might be at maximum (but of course he's just guessing) when the switch is topping out at position 4 (which would be at a setting of 450 Ohms between center tap and ground) - he's just happy that it runs at all (and my replacement of the steering control floor switches with a correctly wired DPDT switch worked like a charm), and is urging me to wait for a test on the lake before trying to get even more speed out of the motor.

Cheers,
-Jon

"My buddy (and co-owner of the boat) thinks that the motor speed might be at maximum (but of course he's just guessing) when the switch is topping out at position 4 ... and is urging me to wait for a test on the lake before trying to get even more speed out of the motor."

Sounds good.  If you do that for a while then see if resistors get too hot to touch or almost.  Then simply replace them with a higher wattage rating.  Also, you might want try lower that 450 ohms, in order to see it the motor then goes up to top speed.  You can easily do that by putting a 500 Ohm resistor in parallel.  Then you'll know.  If it gets somewhat faster, but still not full speed, then try bypassing that 450 Ohm all together (short across it).  Then you'll know it was a rheostat set at about zero Ohms.

>If you do that for a while then see if resistors get too hot to touch or almost.

Well, we finally got the motor mounted to the bow, and my jerry-rigged controls installed on the dash (does anyone have a good solution for installing pots/switches in a surface that is far too thick for the threaded shaft on said pots/switches?  I went really slow and careful with a 7/8-inch paddle bit until I made a hole that didn't quite go all the way through our dash, and then wrestled the mounting nuts onto the switch/pot shafts using a deep-throat socket - painful), and took the boat out on the lake - the resistor board was installed too far up under the dash to really feel if they were hot, but we trolled for a good 10-20 minutes at the highest speed my 750 Ohm stepper would allow (position 4 or above) and nothing started smoking ;-)  

The top speed thrust seemed about right - noticeably faster than our old 28# transom troller, but not incredibly powerful (I think the new motor is rated around 36#).  It seems well suited to our little 14' fiberglass boat, and the mounting rig makes dealing with a trolling motor so easy - after using a bow-mount one time, I can't believe I ever put up with a transom mount.  

I bought a few more resistors to test - a 470, a 560, a 680, and 5 100s - I think those  (in various combinations) should give me enough values (including the 500 you recommended) to test on either end of my current (current as in now, not amps) range (750).

It might be a week or two before I manage to get around to it again - you know how it is - once you've got something working, it's hard to break it back down.  (I have the same problem with getting around to painting my MAME cabinet - it's fully functional, so getting around to actually painting it never seems to happen - it's been about a year since we got it up and running - you get close to it and it seems to say "Why paint me when you can play me?" ;-)

Reading all the above, I have to laugh at myself - I sound like a hillbilly - you'd never know my specialties are unix OSs and IP networking ;-)  I just enjoy working with my hands almost as much as I do setting up something like apache and and a bind/DNS server, then writing a custom script to update my local DNS zones via a dyndns client so my buddy (and his friends) can download the video of him wakeboarding that I recorded with a one-time-use video camera from CVS that I hacked to allow unlimited uses (I didn't write that hack - I downloaded it).

Cheers,
-Jon

I wrote this to myself a while back, and just found it again:

"I'm wondering if using a potentiometer larger than expected might be OK in the end - the Minn Kota design engineers should have expected the resistances of their deployed circuits to rise over time (through eventual water exposure and corrosion), and as such I would expect the motor to underperform under increased resistance, rather than overperform and burn out the motor (unless the design engineers were driven by profit [engineered failure], rather than typically sound principles of making things last as long as possible)."

Any thoughts?
-Jon

Jon,

"does anyone have a good solution for installing pots/switches in a surface that is far too thick for the threaded shaft on said pots/switches?"

When I used to build radios, I would make a mounting bracket from a small piece of sheet metal.

All you need is a piece a little wider than the shaft of the pot and maybe two inches long.  Drill three holes along the center-line:  One hole for the shaft near one end and two holes for two small machine screws (that would set apart from each other after you make the bend in the next step).  Then bend the metal at a right-angle, mount the bracket with the two screws, and mount the pot with its mounting nut.

Note:
When I was a kid, I had all kinds of things I could salvage from.  Maybe you do too.  If not then use a tin snips on the top of the next can you open, in order to get a piece about 3/4 inch wide by 2 inches.  If you want to make it more sturdy and eliminate most of the sharp edges,  you could make it a little wider along the long edges and fold them completely over about 1/8 inch using a vise and a hammer (and then a pliers to finish the fold).  You sound like a person who can improvise too.

How's the Captain and the motor?