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bmw625
04-07-2008, 01:09 PM
can someone here tell me the difference in potentiometer ratings means I.E. 1k,10k as to a "fine" or " coarse" adjustment ?
TIA, brent:confused:

jim_geib
04-07-2008, 01:53 PM
can someone here tell me the difference in potentiometer ratings means I.E. 1k,10k as to a "fine" or " coarse" adjustment ?
TIA, brent:confused:

Brent,

The 1k stands for 1000 and 10k= 10,000 ohms as in restance. If you are using these on a speed control you will have to use the listed ohm's. I try to get a ten turn pot. it takes ten turns to go from 0 to 1k ohm. They have a finer speed control.

SGW
04-07-2008, 02:31 PM
If this is for a VFD, the manual should tell you what resistance value you need.

bmw625
04-07-2008, 04:04 PM
yes this is for the vfd on the mill. So for a "fine" adjustment, I have to use a multi turn pot ?
I don't do too much electrical stuff, but so far I haven't let the smoke out of anything ! :D
thanks,brent

SGW
04-07-2008, 05:28 PM
Yeah, that would be the way to get finer adjustment. Are you currently using a 1-turn pot and don't like it?

bmw625
04-07-2008, 06:04 PM
yes,it seems I just barely touch the thing and it changes more than I want.
Maybe I'm not steady enough !
brent

SGW
04-07-2008, 06:35 PM
Do you have the correct size potentiometer now? I assume you do, but if that was off I expect it could mess up the controlling action.

What frequency range do you have? I've now got my VFD set to 10Hz-60Hz and find the 1-turn pot acceptable, but when I was experimenting with 10Hz-120Hz I found the control was pretty coarse since for a given change in the pot there was 2X as much change in frequency.

bmw625
04-07-2008, 07:05 PM
that is my situation--- trying to do the 10 to 120 hz ! I can't seem to find a decent priced one online-- 10k,2 watt multi turn - newark shows one for about $15,but they charge an additional handling fee - $ 17 I don't like THAT !
brent

mayfieldtm
04-07-2008, 07:47 PM
Newark is always a little pricey.

Try Digi-Key.. http://www.digikey.com/

2 watts is overkill, most any lesser wattage will do.

I use a single turn.

Tom M.

mechanicalmagic
04-07-2008, 08:00 PM
I have a 10k single turn on my big drill press, 0-90 Hz. I can adjust to 1 Hz with no problem.

Maybe your pot is worn out?

Mine is nothing special, got it from one of the surplus dealers, probably under $2. Linear taper (not Audio taper).

b2u44
04-07-2008, 08:04 PM
I use Mouser electronics any time that I need electronic parts.

http://www.mouser.com/

I find that their price is good, but like Digikey, their selection can be overwhelming. Unlike Digikey, there is no minimum order.

As Tom said, 2W seems like overkill, but it's better to have more power overhead than not enough.

jim_geib
04-07-2008, 08:58 PM
that is my situation--- trying to do the 10 to 120 hz ! I can't seem to find a decent priced one online-- 10k,2 watt multi turn - newark shows one for about $15,but they charge an additional handling fee - $ 17 I don't like THAT !
brent
I got mine from e-bay, they were $2.00 to $3.00 each, If you don't mide using e-bay

RobbieKnobbie
04-07-2008, 09:31 PM
Linear taper (not Audio taper).

That's a really good point: maybe you have an audio taper pot. Your adjustment will be too coarse at one end and too fine at the other.

It's not the cheapest source, but automation direct has really nicely mounted pots set up for mounting in a standard 22mm pushbutton hole. I used one for the controls on my mill and I'm completely happy with it, especially compared to the "improvised" pot (standard industrial type from digi key or jameco - I forget) that I had on my previous mill.

bmw625
04-07-2008, 10:08 PM
it is linear taper, I think the problem is the wide hz band I'm trying to use.
brent

bmw625
04-07-2008, 10:28 PM
I just checked automation direct, their sold out of the 10k ones !! dang!!
brent

Ghop Shop
04-07-2008, 11:28 PM
Try a larger knob.

GHop Shop

darryl
04-07-2008, 11:32 PM
You can 'shrink' the range of the existing pot by adding a resistor in series to either or both end lugs. If you find that the range you need is near the middle of the pot, then this can work. Try a value of about 4700 ohms off each end of your existing pot. That will double the resolution of that pot, but it will mean the adjustment won't go below about one third, or above about two thirds.

