Rotary Switches power rating for decade box

[Tomer]

Hi All and thank you for taking the time to read and help.

I am planning to build a Decade resistor box. The resistors will be power rated for 3W and 0.1 tolerance.
1) that's because i believe that 3W will cover most needs.
2) For some reason they are cheaper (compared with 1W on selected ohm's)

I am trying to understand what is important when chosen a rotary switch if I want it to match the box specification
I am referring to the power rating values of the switch. I already know that it needs to be 1 deck, 1 pole, 10 positions and non-shorting.

Thank you
Tom

[Gyro]

I think you want 'shorting' otherwise you'll break the circuit every time you change value and if working at significant voltage (hopefully not), you could also get contact arcing.

With shorting wafers, worst case current rating will be the current that resistors in the lowest decade take when dissipating 3W. You can probably economize on higher decades but you probably want the same 'feel' on each decade anyway.

This is probably going to cost you a bit with 0.1% resistors. You might want to look for an old, good quality one on ebay.

[Tomer]

Thank you for you help
1) i am not fully understand the shorting / non shorting. Thank you for alerting on this. I need further reading on the subject.
2) Do you think 0.1% is overkill ? I am planning to study/build pro audio gear, and general use.
The resistors adds up to about 80$ currently.

Thank you
Tom

[TimFox]

The other name for a "shorting" contact is "make before break", so the moving contact moves to the next fixed contact before losing connection to the previous fixed contact.  This means there will never be an open circuit as you move the switch from one position to the next.

[Gyro]

Yes, 'make-before-break' is a much less confusing term.

With regard to resistor tolerance, it's really down to how much you want to blow on resistors (and cost of resistors that you blow  ). If you're wanting to extend it's use to more precise stuff, then it's worth it. If you're simply using it to select the value of a 1% resistors in a circuit then it's probably overkill, probably one that only you can answer.

An alternative strategy would be to use 0.1% for the higher decades and then drop to 1% or lower for the less critical low decades, maybe even selected 5% on the lowest if you're using 3W wirewounds. The other question to ask yourself is 'do I need 3W resistors on the higher decades?' that is determined by the highest voltage that the box is ever going to see at those resistance levels.

Only you can answer those questions for your intended use but you may be able to save yourself a significant amount of cash (assuming you want to).

[Brumby]

The other question to ask yourself is 'do I need 3W resistors on the higher decades?' that is determined by the highest voltage that the box is ever going to see at those resistance levels.

Indeed.  To get 3W dissipation:
1K     => 55V
10K   => 173V
100K => 548V
1M    => 1732V

I know if I was playing around with 500V-1kV or more, I probably wouldn't be using a substitution box.

[Tomer]

Thank you guys for all the details and great info !
I am apologize for my late response.
I think I need to give some more thinking about the box and common uses for me.

Thank you
Tom

[amspire]

When you are talking about 3W resistors, I just wonder if you are going to have a lot of low value resistors such as 1 ohm or even 0.1 ohm?

The thing is that cheap rotary switches can have significant contact resistance - I have often seen 20 to 50 milliohms. If you have 7 decades, that can be 350 milliohms of switch resistance alone.

A real quality switch can have 1 milliohm contact resistance or less, but they can also cost a lot of money. Hundreds of dollars for one switch is not unusual.

If you need accurate low value ohms, you may be better off with one box with 3 seperate chains of series resistors:
10 x 0.1
10 x  1 
10 x  10 

with a terminal for each resistor junction. These resistors can be 3W.

You could then probably make a decade box with much cheaper lower power resistors for doing values in the 100 ohm -100Mohm range.