Why the crazy price for a decade resistor/resistance box?

[Terry01]

I've been looking to pick one of these up but the prices are rediculous! Why are they so expensive when you can buy all the parts 1000s at a time for pennies? Usually eBay and the likes are flooded with this type of thing for next to nothing

 

[(*steve*)]

If you imagine a 3 decade resistance box. If all decades are to be significant then the most significant decade needs values well within +/- 1%. That's not too hard, but for 4 decades it's 0.1%, 5 decades 0.01%, and so on. These resistors start to get very expensive.

Cheaper decade resistance boxes might only be significant to 2 or 3 decades.

Then you need to look at the smallest decade. If it's 0 to 9 ohms, then you need to make sure that the sum of all the contact resistances are well under 1 ohm -- that's not too hard, but it gets harder as the smallest value gets smaller and smaller.

If you want to set on of these to 12345.6 ohms and have it read 12345.60 ohms, then you'll be paying a pretty penny.

 

[kellys_eye]

Relatively high wattage, high stability and close tolerance resistors aren't 'cheap' - which is what you (ideally) want for a decade resistance box. Add to that a set of decent multi-position rotary switches (have you seen the quality of some of the better ones ) and slap it all in a robust box and you're looking at the best part of £100+ (and then some) just to MAKE it. Add 100% margin and you're beginning to see why a DECENT box costs a lot!

Yeah, we can all knock a usable version up for a lot less but it will have its limitations.

 

[kellys_eye]

If all decades are to be significant then the most significant decade needs values well within +/- 1%. That's not too hard, but for 4 decades it's 0.1%, 5 decades 0.01%, and so on. These resistors start to get very expensive.

I thought tolerances weren't 'divisible' that way i.e. if you want a 0.1% tolerance decade box all you need are 0.1% tolerance resistors across the range.

 

[AnalogKid]

There is a big difference between 1 piece of what a hobbyist needs to putter around at home, and hundreds of units that 100 different professionals doing 100 different things need at work. The test equipment needed to document the accuracy of a resistance box costs way more than the resistors for 100 boxes.

Also, think about the test time (which costs money) - someone has to verify that every resistance combination is correct on every box, to make sure someone else didn't get just 1 resistor in the wrong place.

ak

 

[(*steve*)]

I was just pointing out that if you have 4 decades and you dial up 1002 then the 1000 needs to be better than 0.1% accurate (0.01%) would be better) or the last digit won't resolve. And it's the highest value resistors need to be the most precise.

In the above example, a 1% resistor for the 1000 could give you anything from 990 to 1010, which makes the unit resistors fairly pointless.

 

[shrtrnd]

... they're expensive because you want one. I have the same problem. I see lots of stuff cheap that I don't want, but when I look for what I do want it costs an arm and a leg.
In your situation, though it can take a while, I start looking at places that have what I want, and watch for surplus or liquidation sales they might have (or vendors for places like that, who they would use to liquidate their assets).
My decade boxes, ratio & decade transformer boxes I got from enterprising employees of companies that folded or liquidated unused assets, and then listed them for sale (ebay and etc,....).
Sure, it can take a while, but the items show-up sooner or later.
Find-out the big electronic companies in your area and who liquidates their discards, and check with the liquidators.
Everybody here already explained WHY the resistance decade boxes cost so much. I'm just telling you what I would do about the problem.

 

[Ratch]

I was just pointing out that if you have 4 decades and you dial up 1002 then the 1000 needs to be better than 0.1% accurate (0.01%) would be better) or the last digit won't resolve. And it's the highest value resistors need to be the most precise.

In the above example, a 1% resistor for the 1000 could give you anything from 990 to 1010, which makes the unit resistors fairly pointless.

Selecting a large resistance value does not justify the expectation that the total resistance will be within the tolerance of the lowest resistance value that could be selected (one ohm)? For instance 10% of 1 ohm is 0.1 ohm. For a 10 k resistor to be within that value, its tolerance would have to be 0.1/(10 k) = 0.001% . Don't even ask what it will have to be in the megaohm range. Selecting a small value after selecting a large value should give a 10% accurate increment, however.

Lets see what the value could be on a 4 decade box with 1% resistors. Selecting the value of 1111 gives a worse error of:

which is 11.11/1111 = 1%. Of course, if some of the resistors selected are low in value, then the higher resistance values will be compensated and the accuracy will improve. The fewer resistors selected in the box, the less chance of value deviation.

Ratch

 

[AnalogKid]

Start with your salary or hourly rate at your day job. For salary, divide by 1920 for an approximate hourly rate. If you get benefits at work such as health insurance, increase the hourly rate by 10-20%. This is the total hourly rate for the project.

Keep track of every *minute* you spend doing the tasks necessary to make a resistance box.

Decide on the accuracy (resistance tolerance) and precision (number of switch choices).
Draw the schematic.
Select the resistors, switches, connectors, and enclosure.
Select the best vendor for each.
Place the orders.
Receive the orders.
Inspect each shipment for accuracy.
Pay for the parts.

Create mounting holes in the enclosure for every switch and connector.
Assemble the mechanical components.
Install the resistors.
Assemble the device.

Select a resistance meter that is more accurate than the best-case accuracy of the resistance box. The standard engineering rule for this type of test equipment is 10x; if you need to measure a 1% resistor accurately, a pro would need a 0.1% accurate meter. You can get away with less.

Select the meter vendor.
Purchase the meter.
Receive the meter.
Inspect it to make sure it works.
Pay for the meter.
Use the meter to verify the accuracy of every possible resistance combination.

Add up all of the times. This is the project time.
Multiply the total time by the total hourly rate. This is the labor cost.
Divide the total time by 720. This percentage is used for the overhead.
Add up your rent or mortgage, and utility bills for one month.
Multiply that by the overhead percentage to get the project overhead cost.

Add the parts cost, test equipment cost, labor cost, and the overhead cost. That is what it cost you to build one resistance box. This does not include any profit, workman's compensation insurance, safety agency certification, advertising, packaging and shipping, and all the other stuff it takes for a company to produce a deliver a product.

ak

 

[Terry01]

HOLY SMOKES!! No wonder everything gets subbed out to China!

I'll keep an eye out for a decent 2nd hand one

Dogs just farted too,,,,,he's howling!

 

[kellys_eye]

... they're expensive because you want one.

Testify....

Goes with the adage that the stuff you sell will be sold for pennies - until you see someone selling one a week later that gets £££'s for the same item. Grrrr......

Dogs just farted too,,,,,he's howling!

Yay internet.......