Resistance Decade box

[geekygenius]

Hello, I have decided to build a resistance decade substitution box. I have decided on using this as my selector. I'm planning on buying two of them. I know I want 7 of the digits to be in the 1ohm to 1Mohm range, The only problem I have, is that I don't know if I should have the eighth digit be .1ohms, or 10Mohms. Not having as much experience as many of you here, I was wondering which you would find more useful? If you were to build this, which decade would you choose? I'm planning on using 1% resistors.

Thanks!

 

[john monks]

I think I would go up by 10Mohm range. The reason is that the leads and switches alone may give you almost 1 ohm of resistance. So trying to select in that range strikes me as being a bit silly.

 

[gorgon]

10Mohm would be my selection, as long as the switch is up to it.

A 0.1 ohm range will normally be used with high currents, normally not supported by a small 'logic' switch as described.

A possible problem you may experience, when using high value Mohm resistors are the quality of the mechanical switches used. Some switches need a minimum current to 'make contact', or break through the oxide layer on the contacts, before full connection is achieved. This may result in an undefined resistance value in the high range.

 

[woodchips]

There has to be a reason why most commercial units are mostly 4, sometimes 5, decades. When I use them it seems either to be for high or low resistance. So, I would make one from 1R to 10K ohm and one from 100R to 1M ohm. If these don't work out can always change the resistors or make another.

Bob

 

[duke37]

The reason for limiting the number of decades is the accuracy of the resistors Consider changing from 99999.9 to 100000.0. Will the resistance go up or down?