Figuring out value of Pot to use

[MattAnonymous]

How do you figure out the size of potentiometer you should use for a circuit?  I'm looking at rigging up cruise control for my car.

[niftydog]

shudder...

[MattAnonymous]

I know it's a noob question, but everyone looks at these forums and not the noob section.

[niftydog]

not so much that, but a DIY cruise control?!? ooooo kaaaaayyyyyyy....

I look at the noob section!

Basically, you want to consider input and output impedances, voltages and currents and pick a pot that doesn't load the circuit down, but gives you appropriate control over what you want to control. Often, a good solution is to hook the wiper to a buffer, so that the current through the wiper is limited by the input impedance of the op amp.

Got a schematic? How's it going to work exactly?

[mikeb]

Might I gently suggest that if you need to ask such a question it probably wouldn't be a good idea to be messing with electronics that could lead to your car getting stuck 'cruising' at 100km/h (ok mp/h)? "Real" cruise control systems have all sorts of backups and fallbacks in case things fail or screw up - are you going to spend the years of research necessary to include these things also?

Also surely this is OT....

Mike

[niftydog]

I have enough trouble setting the pot on my delay pedal to the exact setting that I want... imagine trying to set your cars speed with a carbon track pot! Wowsers!

Can't you buy aftermarket cruise control systems?

[guitarhacknoise]

how about a 2x4?

once my little honda warms up, it want's to stall at red lights or when ever you step on the clutch, so i hafta lean over and press the accelerator with my right hand to keep it idling.
(this just popped in my head)
I guess I should get it fixed.

WAIT! you don't happen to have an electric car do you?
j/k
-matthias

[mikeb]

I don't see what's so hard about holding your foot in place anyway. My calves are very shapely from workouts on the clutch and accelerator in my Kombi.  

Mike

[R.G.]

It depends on the circuit you have it in and how it's used.

Critical Factors:
(1) is it used as a voltage divider, current divider, or variable resistor?
Voltage divider (volume control style) gives you a fraction of the available voltage. This only remains true if the circuit following the pot eats little enough current. Current divider use (that is, signal fed into the wiper, output from the "hot" lug) steers the available current either to ground or to the load. In this case, the driving circuit needs a high impedance, and the following circuit a quite low one. As a variable resistor, you need to just calculate the min and max values the resistor can have, and usually put in a fixed resistor for the min.

(2) how much current can the source driving the pot produce?
For voltage dividers, the pot needs to be high  enough resistance to not load down the source. Example: 12AX7 tubes can't really drive much less than 100K on their plates. Anything much less than 500K to 1M as a volume pot would seriously alter the loading on the plate.  And the loading on the wiper needs to be greater than 500K to 1M to avoid loading down the pot wiper, which is driven from an impedance as high as 250K for a 1M pot.

For current dividers, the answers are simpler. The pot impedance needs to be at least ten times the loading impedance of the driven load, so that the pot, when turned full up, will not overwhelm the current going into the driven load.

For variable resistors, you just have to not overheat the pot or burn out the wiper contact, so read the datasheet.

(3) how much current can the load after the pot eat?
Voltage dividers just need to not be overloaded. For current dividers, the source sets the max current, so this doesn't matter as much

(4) what is the pot dissipation?
Pick a pot you won't burn up.

(5) what frequencies are involved?
At high frequencies the parasitic capacitances around the pot can seriously limit the performance. Note that with high value pots, like 1M, the capacitances around the pot of a few pF can limit high frequency response.

Sorry - that's just a guide to the design. Each of those is a few pages of typing to cover.

[Mike Burgundy]

Now that's a real answer.
I Just HAVE to say I think cruise control of any sort should be banned, but that's just my beef...

[niftydog]

Interesting, I've never heard any such opinions on cruise control.. .what's your beef?

[bazzwazzle]

his two cents

[MattAnonymous]

daminit, this electronics stuff gets confusing, that's why I'm a mech eng. major.  If it doesn't work, you can walk over, look at it, touch it, and fix it.  Try that next time you fry an IC.  If you succeed please post pics.  As far as the need for cruise goes, try about a 4 hr.+ drive at a constant speed.  When your foot burns so bad from being in that position that you contemplate its ability to burn through the rubber pad on the accelerator, you'll know what I mean.  Why do you use your hand for the accelerator?  press the break and accel. with the same foot if you have to.  it'll look like this / .  or use your hand for the e-brake. or, even a little more productive, fix your god forsaken car.  Thanks RG, for just putting aside my total lack of knowledge and motivation and aswering my question.  That's what questions are for, aren't they?

Moral, fix your car, burn your foot, answer my question, it's all the same to me.

[sir_modulus]

let's just hope noone pulls the, "Look! I've found the volume control, now lets crank up the stereo" thing  :mrgreen:

[guitarhacknoise]

Moral, fix your car, burn your foot, answer my question, it's all the same to me.

       
yeah, the balancing act......
I figure an 83 honda accord just aint worth a $350 rebuilt 3bbl. carb, (that's with out labor) ever see the vacuum rig on one of these?

[Sic]

. . .

[bwanasonic]

Hmm, I'd be pretty suprised if such a DIY *cruise-control* would be legal in most states in the US. Don't tell your insurance agent for sure.

Kerry M

[Mike Burgundy]

I don't know if this works in teh States, but over here you can just contact the guys that give out road-worthiness-crertificates (ahem), and simply ask them what the rules are.
That might easily sort out the feasability of this project.
Nifty:
I have this thing against drivers getting lazy. I can imagine it's different  when you have to drive hundreds and hundreds of miles on a relatively quiet, straigh highway, but we don't get those here. I know of/experienced some head-to-tail collisions on the highway - at speed -  and all except one were CC-related.
People tend to forget the car will go, and keep going UNLESS they do something about it. False sense of security, apparently. Concentrating ever so slightly on keeping your speed relatively constant yourself keeps you more aware of your driving, in my opinion. That's all.

[niftydog]

yeah, I can understand that. I guess you're especially against it due to your personal experience! But I'd hate to see responsible drivers get punished for the actions of a few Darwin award contenders.

(reminds me of the guy who sued the makers of his motorhome because they didn't make it clear that the cruise control system only controled the speed. Thinking it was like auto pilot, he set it once he got on the highway and went in back to make a cup of coffee...)

From my place, the capital city of my neighbouring state (north) is 3.5 hours drive away. Going the other way (south) it's 12 hours drive. Going east, it's like a whole days drive!

That's not even the half of it. From my girlfriends place to my work is about 25-30 kilometres. Mostly what we call "parkway" which is wide, freeway type road with a speed limit of 90km/h. Hence, I use my cruise control every morning on the way to and from work!!

Different story in a big city though!