Author Topic: DIY Cheap resistor decade substitution box  (Read 38304 times)

0 Members and 2 Guests are viewing this topic.

Offline vtl

  • Regular Contributor
  • *
  • Posts: 136
  • Country: au
DIY Cheap resistor decade substitution box
« on: July 22, 2011, 02:39:34 PM »
After seeing these adjustable resistor substitution boxes I decided that they would be a nice addition to any person fiddling with electronics regularly. I don’t particularly like potentiometers for this purpose as you usually have to have a pretty decent quality one otherwise it can be hard to dial in a precise resistance.
After looking on the interwebs, it became clear that these things are quite expensive, usually utilizing expensive rotary switches. We’re talking on the order of $50-$200 for what really is just a resistors hooked up to some switches. Surely there’s a cheaper way?

I saw these thumbwheel switches on eBay for a reasonable price, just $5 for 10. These can be stacked and locked together, with the numbers ranging from 0-9. The clips aren't particularly strong so glue may be needed to keep them together.



Design
 
Looking at the spacing it seems like it could be ideal for 0603 resistors. I decided not to go this route for now, I had a look at element14 and when you take into account minimum order quantities, the costs can rise pretty quickly when buying precision resistors.



The biggest challenge to a resistor box is making it accurate. The box just puts the resistors in series to make up the value so the error of the resistors can add up pretty quickly and give you pretty wild deviation from what number you dial in. Now accuracy was not a large factor for me, as long as it was somewhat correct that would be fine for me.
I wanted a box that would do 1R, 10R, 100R, 1K, 10K, 100K and 1M. Even if I had got 0.05% resistors, at 100k this could produce an error of 50ohms, quite large considering this device is supposed to do 1R steps. Clearly the larger resistors require a very good tolerance or you’ll end up with large offsets from the value entered.
So I concluded that this box would not be intended on being a precision instrument. It would be just a nice thing to have for circuits that require irregular resistances and I could just hook it up to a multimeter if I needed more accuracy. It would be similar to a pot but with a huge range and very fine and precise adjustment.
On eBay they sell resistor kits pretty cheaply. A while ago I bought 2000 1% assorted resistors for $11 which is quite nice. I decided the accuracy would be terrible no matter what tolerance I use so I figured 1% should be good enough for hobby usage.
These thumbwheel switches are much more compact than those huge resistor decade boxes I’ve seen. Instead of making up some kind of housing, the resistors should fit directly on the PCB and should not increase the overall size at all. The wiring is fairly simple but it may not be immediately apparent when looking at the switch. You have a common pin and 10 other pins corresponding to each digit. For a 10M (9,999,999R), 7 digit box this will require 63 resistors, with 9 resistors per digit.




 
This is a simplified drawing of the switch with only 6 contacts drawn. You would measure the resistance between the common and pin 0. In this case the wiper is switched to 3 so the measurement would be 3xR. You can daisy chain these up to provide more resistance, just attach the common to pin 0 of the next decade.







 
And here is the end result. It can be quite time consuming I guess if you work fast you can do 1 switch every 10-15 minutes or so. Its probably a good idea to check each switch before combining them up, I ended up gettign a few soldering errors and reworking it when its all glued and soldered together can be a bit tricky.
 

Accuracy
The accuracy will depend greatly on what value is shown on screen. If you use only 1 resistor from each decade then this will be highly accurate, but if you use many resistors then the error will add up. So I’ve tested by setting each range to 9 and recording the reading. REL was used when the resistor box was set to 0 to null out the resistance of the test leads




 
The percentage error looks quite bad at the lower resistances but the actual absolute values are quite small. We’re talking less than 1.5ohms at the errors of greater than 1%.
While probably not lab grade equipment I’m pretty pleased with the accuracy results. In general, when dialling in high values you tend to need to trim it out slightly. For this device, most of the time the device will give a value less than the desired value. I suppose it is possible to improve the device by adding some trimmers for each decade and might be handy if you’re just building up a 1K box.

Overall cost estimate:
The parts cost is pretty cheap for this:
$5.45 for 10 thumbwheel switches
$.0055*63= $0.35 for 63 1% resistors
Prices in AUD

So surely this beats the hell out of those crazy expensive rotary switch boxes in terms of price and portability.

So go out and build your own, I’d be interested to see what kind of error if 0.05% or better resistors were used.

Offline gregariz

  • Frequent Contributor
  • **
  • Posts: 510
  • Country: us
Re: DIY Cheap resistor decade substitution box
« Reply #1 on: July 22, 2011, 03:58:48 PM »
If only you had posted a day earlier! I finished a couple builds last night.

The only tip I have is make sure you clean the flux off well as you'll see it on the Meg Ohm switch settings.

Offline IanB

  • Super Contributor
  • ***
  • Posts: 5256
  • Country: us
Re: DIY Cheap resistor decade substitution box
« Reply #2 on: July 22, 2011, 05:54:29 PM »
Very neat idea and good job--but I'm a bit confused about your accuracy estimations.

