# Pot boxes



## frodus (Apr 12, 2008)

it doesn't need to be that big, the controler is only sensing the voltage on the wiper, its not really a load.

but I wondered the same thing, why not fab my own. A guy I know is using a throttle body off another car, and the throttle position sensor is a 5k pot. sure, its a little on the big side, but you have the stock throttle linkage, spring and curve that a real throttle does.

I was just going to take the throttle pully off my carbs and mount to a 5k pot and put a large spring on it. I dont want to spend 50 bucks on something from Curtis that I can build myself.


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## TheSGC (Nov 15, 2007)

There happens to be a major difference between the Curtis pot and a normal 5K pot. The Curtis is calibrated to work with the normal throttle cable with a 90 degree turn radius, where as a normal 5K pot uses a 300+ degree turn radius. I too thought about building my own, bought a 5K pot from the local Radio Shack, sat down and started to design it and realized that the amount of work needed to gear it so the pot would even work with a normal throttle cable was not worth it and the $66 Curtis PB5 was a very acceptable price. It's also one of the parts of the conversion thats better left to the tried and proven equipment.


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## frodus (Apr 12, 2008)

"calibrated" hahahaha... you're giving them too much credit. They just buy a pot that has ~90 degree throw, put it in a box, throw a spring on it and put an arm on the pot. They're not creating magic.

I wasn't talking about a radiocrap pot either, I was talking about getting an actual pot from digikey or mouser to fit the job. Hell, you could even use a linear pot, lots of people do.

If you use a pulley on a 300+ degree pot, depending on the throw of the throttle, you calculate circumference and make/get a pulley that rotates 300+ degrees with an xx inch throw of the throttle cable. Its not rocket science, but 50-60 bucks seems a bit much for something so simple, that I'll just do it myself, or buy a throttle body with a pot in it, cut it down, and reuse.


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## KiwiEV (Jul 26, 2007)

frodus said:


> "calibrated" hahahaha... you're giving them too much credit. They just buy a pot that has ~90 degree throw, put it in a box, throw a spring on it and put an arm on the pot. They're not creating magic.
> 
> I wasn't talking about a radiocrap pot either, I was talking about getting an actual pot from digikey or mouser to fit the job. Hell, you could even use a linear pot, lots of people do.
> 
> If you use a pulley on a 300+ degree pot, depending on the throw of the throttle, you calculate circumference and make/get a pulley that rotates 300+ degrees with an xx inch throw of the throttle cable. Its not rocket science, but 50-60 bucks seems a bit much for something so simple, that I'll just do it myself, or buy a throttle body with a pot in it, cut it down, and reuse.


Hey if you do figure a way to do it cheaply and reliably, chuck the instructions up here with pictures - you'll save the EV community thousands!


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## frodus (Apr 12, 2008)

KiwiEV said:


> Hey if you do figure a way to do it cheaply and reliably, chuck the instructions up here with pictures - you'll save the EV community thousands!


I want to keep the kill sw from my motorcycle, so I'll be building one soon. I'm going to use a magura for now, but sure thing.

I mean, sure, some people are loaded, and don't have time to do it themselves, but some people like me, want to do it themselves.

I've got a motor, controller, batteries, charger, motorcycle, cable, throttle and I've barely spent over $800 and its all new or surplus new except for the motorcycle. Gotta save money where we can. Hell, we might just offer up a kit or something (I work with a company called Synkromotive).


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## BHall (Aug 1, 2007)

Ok, why not get a 1 turn/rotation 20k pot and only turn it 90 degrees. (360deg/90deg = 4, 20k/4 = 5k) A stopping mechanism would also be very easy to create to keep it from going past 5k. I guess that would be too easy also.

Brian


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## frodus (Apr 12, 2008)

BHall said:


> Ok, why not get a 1 turn/rotation 20k pot and only turn it 90 degrees. (360deg/90deg = 4, 20k/4 = 5k) A stopping mechanism would also be very easy to create to keep it from going past 5k. I guess that would be too easy also.
> 
> Brian


Well, you can't do that, because its not measuring the 0-5k resistance, its measuring the voltage on the wiper beween 0 and whatever voltage.

If its 5 V, and you have a 5k pot, it goes from 0-5V. 

If you do what you say, and use a 20K pot, it SHOULD work ok, but you'd only get 0-1.25V (25% throw). 

Long and short of it, get the right part for the job...


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## DVR (Apr 10, 2008)

BHall said:


> Ok, why not get a 1 turn/rotation 20k pot and only turn it 90 degrees. (360deg/90deg = 4, 20k/4 = 5k)


Actually I've had a bit of experiance with this. If you check my web page Iv'e built a few PC steering wheels for racing simulators and the pedals are basically the same thing used in an EV but PC controls use 50-100Kpots.
The problem is about resolution and is 2 fold.

