DIY Cheap resistor decade substitution box

[KJ6EAD]

Those switches appear to be from this Chinese company.
http://yqhrie.en.ec21.com/Coded_Switch--1512901_1518592.html

They look like a clone of the Omron A7 series switches.
http://www.ia.omron.com/product/42.html

You might be able to complete your switch assembly by using Omron end caps.

[metalphreak]

hmm, the ones i ordered(think from the link tronix gave) and i got mine today, but they only have 5 contacts, it appears for like 3, it connects common to both 1 and 2, like 7 it connections common to 4 + 2 + 1. definitely odd/different

I just received mine today, and they are the same They are binary encoded versions (BCD) instead of decimal. The description clearly says decimal, and the pictures all show decimal contacts. It's a shame because there's only the one person selling them on ebay. Don't be fooled by all the usernames... they're all UXCELL. I bought mine from 'milancentral' and it has a UXCELL coupon in the package xD

I've emailed to see if they can send the correct ones. Shall see what happens.

BTW, I found all the specifications here: http://www.easternradio.co.in/switches.html


FYI, the BCD ones are actually suitable for building a capacitance box

[Bored@Work]

FYI, the BCD ones are actually suitable for building a capacitance box

Or a constant current source http://www.edn.com/article/470718-Programmable_current_source_requires_no_power_supply.php

[alm]

Always try to verify important specs in the datasheet based on the part number. Vendors are sometimes lazy and copy the description from one member of the family to all, or copy the wrong specs. The part number is usually correct, or at least you have a good chance to receive a refund if it's wrong.

[amspire]

Love all the equations for the accuracy!

All inaccurate unfortunately.

If the decade box is made from 1 % resistors, then ignoring contact resistance, the accuracy of the decade box is 1%, the linearity is 2%.

The use of statistical tools are not really relevant, because 10 resistors made sequentially in a production run could all have similar errors and not random errors over the full +-1%, and more importantly, the resistance dialed up in the decade box will in many cases be a single resistor (1K, 10K, 100K, 1M, etc) and the accuracy of that single resistor is 1%, even if all the other resistors in the box had perfectly random error distributions. So you cannot quote less the 1% accuracy.

The accuracy cannot be over 1%.  Say every resistor was exactly 1% high - the worst case situation. This will give the maximum possible error.

If you dialed up 99,999,999 ohms, the what would the error be?

1% high.

So the accuracy cannot be over 1% and it cannot be under 1%.

[IanB]

If the decade box is made from 1 % resistors, then ignoring contact resistance, the accuracy of the decade box is 1%, the linearity is 2%.

As other people have pointed out, that is not the whole story.

Suppose you dial in 999 ohms, which is made up from 9 x 100 + 9 x 10 + 9 x 1, and suppose this has the maximum positive error of 1%, then the  actual resistance will be 1009 ohms. Suppose you now increase the resistance setting by 1 ohm to 1000 ohms, only this is now made up from 1 x 1000 and it has the maximum negative error of -1%. The new actual resistance will be 990 ohms. Increasing the nominal resistance by 1 ohm caused a decrease in actual resistance of 19 ohms, a relative error of -1900%!

[gregariz]

Love all the equations for the accuracy!

... I think I could count on one hand the number of times in the last 15 years I needed a precision resistor box. 5% resistors will be fine thanks.

[ejeffrey]

If the decade box is made from 1 % resistors, then ignoring contact resistance, the accuracy of the decade box is 1%, the linearity is 2%.

As other people have pointed out, that is not the whole story.

Suppose you dial in 999 ohms, which is made up from 9 x 100 + 9 x 10 + 9 x 1, and suppose this has the maximum positive error of 1%, then the  actual resistance will be 1009 ohms. Suppose you now increase the resistance setting by 1 ohm to 1000 ohms, only this is now made up from 1 x 1000 and it has the maximum negative error of -1%. The new actual resistance will be 990 ohms. Increasing the nominal resistance by 1 ohm caused a decrease in actual resistance of 19 ohms, a relative error of -1900%!

This is what is meant by 2% (differential) nonlinearity.

[JuKu]

Talking about accuracy: Do you know do they make 0.5% or 0.1% resistors? They don't, they just make resistors and measure what they got. In other words, if a factory makes 5%, 1% and 0.5% resistors, then 5% parts are pretty much guaranteed to be at least 1% off. Likewise, in this case, 1% resistors will have 0.5% to 1% error. As said, a 1% resistor box is pretty handy, and if you ever need better than 1% in a decade box, just find a trim pot:-).

[alm]

Talking about accuracy: Do you know do they make 0.5% or 0.1% resistors? They don't, they just make resistors and measure what they got. In other words, if a factory makes 5%, 1% and 0.5% resistors, then 5% parts are pretty much guaranteed to be at least 1% off. Likewise, in this case, 1% resistors will have 0.5% to 1% error.

Funny, since the majority of through-hole 5% resistors are carbon film, and the majority of 1% resistors are metal film. I'm sure selection is taking place, but the production processes have improved enough that just producing resistors with 1% tolerance has a good yield. More accurate resistors often have a better tempco and sometimes different materials.

