# Precision resistance box

#### DarthVolta

Joined Jan 27, 2015
521
I want to make a switchable resistance box, and rotary switches are nice, but I want to practice on analog switch IC's and digital, and they are cheaper than the rotary switches, and I could even and use an MCU and add a display and program the resistance that way. At least for the higher ranges right?

What sort of analog switches would be good enough to not matter in a resistance box? Or is that a bad idea if you really still want the precision?

Thanks to SamR I'm making an order list from Tayda, so what % resistors fo people usually use ? I can always replace the resistors later with better ones, I'll at least get 1% resistors.

#### ronsimpson

Joined Oct 7, 2019
1,125
analog switch IC's
Read the data sheet. Most analog switches can not have voltage on the resistors above or below the supply. Many switches have 50 ohms inside the switch. Read the data sheet.

I got switches like these. Wired 9 1% resistors to the PCB on the back.
0=short, 1=1k1%, 2=2k1%, ,,,,,9=9k 1%. Get 1 ohm, 10 ohm, 100, 1k, 10k, 100k, 1meg (9 of each) (get 10 because one will hide on the floor)
The switch part can take 200V and the analog switch can not! The switch resistance is a fraction of a ohm.

#### SamR

Joined Mar 19, 2019
2,932
I assume you mean a resistance decade box? Good Research Grade ones are very expensive. That said, here is a decent and relatively inexpensive one if a bit kludgy to change resistance. 1/2W 1%: https://www.amazon.com/Electronics-...d=1&keywords=decade+box&qid=1592190227&sr=8-8

There are also a couple of capacitor decade boards very similar but with dip switches so a bit easier to use. There is also the decade box kits by Elenco.

They are a bit handier than swapping out parts when playing around with a circuit.

Edit: I did upgrade the banana jack sockets on one of them as it came with crap connectors.

If not in a hurry they are much cheaper on AliX.

#### crutschow

Joined Mar 14, 2008
26,058
The problem with such a resistance box is that the overall resistance tolerance is determined by the largest resistor connected, so for example if the largest resistor connected is10k±1%, then no matter what other smaller resistors you put in series with that, the tolerance is no better than 1% of 10k or 100Ω.
You thus probably want to use no worse than 0.1% tolerance resistors, at least for the larger values.

#### DarthVolta

Joined Jan 27, 2015
521
I guess making a substitution box. But I do want to make a little resistance/cap reference box too, at some point.

Right, I should wait till next month and bulk order the parts, after I decide on a version to make. Some nice bananna jack/plugs posts got here today, I could make a mix of 5% and 1%, but I rather do better than that. I see 1 on amazon w/1W resistors, I guess that's worth it too as a substitution box for inserting into a circuit, and for more thermal mass......I don't mean the amazon price for it tho. But after I order the parts, sw's ,w/shipping it will be close

Last edited:

Joined Jan 15, 2015
5,733
What sort of analog switches would be good enough to not matter in a resistance box? Or is that a bad idea if you really still want the precision?
Some of the finest decade resistance boxes ever made were likely the Gen Rad (General Radio) 1433 series. The new cost was about $2K to$3K for a single box. Precision decade boxes use pretty expensive rotary switches and if you ever see one up close and personal you will know why.

"The 1433 Series employs stable, very-low-resistance switches with silver-alloy contacts. A special design keeps zero-resistance to less than 1 mΩ per decade. Self-cleaning keeps the silver contacts from becoming tarnished when unused, or when only low currents are passed through them".

Granted these were often lower level lab standards but it is worth noting that less than 1.0 mOhm is a pretty tight tolerance on a switch. Especially a switch passing a very low current. Some good images of a decade switch and the precision wire wound resistors.

Anyway yes, you can buy cheap decade resistance boxes using slide switches but when we throw "precision" into the mix things get complicated and expensive.

Ron