5k potbox for Curtis... Log or lin?

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colinmcc
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Joined: 12/06/2008
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I'm about to install a Curtis control in my Pontiac Firefly (Geo Metro in US), I want to make my own pot-box/speed control which I know should be 5Kohms, but can't find out if it should be a 'logarithmic' or 'linear' pot. I've looked at the tech manuals for various controllers, but none mention this point.

Any thoughts?

Thanks,

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Pontiac firefly Convertible, 8"Advanced DC motor/Curtis/132V.

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Johnny J
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Re: 5k potbox for Curtis... Log or lin?

colinmcc wrote:

I'm about to install a Curtis control in my Pontiac Firefly (Geo Metro in US), I want to make my own pot-box/speed control which I know should be 5Kohms, but can't find out if it should be a 'logarithmic' or 'linear' pot. I've looked at the tech manuals for various controllers, but none mention this point.

Any thoughts?

Thanks,

------------
Pontiac firefly Convertible, 8"Advanced DC motor/Curtis/132V.

You should definitively use a linear pot, the problem with normal pots are that from one end to the other you have to turn it around 280 degrees, this makes it difficult as you need gearing to use it.
Try to find a pot that only needs around 80 degrees from end to end, much easier to connect.
Might be difficult to find in your local shop though...

Best regards

Johnny
Converted Ford Fiesta -96

frodus
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Re: 5k potbox for Curtis... Log or lin?

like this?
http://www.tecknowledgey.com/catalog/product_info.php?cPath=54_55&products_id=464

"A 0 to 5K ohm potentiometer with 0 to 40 degree throw range. Same potentiometer used in the Curtis PB series throttles."

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colinmcc
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Points: 4
Re: 5k potbox for Curtis... Log or lin?

Thanks for the information and the link to the 5k pot, I have ordered one. I hadn't actually thought about the range of rotation of the pot although now I have, the range limit of 40 degs seems to make sense.

(I could get a normal 5Klin pot from this supplier; http://www.tubedepot.com/rv24a-10d2-15r1-b.html
for only a couple of $... Don't know where I'd get the gears to adapt the rotational travel though! I'm going to see if I can source a slider type pot as used in audio mixing desks, they usually have about 3 inches of linear travel.)

I was actually hoping to get a dual (ie stereo) pot and use the second pot to drive a voltmeter which I was going to calibrate and then make up a dial for to show motor rpm. (I think the advanced 8" DC motor I have revs at about 3,500 rpm flat out, commercial car tachos seem to max out at 10k, which would be prettier but a waste of dial space, and, for a tach Id have to generate the correct number of pulses per rev with a hall effect switch/opto isolator/ magnet/whatever on the flywheel or some such fudge) I could still do that with two pots mounted shaft end to shaft end, one rotating clockwise, the other anti, and the lever as part of the center link...??? If they are linear pots then that should work. I have a classic old English Myford metal lathe in my workshop, so fabricating the coupler/lever is no problem.

My other thought was to get a rotary switch wafer, these can be stacked on a shaft and each could have the appropriate resistors soldered in. With a 10 way wafer for example I should have 10 discrete steps ie speeds, but I'd need that full rotational sweep to make it practical. Then I could wire a second wafer to some led displays and have a digital speed gauge..

My background is building audio incl tube amps, so I understand making high resolution/precision resistor ladders. Hey! Could I make a tube based controller, I seem to recall some very very high output radio transmitting tubes?! Must broach the idea with my local 'ham'.... Thinking now,,, 144v is probably too low to get the power, needs way higher voltage, less amps to get xmitter tubes really singing.

But a tube based controller would be funky, just imagine the glow under the hood;-) (and solve my problem of how to heat the car! ;-) ;-)

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Pontiac firefly Convertible, 8"Advanced DC motor/Curtis/132V.

colinmcc
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Joined: 12/06/2008
Points: 4
Re: 5k potbox for Curtis... Log or lin?

Tried finding a 5k Slider type pot... No joy, but I can find a stereo 10k, with 60mm of travel. Put the two tracks in parallel and I'd have 5k.

http://www.maplin.co.uk/module.aspx?moduleno=2215

Looks rather flimsy though, I wonder how it would last it it was linked to an accelerator and then constantly being 'slid' back and forth..?

In terms of 'guaranteed reliability' I suspect that the 40degs, 5K Curtis pot at $25 + postage is the sensible choice,

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Pontiac firefly Convertible, 8"Advanced DC motor/Curtis/132V.

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