Decade Substitution Boxes-Do you actually use them? - Page 1
Forum section: electronics / general

author: FenderBender
author_rank: Super Contributor
timestamp: July 05, 2012, 03:39:13 am
content: I have a full pallet of stuff to do this summer, electronics and non-electronics stuff...one of those things was to make resistance and capacitance substitution boxes.Do you guys find yourselves actually using your boxes? Or perhaps the better question would be, is it worth spending the ~$50 on parts for one of these boxes, in terms of actual use?I could see where they might be useful. Tweaking a value up or down and see how the circuit behaves, perhaps while you view some sort of meter or scope. I'm not sure if the whole 'precision' box is necessary. Since these are most likely used while prototyping, there's already a lot of uncertainties and variables in prototyping. Stray capacitance, inductance or resistance in the breadboard or your wiring. That's not to say that you should use 10% resistors, but you don't have to go mad buying 0.01% Caddocks for your box.Sorry bit of a tangent there...But anyway. What's your opinions on actual use. I think they are 'cool to have', just not sure if its worth my time and money.Post away...if you want.

author: vk6zgo
author_rank: Super Contributor
timestamp: July 05, 2012, 04:10:49 am
content: These "precision" boxes are really only useful with DC & quite low frequency circuits.The connecting leads add stray capacitance & inductance,even when the internal circuitry is designed to minimise strays.Most substitution boxes sit in the corner of the shelf gathering dust.because of these considerations.Re your "tangent":It has always been my understanding that the essence of good design is to make things which will operate within specifications using normally available standard tolerance components.Neither active devices,or other passive components are specified to such close tolerances as resistors are.Another point to consider,is that "close tolerance" resistors are not necessarily "high stability"components.In many,probably most,cases they are,but not universally,so it may sometimes be better to choose,say,a 1 % tolerance "high stab",which still maintains that degree of tolerance in 10 years,than a 0.1% tolerance resistor which changes value with age.

author: FenderBender
author_rank: Super Contributor
timestamp: July 05, 2012, 04:41:35 pm
content: Right. Well I put 'Caddocks' as I am fairly sure they are very stable with temperature and over time. Just an example.I wonder if you made a few substitution boxes if you could avoid having to stocking resistors at home!Just dial it in.Well as I said, I think they are interesting, but I'm not sure how useful.If I was to use one, I don't think I would be using it to tweak a circuit. Like instead of using a standard 470Ohm resistor, I dial in 471.7Ohms. In the real world, a resistor with such a resistance would be hard to find and probably expensive. Rather just changing resistor values to see how a circuit responds. Not for actual "tuning".Yeah I probably not making sense.

author: vk6zgo
author_rank: Super Contributor
timestamp: July 06, 2012, 12:56:31 am
content: One way of looking at it is,if you make or buy some,use them for investigating the effects of changes,as you suggested,then put them on the shelf,& use them once or twice in the next few years,you have had the learning experience,& they will have been handy to have from time to time.One use which comes to mind,is selecting series resistors for a maybe unknown analog milliammeter to read voltage,though that is less likely these days.Even discovering the limitations of these devices could be a learning experience!If you can build or buy fairly cheap,they won't owe you much,& if you find they just sit on the shelf,you can sell them on eBay.

author: Short Circuit
author_rank: Frequent Contributor
timestamp: July 06, 2012, 02:36:26 am
content: Quote from: FenderBender on July 05, 2012, 03:39:13 amI have been in electronics for some 25 years, 12 of which making a living out of it.Don't recall ever having wanted to have one of these at hand. So waste of time and money imho.

author: David_AVD
author_rank: Super Contributor
timestamp: July 06, 2012, 03:08:49 am
content: Same for me. I don't recall any time in the past 30+ years "A decade box would have been handy".

author: vk6zgo
author_rank: Super Contributor
timestamp: July 06, 2012, 03:32:17 am
content: That was my first reaction,too---I've probably used them 2 or 3 times in my whole career.FenderBender seems interested in them,though,so there's no real harm in them,is there?

author: Rufus
author_rank: Super Contributor
timestamp: July 06, 2012, 03:32:44 am
content: Twiddling a pot and measuring it will cover half of what a resistance box can do.A lot of circuits are not going to be happy substituting a couple of feet of wire and a box full of switches for a resistor. Even less likely to find a circuit happy with a similarly substituted capacitor.If you are making or testing something which measures resistance or capacitance being able to dial in a value is useful. If I had a resistance box I would have used it to simulate thermistors in the past but not often enough to make me want to spend much on one.

author: vk6zgo
author_rank: Super Contributor
timestamp: July 06, 2012, 03:41:55 am
content: Quote from: Rufus on July 06, 2012, 03:32:44 amAs I said above,FenderBender still seems interested.I have known a few people who use them a lot,so maybe his experience will differ from us.

author: free_electron
author_rank: Super Contributor
timestamp: July 06, 2012, 04:20:29 am
content: The only time i have used those is to align an old analog ac ohms meter... You needed to pingpong with 3 values... Then its easier to flip switches than to swap resistors.For anything else. I have a few bourns multiturn potmeters laying around ( not trimmers ! Real potmeters ). Solder those in , adjust , desolder one leg and measure. Done. And the real multiturns are 3 watt as well.

author: SeanB
author_rank: Super Contributor
timestamp: July 06, 2012, 04:45:31 am
content: I used them only to set offets on opamps, as they were around. On the test board ( i took one and made it a dedicated test part so as to have a known good board) i took a piece of veroboard and mounted 6 10k single turn trimmers on it, and superglued and dead bugged the wires to the correct locations. Offset nulling on these boards was by 2 discrete resistors soldered in, the designers also did not have a high opinion of presets.

author: David_AVD
author_rank: Super Contributor
timestamp: July 06, 2012, 04:46:15 am
content: On the odd occasion I need to tweak a resistor value, I use a pot or extend a couple of wires out to the resistor so I'm not killing the pads from fiddling around a lot.

author: Psi
author_rank: Super Contributor
timestamp: July 06, 2012, 05:06:05 am
content: I use one of those old DSE rotary resistor selectors when trying to tune analog circuits.But i wouldn't say i use it lots.A pot works equally well in most cases.

author: David_AVD
author_rank: Super Contributor
timestamp: July 06, 2012, 05:27:46 am
content: Quote from: Psi on July 06, 2012, 05:06:05 amI remember those. I think I burnt mine out about 20 years ago! LOLFrom memory the selected value varied somewhat whenever I looked sideways at it.

author: Unknown
author_rank: Unknown
timestamp: July 06, 2012, 05:34:40 am
content: Note that power ratings for pots are only valid when the pot is set to its maximum resistance, so it uses all of the resistive material. If you set the pot to 10% of full scale, then current flows through only a small part of the material, so it can dissipate much less power. This maybe bad if you have it connected to a voltage source.