1% is 1%, whether it be 1% of 1 ohm or 1% of 100k. The circuit doesn't know the difference, and the 1% 100k resistor is no less accurate than the 1% 1 ohm resistor. (Consequently, a resistance value of 134,968 ohms, say, is rather meaningless and also not likely to be useful in any practical application.)

Also, the error doesn't "add up" when you put resistors in series. It remains exactly the same at 1% no matter how many resistors you combine. (The variance changes, but the mean error doesn't.) This means that if you put ten 1k, 1% resistors in series you will get a 10k, 1% resistor. Therefore, I would say that if you have an error of 2% on your 9 ohm value, it is either because the resistors themselves are out of spec or you have an inaccurate measurement. (At low resistance values you also have to worry about extra circuit resistances like switch contact resistances getting in there, but your measurements don't indicate that problem.)
I'm not an EE--what am I doing here?

Offline vtl

  • Regular Contributor
  • *
  • Posts: 136
  • Country: au
Re: DIY Cheap resistor decade substitution box
« Reply #3 on: July 22, 2011, 07:00:15 PM »
Very neat idea and good job--but I'm a bit confused about your accuracy estimations.

1% is 1%, whether it be 1% of 1 ohm or 1% of 100k. The circuit doesn't know the difference, and the 1% 100k resistor is no less accurate than the 1% 1 ohm resistor. (Consequently, a resistance value of 134,968 ohms, say, is rather meaningless and also not likely to be useful in any practical application.)

Also, the error doesn't "add up" when you put resistors in series. It remains exactly the same at 1% no matter how many resistors you combine. (The variance changes, but the mean error doesn't.) This means that if you put ten 1k, 1% resistors in series you will get a 10k, 1% resistor. Therefore, I would say that if you have an error of 2% on your 9 ohm value, it is either because the resistors themselves are out of spec or you have an inaccurate measurement. (At low resistance values you also have to worry about extra circuit resistances like switch contact resistances getting in there, but your measurements don't indicate that problem.)

Ah yeah thats right regarding the numbers, but what I meant was that the absolute values in ohms do add up. I didn't say 9 1% resistors in series = 9% tolerance.

Percentage errors will roughly stay the same but when youre building a device thats supposed to have 1ohm resolution, youll never get near that kind of precision at the higher values which is the point I was trying to get across.

Offline hacklordsniper

  • Frequent Contributor
  • **
  • Posts: 597
  • Country: hr
  • Don't turn it on, take it apart!
    • HackLordSniper
Re: DIY Cheap resistor decade substitution box
« Reply #4 on: July 22, 2011, 07:18:32 PM »
Just a quick note. I bought few of these "resistor kits" and some of them had all resistors labeled wrong and i guess they were factory discarded  and resold by clever Chinese guy :)
Oh, the joy of sending various electronics to silicon heaven

Offline Kiriakos-GR

  • Super Contributor
  • ***
  • Posts: 3526
  • Country: gr
  • User is banned.
    • Honda AX-1 rebuild
Re: DIY Cheap resistor decade substitution box
« Reply #5 on: July 22, 2011, 08:46:11 PM »
I am aware that this thread are about cheap decade boxes.

But I have to add a link of my own decade box.
It is a good example  about making with your hands, one box that in the market retails at 1500$,
with the lowest possible cost.

The low in Watt decade boxes, they must be easy to repair too.
The low in watt resistors, they can easily get damaged for high current.

And so its not wise to follow dirt cheap solutions, but to play safe by making something better.
And even by looking at your home made box, you will have with it and all the memories by the experiences that you earned by making it, with your own hands.  :) 

http://www.eevblog.com/forum/index.php?topic=1148.0

Offline IanB

  • Super Contributor
  • ***
  • Posts: 5256
  • Country: us
Re: DIY Cheap resistor decade substitution box
« Reply #6 on: July 23, 2011, 08:15:34 AM »
Percentage errors will roughly stay the same but when youre building a device thats supposed to have 1ohm resolution, youll never get near that kind of precision at the higher values which is the point I was trying to get across.
Actually you will have 1 ohm resolution, but you won't have 1 ohm accuracy. Although as I was saying, I can't think of a 100k resistor where 1 ohm accuracy is of practical use. (Maybe a high precision voltage divider or bridge circuit, but in that case a trim pot would likely do the job.)
I'm not an EE--what am I doing here?

Offline ejeffrey

  • Super Contributor
  • ***
  • Posts: 1363
  • Country: us
Re: DIY Cheap resistor decade substitution box
« Reply #7 on: July 23, 2011, 08:45:15 AM »
Actually you will have 1 ohm resolution, but you won't have 1 ohm accuracy. Although as I was saying, I can't think of a 100k resistor where 1 ohm accuracy is of practical use. (Maybe a high precision voltage divider or bridge circuit, but in that case a trim pot would likely do the job.)

Not quite.  Unless you take special precautions, 10,000 ohms will not be within 1 ohm of 9999 ohms, nor will it even be greater.  In the language of DACs, typical resistor decade boxes have poor DNL and are not monotonic.

