A fat bad ass vintage multiturn pot, is there anything I should aware of ?

[BravoV]

I always thought that I've seen enough different types of potentiometers until I bought this one, just received it recently, almost lost my jaw when I saw and held it in my palm, I wasn't expecting it will be this huge. 

Its the vintage Beckman Helipot 10K, 15 watt rated,  15 turns and with 0.02% linearity, also it looks like its designed to be hooked up with servo motor since the shaft is using a ball bearing and its made by Fafnir in usa, I guess it must be > 30 years old.

To other experienced fellows, just curious, apart from very good linearity, are there any other special care or treatment should I aware of since this is quite old piece of equipment ? From appearance, I believe its nos and the gold plated terminals are never soldered. I will let the pictures tell the tale since I'm speechless bout it's size.


Ok, how big is big ? The one 5 watt in the middle is already considered quite a big pot isn't it ? Wait a minute, no, the pot in this pic is not the one I'm talking about, wait till you see the 2nd photo.




A picture worth lots of words. 




Its even fatter than a soda can.




Other more details shots with ruler to give you the idea of the size are at below thumb nail sized pics.



.

[ashley.hughes]

That looks awesome nice find

[hacklordsniper]

Haha i have a small screen and seen only first photo while reading the text and tought "omg that is one kick ass pot", seeing the second one i was really suprised 

[blackdog]

Hi, BravoV

How about this "potmeter" ...
The price = 4000$ US 

www.bramcam.nl/Pot-KVD-01.jpg
www.bramcam.nl/Pot-KVD-02.jpg

Price of 0.5L beer is 1 Euro.

Regarts,
Blackdog

[grenert]

How about this "potmeter" ...
The price = 4000$ US 

blackdog, that's no pot in the typical sense.  That's a Kelvin Varley divider!  Very fancy, precision device, and priced accordingly.

[blackdog]

Hi grenert,

I think it is still a potmeter, a voltage divider, just like a normal potmeter.

Have a look inside in my DP1311.
www.bramcam.nl/Pot-KVD-03.jpg
www.bramcam.nl/Pot-KVD-04.jpg
www.bramcam.nl/Pot-KVD-05.jpg

I will use is in a precision powersupply, with 10 ranges, 0-10, 10 to 20 and the last range 100 to 110V.
Maximum current about 100mA.

Regarts,
Blackdog

[alm]

How is a rotary switch with discrete resistors a potmeter? A step attenuator might be a better term.

[blackdog]

Hi Grenert and Alm,

Think of a wirewound potmeter, the wiper jumps (switch) from one resistor to another... 

Kind regarts,
Blackdog

[Conrad Hoffman]

It is indeed an impressive pot, the largest of that type I've ever seen. The big pots used in General Radio bridges are bigger, but they're only single turn, albeit with a multitude of calibration adjustments. It would be interesting to see which type has better linearity. Your's is 0.1% lin, which is darned good, and about the same as the GR bridges. I think you ought to build a bridge!

[Rufus]

to see which type has better linearity. Your's is 0.1% lin, which is darned good, and about the same as the GR bridges.

You are looking at the wrong pot. The big one is labelled 0.02%.

[Conrad Hoffman]

Wow, you're right Rufus, it is .02! Definitely a candidate for a bridge or something really precise.

[slateraptor]

One word: ridiculous.

[BravoV]

Wow, you're right Rufus, it is .02! Definitely a candidate for a bridge or something really precise.

Conrad, what kind of bridge for measurement are you talking about ? Please shed some light, I'm totally a noob on these precision electronic and metrology stuffs and really love to learn. Infact I just started into these metrology stuffs by reading those excellent postings made by you, Richard (amspire), alm, robrenz and others here.


I did some few measurements at those 5 gold plated slugs, and curious why it has these extra fixed resistors at slug number 4 and 5 ? See the pic below for layout and the measurement results.

What is the purpose of these extra slugs labelled number 4 and 5 ? Anyone have any idea ?

[Rufus]

What is the purpose of these extra slugs labelled number 4 and 5 ? Anyone have any idea ?

These pots are mostly intended for servo systems so comparing the voltage on the taps with the wiper would give you end stop signals. Just guessing.

[BravoV]

These pots are mostly intended for servo systems so comparing the voltage on the taps with the wiper would give you end stop signals. Just guessing.

I'm sort of getting it, but the resistance ratio for the stop signal to detect when the wiper is near to the end by comparing those taps is quite weird I'd say.

Thanks for the idea though.

[Conrad Hoffman]

I dunno. Given the linearity, maybe the best use is for a precision DC voltage standard. Ignore the two resistors tacked on to the ends, put a good turns counting dial on it, Apply a stable voltage reference and buffer the wiper with a power opamp or similar. A precision voltage is always handy at the bench.

[BravoV]

I dunno. Given the linearity, maybe the best use is for a precision DC voltage standard. Ignore the two resistors tacked on to the ends, put a good turns counting dial on it, Apply a stable voltage reference and buffer the wiper with a power opamp or similar. A precision voltage is always handy at the bench.

Thats the plan, I thought you're suggesting something more advance, also this ridiculously big pot was an impulse buy, saw it on ebay and grabbed it for under ten bucks.

Btw, I just downloaded your excellent article on building a capacitance brige and currently reading it.

