Making a Precision Selectable Variable Resistor - Page 1
Forum section: electronics / beginners

author: Legion
author_rank: Frequent Contributor
timestamp: May 08, 2014, 02:02:05 am
content: A lot of the time when I'm trying to figure out how a circuit works, I find it useful to replace resistors with pots and watch how the circuit reacts to different resistance values. Only problem is, with a rotary pot I can't accurately tell what the resistance is without disconnecting it from the circuit and measuring it.I have an idea to stack resistors in series in groups of 9 each of 10R, 100R, 1k, 10k, 100k and 1M and connect each stack with a 10 way switch. This way you could select a resistance value provided the resistors are reasonably precise. Schematic below.Two things I was wondering about before I went ahead. When looking for switches, the only thing I've found that would work are 10 way rotary switches, but they're pretty expensive (~$12 each), any suggestions for something more cost effective?For the resistors, I was looking at resistor arrays. I've never used them before, would these work?http://www.digikey.ca/product-search/en?FV=fff40001%2Cfff80007%2C40025%2C40067%2C40069%2C40070%2C40404%2C4046b%2C40936%2C1140050%2C3b80008&mnonly=0&newproducts=0&ColumnSort=1&page=1&stock=1&pbfree=0&rohs=0&quantity=&ptm=0&fid=0&pageSize=500

author: liquibyte
author_rank: Frequent Contributor
timestamp: May 08, 2014, 02:20:26 am
content: I did the same thing but used cheap dip switches. Worked great. 10 each of 1R, 10R, 100R, 1K, 10K, 100K, 1M and 7 nine position dip switches and a piece of perfboard.

author: quarks
author_rank: Frequent Contributor
timestamp: May 08, 2014, 06:13:54 am
content: There are some more options besides rotary and dip switches.But all is depending on the level of accuracy, quality and convenience you look for and your budget.- headers (very cheap but good/useful, look for Gerry Sweeney solution)- jumper cable (for perfection see beautyful robrenz Versa-Ohm Box solution)- Decimal thumbwheel Switch switch (see many professionlal and DIY solutions)- relays or motorized switches, if you need automationAlso a Hamon divider design could be interesting (see ESI SR 1010 and fantastic Conrad Hoffman articlehttp://conradhoffman.com/HamonResistor.html)Exept the one from robrenz (which I hope he will show us more about it and his genius skills) I have tried all or even still use them. If there is interest and if I find some time I can show some more details (especially Burster and ESI are awesome in any respect).BTW my personal DIY favorite is the decimal decoder switch solution (very high quality possible, combined with conveninet "digit readout")

author: krish2487
author_rank: Frequent Contributor
timestamp: May 08, 2014, 11:56:53 am
content: I remember Gerry on the forum coming up with his take to this problem.Heres the linkhttp://gerrysweeney.com/seven-decade-programmable-resistor-a-low-cost-solution/Maybe it already addressess your issues..

author: Legion
author_rank: Frequent Contributor
timestamp: May 08, 2014, 02:31:40 pm
content: Thanks for the suggestions. I like the Gerry Sweeney and robrenz devices. I'm kind of torn, because the cheapest I could make something similar to Gerry Sweeney's is $70. It'd be bulky, on perfboard, using through hole components. If I go SMT, I'm not sure how much it would cost, but it'd be $70 - $25(perfboard) + $x(custom PCB). He's asking for $20+shipping. So it'd definitely be cheaper just to buy his. On the other hand it does offer value as a learning experience as I've never had PCBs made or soldered SMT.

author: quarks
author_rank: Frequent Contributor
timestamp: May 08, 2014, 03:36:47 pm
content: if you are interested, I have some more of thesehttps://www.eevblog.com/forum/buysellwanted/fs-very-high-quality-hartmann-thumbwheel-switches/msg324329/#msg324329