Offline george graves

  • Frequent Contributor
  • **
  • Posts: 751
  • Country: us
Re: DIY Cheap resistor decade substitution box
« Reply #8 on: July 23, 2011, 09:00:51 AM »
Looks great!  Nice work.

Thanks for posting that up.  I just ordered 20 of the switches, and will make a box or two when I get the chance.  Seems like a really great budget/alternative.  I could never pull the trigger on the $100 worth of parts.  This is great.

Offline gregariz

  • Frequent Contributor
  • **
  • Posts: 510
  • Country: us
Re: DIY Cheap resistor decade substitution box
« Reply #9 on: July 23, 2011, 09:14:46 AM »
Looks great!  Nice work.

Thanks for posting that up.  I just ordered 20 of the switches, and will make a box or two when I get the chance.  Seems like a really great budget/alternative.  I could never pull the trigger on the $100 worth of parts.  This is great.


If you don't need so many resistor levels, or precision resistances.. another cheap option is

http://www.elexp.com/tst_subk.htm

Offline IanB

  • Super Contributor
  • ***
  • Posts: 5256
  • Country: us
Re: DIY Cheap resistor decade substitution box
« Reply #10 on: July 23, 2011, 09:17:55 AM »
Not quite.  Unless you take special precautions, 10,000 ohms will not be within 1 ohm of 9999 ohms, nor will it even be greater.  In the language of DACs, typical resistor decade boxes have poor DNL and are not monotonic.
Oh, you are correct of course, thanks for pointing that out. Oops.

(Goes off to think about putting 10,000 one ohm resistors in series...)
« Last Edit: July 23, 2011, 09:21:01 AM by IanB »
I'm not an EE--what am I doing here?

Alex

  • Guest
Re: DIY Cheap resistor decade substitution box
« Reply #11 on: July 23, 2011, 09:28:08 AM »
Relevant, if not mentioned already:

http://www.bourns.com/data/global/pdfs/3680.pdf
Bourns 3680 High-Precision Variable Resistors

Offline Kiriakos-GR

  • Super Contributor
  • ***
  • Posts: 3526
  • Country: gr
  • User is banned.
    • Honda AX-1 rebuild
Re: DIY Cheap resistor decade substitution box
« Reply #12 on: July 23, 2011, 09:31:02 AM »
Tell us the price Son !!   ;D

Offline Conrad Hoffman

  • Frequent Contributor
  • **
  • Posts: 308
  • Country: us
    • The Messy Basement
Re: DIY Cheap resistor decade substitution box
« Reply #13 on: July 23, 2011, 10:15:58 AM »
I use resistance boxes constantly and have acquired quite a few over the years. Switch costs are always the primary problem when you build one, followed of course by the resistors. General Radio Corp made their own switches and wound their own mica card resistors- very precise and handle quite a lot of power if you don't mind the inductance. For low values it's easy to wind your own resistors if you get a roll of constantan or other solderable resistance wire.

You might get some more ideas from some stuff I've written on KVDs and precision resistor dividers. Look at the Mini Metro Lab and the Hamon Divider. With nothing special you can do ratios at the 10-100 part per million level. Absolute resistance value is a bit more difficult because you need at least one known standard. I also like the BCD methods because it takes fewer parts for both an R box and a C box. I suppose nobody builds L boxes, but you certainly can.

Offline tomlut

  • Contributor
  • Posts: 9
Re: DIY Cheap resistor decade substitution box
« Reply #14 on: July 23, 2011, 11:04:18 AM »
Very neat idea and good job--but I'm a bit confused about your accuracy estimations.

1% is 1%, whether it be 1% of 1 ohm or 1% of 100k. The circuit doesn't know the difference, and the 1% 100k resistor is no less accurate than the 1% 1 ohm resistor. (Consequently, a resistance value of 134,968 ohms, say, is rather meaningless and also not likely to be useful in any practical application.)

Also, the error doesn't "add up" when you put resistors in series. It remains exactly the same at 1% no matter how many resistors you combine. (The variance changes, but the mean error doesn't.) This means that if you put ten 1k, 1% resistors in series you will get a 10k, 1% resistor. Therefore, I would say that if you have an error of 2% on your 9 ohm value, it is either because the resistors themselves are out of spec or you have an inaccurate measurement. (At low resistance values you also have to worry about extra circuit resistances like switch contact resistances getting in there, but your measurements don't indicate that problem.)

Ah yeah thats right regarding the numbers, but what I meant was that the absolute values in ohms do add up. I didn't say 9 1% resistors in series = 9% tolerance.

Percentage errors will roughly stay the same but when youre building a device thats supposed to have 1ohm resolution, youll never get near that kind of precision at the higher values which is the point I was trying to get across.

I may be wrong but i think the total tolerance is the geometric mean of the individual tolerances.

So %Total = sqrt(%1^2 + %2^2 + %3^2 + ... %n^2)

Or if all the tolerances are the same:

%Total = sqrt(n.t^2) where

n = number of resistors
t = resistor tolerance

So for 10 x 1% resistors in series you are looking at a total tolerance of 3%.


Share me

Digg  Facebook  SlashDot  Delicious  Technorati  Twitter  Google  Yahoo
Smf