[ejeffrey]

What is the purpose of these extra slugs labelled number 4 and 5 ? Anyone have any idea ?

Given the odd values, I would expect it was some sort of manufacturing trim, but for what I don't know.  Seems unlikely it was trimmed for a manufacturing specification of exactly 10035 ohms, so perhaps it was to compensate for some offset elsewhere in the circuit for which it was built.  Something like if you have a buried zener voltage reference with a very large part-to-part variation you could adjust the fixed resistors so that the potentiometer always went over a specified voltage range.

[amspire]

What is the purpose of these extra slugs labelled number 4 and 5 ? Anyone have any idea ?

Given the odd values, I would expect it was some sort of manufacturing trim, but for what I don't know.  Seems unlikely it was trimmed for a manufacturing specification of exactly 10035 ohms, so perhaps it was to compensate for some offset elsewhere in the circuit for which it was built.  Something like if you have a buried zener voltage reference with a very large part-to-part variation you could adjust the fixed resistors so that the potentiometer always went over a specified voltage range.

I think the pins are associated with the way the pot is made.  I think they didn't want the wiper going all the way to the end of the windings, so 2 and 3 are the actual welded ends of the wire. 4 and 5  may be internally adjusted fixed wipers or taps that match the ends of the main wiper travel. They might even let the moving wiper go slightly beyond the positions of pins  4 and 5. If you are happy with a 1% to 99% range, just stick to pins 2 and 3.  If you need to go from 0% to 100%, still apply the main voltage between pins 2 and 3, but reference the wiper to the voltages on pins 4 and 5.

Richard.

Edit:  Just to explain myself better.  If you applied 10V across a conventional pot, it is fine, but the performance gets dodgy at the ends of the travel where it jumps from the resistive element to the metal terminations at the ends of the travel.  With your pot, if you applied a stable voltage source to pins 2 and 3 and adjusted it for exactly 10.00V between pins 4 and 5, then I suspect you will find this pot does at least a true 0.00V (referenced to pin 4)  to a true 10.00V with no non-linear performance at the ends of the travel. It may even allow something like -0.01V to 10.01V referenced to pin 4.

Can you just apply the voltage between pins 4 and 5 instead? No, you shouldn't - it will degrade the linearity at the ends of the travel.

[BravoV]

Edit:  Just to explain myself better.  If you applied 10V across a conventional pot, it is fine, but the performance gets dodgy at the ends of the travel where it jumps from the resistive element to the metal terminations at the ends of the travel.  With your pot, if you applied a stable voltage source to pins 2 and 3 and adjusted it for exactly 10.00V between pins 4 and 5, then I suspect you will find this pot does at least a true 0.00V (referenced to pin 4)  to a true 10.00V with no non-linear performance at the ends of the travel. It may even allow something like -0.01V to 10.01V referenced to pin 4.

Can you just apply the voltage between pins 4 and 5 instead? No, you shouldn't - it will degrade the linearity at the ends of the travel.

I think this is a better explanation so far, agree on the sudden sharp resistance change when the slider hit both ends of the resistance slide.

This brings another challenge and complexity, cause the reference voltage needs to be stretched beyond from the original source while still maintaining the same accuracy. 


Edit :

Just fyi, although this is not very scientifically correct method, tested it's resolution with a really big knob with my ultra sensitive finger  , the finest resolution when the slider jumps between the winding is less than 2 ohms on every jump, it varies approx. 1.2 to 1.8 ohms on each step. It needs a truly concentration to feel the jump since its has a very faint click.

[amspire]

This brings another challenge and complexity, cause the reference voltage needs to be stretched beyond from the original source while still maintaining the same accuracy. 

Luckily that is not a problem. Most references allow for a trimming of the output voltage.

For example, the REF01 10V reference allows a +/- 3% adjustment, and all you need for this pot is + 1%.

[BravoV]

Luckily that is not a problem. Most references allow for a trimming of the output voltage.

For example, the REF01 10V reference allows a +/- 3% adjustment, and all you need for this pot is + 1%.

Richard, I've been thinking using a vref that has those kelvin sensing wires like MAX6126 like this, what do you think ? Too bad these vref doesn't have 10 volt version.

[amspire]

That Max chip looks good. That circuit should work for it.  But something like a 10V REF01 will work very well.  The voltage drop across the 50+ ohm resistors will be very stable as it is the same wire as the rest of the pot, so you would get the full REF01 stability out of it for the adjusted 10V. The output amp will need a slight negative supply or it will not be able to work well near 0V.

Make sure you confirm my ideas about pins 4 and 5 before you go to far. Just connect pins 2 and 3 to a supply and see if you can adjust for 0v between the wiper and pin 4, and also 0v between the wiper and pin 5.

Did the pot come with a vernier dial? If not, how are you going to adjust it accurately?

Richard.

[BravoV]

Make sure you confirm my ideas about pins 4 and 5 before you go to far. Just connect pins 2 and 3 to a supply and see if you can adjust for 0v between the wiper and pin 4, and also 0v between the wiper and pin 5.

Thanks for reminding me that, currently I'm out of town and will check the resistance at those pins when the wiper at the edge.

Did the pot come with a vernier dial? If not, how are you going to adjust it accurately?

Don't worry, I have that covered and it will be pretty pointless if this pot is not equipped with precision vernier dial, a mint nib beckman 15 turns duo-dial knob is on it's way too.