1) If you limit the travel of the pot and only use to portion of resistance that you need, you dont get fine enough resolution and the pot becomes very inacurate. Its like getting too close to a big TV, the pictures still there but its grainy.

2) Not all pots are created equal. Getting your hands on a Quality pot that has a high cycle life can be difficult. Remember that in a car your foot rarely stays still, your constantly varying the position of the pot and it clocks up its cycles quickly. With PC controllers I've found that after a few months of regular use the resolution becomes very sloppy with spikes and troughs all over the place. I regularly buy out the complete stock of 100K linear pots at AZtronics and treat them as a consumable. OK in a game. Not a good idea in a real car!!

Personally I wouldn't hesitate to build a pedal assy for a PC but I wouldn't do it for an EV. I'm a Fitter/Machinist by trade and at the $100 that a potbox costs, I wont bother trying replicate it .


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## DVR (Apr 10, 2008)

frodus said:


> "calibrated" hahahaha... you're giving them too much credit......................... Hell, you could even use a linear pot, lots of people do......



Hell, you _*HAVE*_ to use a linear pot!!!

If you dont know the difference between a Linear pot and a logrithmic pot you best leave this alone.

Yes the pots ARE calibrated, if they wernt, no 2 pots would be the same. You don't give them enough credit.


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## BHall (Aug 1, 2007)

DVR, I think frodus may be thinking of a slide pot. And yes you would definatily want to use a linear rotational pot over a logarithmic.

I can understand the resolution part of using 1/4 of a 20k pot but the voltage being measured instead of the resistance I am not buying. I will be using a Zilla 1K LV controller and all the wiring diagrams and schematics for the Zilla's only show two connections to the throttle pot. As a matter of fact here is a direct quote from the Zilla manual "*The standard unit uses a regular 5K ohm, two wire potentiometer of the type used by Curtis and other controllers. The potentiometer is not supplied with the controller package. On the input to the Hairball, below 150 ohms is off, and over 4.8K ohms is full on. Resistance over 7K ohms will cause a fault condition. (DTC: 1214)*" Looks to me as long as there is a variable 0(150) to 4.8k between the TWO inputs it should work fine. 

Brian


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## TheSGC (Nov 15, 2007)

A high quality 5K 90 degree pot goes for $25 and then the misc metal needed to build the casing is about another $20, so you are already up to $45. 

I am a computer/electrical engineer and I am building my own controller and will probably tackle the battery charger, but I just won't touch the throttle pot. It's just one of those things that can cause serious trouble if is fails or gets jammed. It's more of a mechanical device than electrical anyways.


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## DVR (Apr 10, 2008)

A pot (potentiometer) is a variable resistor. It applies a resistance to the voltage that is being put through it. So putting 5V through a pot you will get a 5 volts at one end of the travel and percentage of 5volts at the other. The amount of resistance is measured in Ohms.

In a linear pot the transition is smooth and linear.

In a logrithmic pot the transition is epodential, not linear. These pots are normally used in audio equipment.

Both radial pots and slider pots are available in linear and logrithmic versions.


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## frodus (Apr 12, 2008)

DVR said:


> Hell, you _*HAVE*_ to use a linear pot!!!
> 
> If you dont know the difference between a Linear pot and a logrithmic pot you best leave this alone.
> 
> Yes the pots ARE calibrated, if they wernt, no 2 pots would be the same. You don't give them enough credit.


linear as in linear slide, not rotational

sure are quick to jump, I'm well aware of the difference, you misunderstood me... as far as linear scale goes... never seen one that isn't (not saying they don't exist, just haven't seen one)

And they don't calibrate them, they either work or they don't, they likely test them for functionality, and install... they buy in bulk, same as components manufacturers do. there is NOTHING SPECIAL about a 90 degree rotation potentiometer other than the resistive print on the slider board only goes 90 degrees, and the shaft has a stop to limit rotation. There's one in my magura, and its a standard easily ordered part. Just kinda small.

I don't give them enough credit, because its just a pot with an arm on it. Period. No one can argue that it isn't.

and why don't you all stop shooting it down, the guy asked if it was possible, i said yes, you are trying to just get him to buy a curtis throttle pot... why can't he just try it himself? He needs to follow good design procedures, get the right pot and make it mechanically sound. Other than that, he'll be fine.


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## frodus (Apr 12, 2008)

TheSGC said:


> A high quality 5K 90 degree pot goes for $25 and then the misc metal needed to build the casing is about another $20, so you are already up to $45.
> 
> I am a computer/electrical engineer and I am building my own controller and will probably tackle the battery charger, but I just won't touch the throttle pot. It's just one of those things that can cause serious trouble if is fails or gets jammed. It's more of a mechanical device than electrical anyways.