If the range you need is bunched up near one end of the pot, put a 4700 ohm resistor in series with the opposite end lug. This will bring your control range closer to the center of the pot, while widening it to some extent. By the way, many pots are scratchy and this will definitely cause problems like you're having. You might try spritzing some control cleaner into it and working it back and forth a few times. Power off, of course.

rdfeil
04-07-2008, 11:54 PM
Hi BMW625,

Not to sound snide but how close are you trying to adjust the speed to, a one turn pot normally is fine for this application. Anyway, on to your wishes... Use Mouser.com as mentioned above, no minimum order and great prices. You don't need a high power pot anything will do 1/2 watt or better. Make sure it is linear taper. Mouser sells some high quality multi-turn pots, search for Bourns and/or ETI. ETI is cheaper while Bourns is higher quality (and MUCH more expensive :rolleyes: ) I would recommend a 3 turn for your application, 5 or 10 turn will give finer adjustment but you will be turning the pot forever to make changes.

Good Luck,
Robin

Paul Alciatore
04-08-2008, 03:19 AM
Pot or variable resistor specs:

Resistance which is measured in Ohms. A "K" after the number means x 1000 so 10K is 10,000 Ohms. Also a "M" after the number would indicate x 1,000,000 or times one million. So a 2M pot would be 2,000,000 Ohms.

The resistance value is determined by the circuit requirements and you should stay close to the value originally specified. Substituting a 25K for a 20K would probably be OK, but using a 1K in place of a 10K may court disaster. Changing the resistance value may or may not change the sensitivity or operation of the control, but it is not the recommended method at the user level.

Power Rating is usually specified in Watts. If substituting you should always use a unit with the same or a higher power rating. The power rating is of less concern at higher resistance values as the power dissipated is determined by the V^2 / R equation so the higher the resistance, the lower the power dissipated in the pot. But at lower resistance values it can be critical. At 100 Volts and 1000 Ohms this becomes 100 x 100 /1000 = 10 Watts. A two Watt pot would smoke in short order.

Resistance Curve is the manner in which the resistance varies when the shaft is turned. Most pots are linear which means the resistance changes by the same amount for each degree of shaft rotation. But there are others; the most common of which is the audio taper which starts at the low end with a small change per degree and increases to a much larger change at the high end. Most speed control applications would want a linear pot.

Number of Turns is the number of times the shaft has to be turned to move from one end of the resistance element to the other. The most common are called "single turn" pots which is usually an actual 3/4 of a turn or about 270 degrees. Other values are common including 2, 3, 10, 20, and 25 turns. If you want a more sensitive control, the best way of getting this is to use a pot with a greater number of turns (but the same resistance and power values). These, of course, will be more expensive.

It is also possible to put a special, multi turn knob on a single turn pot. These are available from electronic supply houses.

http://www.newark.com/16F9386/passives/product.us0?sku=BOURNS-H-22-6A

They are usually 10 turn devices and most have a "calibrated" scale. This kind of device can easily be added to an existing control (pot) for finer control and repeatable settings. These will work better with higher quality pots as low quality units may have a noisy response to small changes.

There are other specs. Tolerance range, Voltage rating, physical size and mounting, etc.

bmw625
04-08-2008, 11:05 AM
rdfeil, I ordered a bourns from mouser-3 turn should be the ticket . I guess I'm to fussy about adjustment, but it just doesn't seem right . much thanks for everybodys input.
brent

rdfeil
04-08-2008, 11:59 PM
BMW,

Glad we could help:p Please let us know the outcome and if it did what you wanted.

Robin

DR
04-09-2008, 12:54 AM
FWIW, most CNC machines use single turn pots for spindle speed control and for feedrate control. Actually, they're more like 300 degree.

This is easily fine enough for it's intended purpose. The multi-turns wouldn't be good, IMO.

darryl
04-09-2008, 01:45 AM
I'm tending to agree- if the adjustment is that touchy there's something not right- most commonly in my experience is that the pot is 'dirty'. Other than that, a multi-turn will reduce the problem, but also I have to agree that a ten turn is too much in this application.

TGTool
04-09-2008, 11:03 AM
That just led me to remember that the donor treadmill I'm cannibalizing has about a 3" knob on the pot. That's probably a cheap way to get leverage and better feel for small angles of rotation and speed change.