As said, a 1% resistor box is pretty handy, and if you ever need better than 1% in a decade box, just find a trim pot:-).

How's the stability, settability and tempco of that trim pot? One thing that does work is put a small value trimpot in series with a resistor that's pretty close (or a large value one in parallel). Because of the small contribution to the equivalent resistance, the error also has less of an effect.

[metalphreak]

I measured about 25 or so 5% SMD resistors and there were plenty that fell within the 1% range. The really high accuracy ones are normally produced the same as 1% resistors, but they individually test and laser trim them to the correct value. Higher accuracy ones are usually made from different materials that have less variability due to temperature (metal vs carbon film for example).

[Vertigo]

couldn't one simply measure all resistors one by one and cherry pick the accurate ones?

[ejeffrey]

couldn't one simply measure all resistors one by one and cherry pick the accurate ones?

Yes, assuming you have a sufficiently good meter -- which just translates to a set of known good resistors to compare against: ohmmeters work by comparing the DUT to a set of precision resistors.

However, it isn't as good as you would like.  Resistors change with time, temperature, applied voltage, and assembly technique: the heat from your soldering iron or stress on the leads can change the resistance permanently.  If you select resistors then solder them into a circuit, they may no longer be matched.

Temperature coefficient of resistance is also a big deal.  Lets say you select some 1% metal film resistors for .1% accuracy.  They still have a roughly 100 ppm/K temperature coefficient, and will change by 0.1% with a 10 degree temperature rise.  No problem you think: I will only use my decade box indoors! I won't seem more than +/-2 C.  Well, keep in mind that resistor power ratings are based on heating to some maximum temperature.  Lets say you have a 1/4 watt resistor with a maximum operating temperature of 125 C -- about 100 degree temperature rise over room temperature.  25 milliwatts of power will be enough to raise the temperature 25 degrees and change the resistance by .1%.

High precision resistors are usually made from more stable materials, built so that the body protects the resistive element from strain during assembly, and thermally oversized to keep the temperature rise to a minimum under operating conditions.

[Guppzor]

Just would like to confirm that the switches listed on eBay here do indeed have 10 contacts (0-9) plus Common. I checked with the seller prior to purchasing, and the units that arrived today are as per the image. Now to get a bunch of resistors...   

[kripton2035]

Just would like to confirm that the switches listed on eBay here do indeed have 10 contacts (0-9) plus Common. I checked with the seller prior to purchasing, and the units that arrived today are as per the image. Now to get a bunch of resistors...   

it's ok : it says
Digital   0,1,2,3,4,5,6,7,8,9
Code Method   Decimal

[IanJ]

Hi all,

Found this thread AFTER I'd started building my own box.........and using the same switches via Ebay........

I'm not so much interested in accuracy but the step change resistance......so at the bottom end are 0.1 ohm, and the top end 10M ohm resistors. 9 switches in total.

As VTL noted, the clips are pretty hopeless so I'd elected to use some thin Alu tape.

Testing done, just got to box it up now.

Ian.

[amspire]

If anyone comes up with a good way of mounting these switches without the endplates, please let us know.

Other then gluing, of course - the last resort of the desperate.

I have not found anyone selling the endplates yet. If the true manufacturer could be located, it would help.

Richard

[IanJ]

Is this them?

http://www.tootoo.com/d-cu14047240-Yueqing_Huari_Instrument_Electron_Co_Ltd_/

Ian.

[kripton2035]

I ordered my switchs (on ebay) also before reading this thread...
they arrived this week and I use 0805 resistors, they fit perfectly between the tracks...
works nice, but I have to put them in a box like Ianp, must look for a box that fits the decade box and 2 banana plugs

I used standard resistors, and the precision is not here... but enought to search for a precise resistor in a filter for example...

[amspire]

Is this them?

http://www.tootoo.com/d-cu14047240-Yueqing_Huari_Instrument_Electron_Co_Ltd_/

Ian.

Ian thanks. They don't seem to have a website, but I tracked this down:

http://shlemore.en.alibaba.com/product/215208027-200521245/KM1_2_3_push_wheel_switch_22x_8.html

They are HKM2 decade switches.

Richard

[IanJ]

Hi,

Here's mine finished, just awaiting a label or two (haven't thought about that yet).

The switches are a tight fit, and with the silver tape the switches aren't going anywhere.

Ian.

[don.r]

I always advise builders of resistance boxes to make the lower ranges 1W or more as they are likely to be used in higher power applications. You also need to look at the switch's ratings. Most can only take 1/8 amp. Also be aware of contact resistance and how it affects the lowest range. This is around 0.1 ohm per switch so a tap at say around the deka-decade may make sense if you want some accuracy in the 0.1 ohm range. This would be about 1% so about the same as the resistors' tolerance.

[Conrad Hoffman]

Good advice on the power ratings; it's amazing how many used resistance boxes turn out to have burned parts when opened up.

[george graves]

Here's my take on them...

You can see the full tutorial here - with dozen of easy to follow pics for anyone not too sure how to wire things up.

http://www.digitalunderpants.com/?p=219

[mianchen]

Good tutorial