I'm an electrical engineer too, helping to develop a controller... what voltage/amps are you doing? fets or IGBT's, high side or low side? series motor? 

As far as the POT still goes... we've used several throttle bodies, works just fine. It wouldn't take but an hour or so in a metal shop to fabricate a mount, and an arm that is compatible with a "high quality $25 pot"... what is the part number of that "high quality" pot? I need to start looking for one to replace my magura on my motorcycle. I want to keep the original throttle/killswitch assembly.


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## frodus (Apr 12, 2008)

BHall said:


> DVR, I think frodus may be thinking of a slide pot. And yes you would definatily want to use a linear rotational pot over a logarithmic.
> 
> I can understand the resolution part of using 1/4 of a 20k pot but the voltage being measured instead of the resistance I am not buying. I will be using a Zilla 1K LV controller and all the wiring diagrams and schematics for the Zilla's only show two connections to the throttle pot. As a matter of fact here is a direct quote from the Zilla manual "*The standard unit uses a regular 5K ohm, two wire potentiometer of the type used by Curtis and other controllers. The potentiometer is not supplied with the controller package. On the input to the Hairball, below 150 ohms is off, and over 4.8K ohms is full on. Resistance over 7K ohms will cause a fault condition. (DTC: 1214)*" Looks to me as long as there is a variable 0(150) to 4.8k between the TWO inputs it should work fine.
> 
> Brian


I wasn't aware that the Zilla was a 2 wire... but still the same story applies... you should use a 5K pot, always, if it says in the manual that it requires it.

I only know how GE, curtis and a few others work. They reference pack ground, have a wiper that goes into the controller, and the other side of the pot gets a ~5V signal, sometimes more or less, depending on controller. The wiper arm in turn shows a voltage drop across it due to the voltage divider. It varies from +V to 0.


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## DVR (Apr 10, 2008)

frodus said:


> linear as in linear slide, not rotational
> 
> sure are quick to jump, no DUH the pot is linear scale... never seen one that isn't (not saying they don't exist, just haven't seen one)


Every time you look at a stereo, graphic eq or a volume knob, your looking at one that isnt a linear pot. truth be know Most of the pots you see on a daily basis are logrithmic



frodus said:


> And they don't calibrate them


Not the pot box, the pot.
As part of the QA process every pot would be checked for calibration at manufacture. If you only use a portion of the pots throw the claibration becomes meaningless.




frodus said:


> and why don't you all stop shooting it down, the guy asked if it was possible, i said yes, you are trying to just get him to buy a curtis throttle pot... why can't he just try it himself?.


This is a forum, things are discussed. If your opinion was the only one that mattered we all would go to frodus.com and get it.
As it is, he asked a question and I and others answered with what we know, and what I know is, what you said was misleading.
Well intentioned but misleading.

I dont really care which way he goes, it's up to him.
Good luck to him. As KIWIEV said If you can do it well, tell us how and we can all learn.


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## JRP3 (Mar 7, 2008)

If anyone has a source for a 90 degree 5 k pot other than the $30+ ones sold as curtis replacements I'd like to see it. I was never able to find one and gave up on building my own throttle.


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## BHall (Aug 1, 2007)

I didn't mean to start any bad feelings among our great and knowledgeable users. (and this time I am not being sarcastic). I will keep searching for a 90deg 5k pot and might mess with trying to make a 360deg work too. I would think that would give some good resolution. I really appreciate all of the feedback. 

Thanks!

Brian


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## ww321q (Mar 28, 2008)

at one time I ran across 90deg pots I'll see if I can find it again ...J.W.


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## MrCrabs (Mar 7, 2008)

BHall said:


> I didn't mean to start any bad feelings among our great and knowledgeable users. (and this time I am not being sarcastic). I will keep searching for a 90deg 5k pot and might mess with trying to make a 360deg work too. I would think that would give some good resolution. I really appreciate all of the feedback.
> 
> Thanks!
> 
> Brian


To answer your original question, most people use the pot-boxes for the convenience/simplicity of installation. Its quicker to buy one, and install it, than building your own... It all depends on how much you want to do yourself.


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## frodus (Apr 12, 2008)

MrCrabs said:


> To answer your original question, most people use the pot-boxes for the convenience/simplicity of installation. Its quicker to buy one, and install it, than building your own... It all depends on how much you want to do yourself.


see, thats a great answer.... first great answer on this thread.....


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## Wirecutter (Jul 26, 2007)

Actually, the solution I like for this problem can be found in the electric guitar effect boxes.

I think it was a wah-wah pedal me and the guys took apart once. Those things also get a lot of cycling, and musicians are a bit picky about pot-induced noise and unreliability. So this wah-wah pedal used an optical solution. You have a light source (LED) and a photocell. In between, you have an opaque film mechanically coupled to the pedal. The film has a wedge shaped hole punched in it. There's a nice linear variation in the "pot", and there are no moving parts in contact with each other, so no wear. No noise, either, unless of course the box is open and you shine a spotlight on it.

How about this little plaguaristic idea?

-Mark


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## MrCrabs (Mar 7, 2008)

Check out http://www.belktronix.com/

http://www.belktronix.com/throttle.html

The throttle control for the Belktronix controller is an optical sensor, and it uses 2 wire serial communications, which are opto-isolated to send the throttle position to the controller.


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## TheSGC (Nov 15, 2007)

frodus said:


> I'm an electrical engineer too, helping to develop a controller... what voltage/amps are you doing? fets or IGBT's, high side or low side? series motor?
> 
> As far as the POT still goes... we've used several throttle bodies, works just fine. It wouldn't take but an hour or so in a metal shop to fabricate a mount, and an arm that is compatible with a "high quality $25 pot"... what is the part number of that "high quality" pot? I need to start looking for one to replace my magura on my motorcycle. I want to keep the original throttle/killswitch assembly.


Ironically its the same throttle used in the Curtis PB potboxs, but I don't know the model off hand. It's back at school, and costs a lot less than what the replacement costs of the Curtis is. We took them a bunch of them apart in class to learn how they work, and there were clear quality and reponse differences between the ten different brands or so that we tested. 

As for the controller, its a 24 to 156 volt, low side, 600 AMP IGBT setup currently running at 2KHz, and uses all three wires on a potentiometer. (My PIC18 skills aren't up enough yet to rely on my programming to drive a car.)


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## frodus (Apr 12, 2008)

TheSGC said:


> Ironically its the same throttle used in the Curtis PB potboxs, but I don't know the model off hand. It's back at school, and costs a lot less than what the replacement costs of the Curtis is. We took them a bunch of them apart in class to learn how they work, and there were clear quality and reponse differences between the ten different brands or so that we tested.
> 
> As for the controller, its a 24 to 156 volt, low side, 600 AMP IGBT setup currently running at 2KHz, and uses all three wires on a potentiometer. (My PIC18 skills aren't up enough yet to rely on my programming to drive a car.)


keep us posted on that controller... we've gone 24V too, just not many appplications.

2Khz seems slow, but again, its an IGBT... Is it audible? How's your isolation? Ringing? Snubbers?

We use a 3 wire as well, but as I'm sure yours is, you can calibrate the "throw".... so a 300 degree throw, doesn't matter much, but the resolution goes away.


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## TheSGC (Nov 15, 2007)

frodus said:


> keep us posted on that controller... we've gone 24V too, just not many appplications.
> 
> 2Khz seems slow, but again, its an IGBT... Is it audible? How's your isolation? Ringing? Snubbers?
> 
> We use a 3 wire as well, but as I'm sure yours is, you can calibrate the "throw".... so a 300 degree throw, doesn't matter much, but the resolution goes away.


It is somewhat audible, but I find it more than acceptable at the moment. The IGBT is not quite isolated from the PWM circuit as they both require the same ground in order for the current sensing and limiting circuit to function. I am using two 300 amp pulse diodes that do an extremely good job keeping the ringing down, and the IGBT as a blocking diode of 1200 volts, so I am not worried about that.

My website has a video of the prototype running. I have actually shipped the guts to another EV'er who wanted in on this project (and funded it) and he is going to make up the proper casing and terminals and then test it in his EV.
http://civicity.blogspot.com


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## frodus (Apr 12, 2008)

congrats... its great to see more people doing stuff themselves rather than wait....


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## Mike D EV (Sep 11, 2007)

For what it is worth.
Just finished making a throttle controller from an audio taper slide pot.
yes I know it is a log scale, but I found that if I used the slowly changing end of the pot for the low speed end of the control, it felt pretty smooth. The vehicle will not be used on the road, and will only get maybe 1-2 more miles of use, so longevity is not a factor.
The operation is smooth and reliable, and it only took 20 minutes to make.
The throttle cable had a bit more travel than the pot, so I made a pedal stop to restrain it to the full travel of the pot.
http://www.99mpg.com/resources/articlesandblogs/pluggingintothesun/thecontroller/all_wired_up.jpg,0


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## DVR (Apr 10, 2008)

If you can enclose/seal them I think a linear slide pots would probably be the best way to go. They are much easier to set up with the full range of throw than a radial pot. The best set of PC pedals that I've made and my sprintcar simulator both used them with brilliant results, but they are prone to getting crap in them.


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