Switch box

[Dannibou]

Hi. I want to build a resistor box and I need a diagram to realize this project. Example: I want to switch 10-20-30-40 ohms with 4 switch BUT if I switch the 40 and the 20 ohms, I want these in serie to get 60 ohms. I have DPDT to do that. Is it possible? Thanks for any help.
Danny

 

[Grossel]

No, you cannot.

However, if you're willing to increase the number of DPDT there may be possible to select them resistors to be parallelled or in serie. It would probably be clumsy anyway.

 

[Nigel Goodwin]

The question is rather vague, but it's simple to do with four SPST switches. You simply put all the resistors in series, and fit a separate SPDT switch across each resistor, each switch can then either short it's resistor, or put it in series with the others. The values you mentioned would allow you to get from 0 ohms to 100 ohms in 10 ohm steps, but would be rather clumsy to set.

 

[Dannibou]

No, you cannot.

However, if you're willing to increase the number of DPDT there may be possible to select them resistors to be parallelled or in serie. It would probably be clumsy anyway.

Thank for the advice. That what I thought. I think an 3PDT would do the job. Agree with this?

 

[Grossel]

Was thinking about Nigel approach, but there is a huge risk of making a short circuit when all switches is set to off.

 

[Dannibou]

OK

 

[Pommie]

If you make the resistors 10, 20, 40 and 80 then you can go from 0 to 150 ohms in 10 ohm steps using Nigel's method. Make the switches so that down is off and then all up is zero and for any other value just add as needed. EG, for 100 ohms you need 80 + 20 down, 70 = 40+20+10 etc.

Mike.

 

[Dannibou]

Here is the diagram I did but it won't work as I wish Note that the G and Arrow are probes input

 

[Mickster]

Taking the suggestions from Nigel and Pommie, the below should give what you require (+ an additional 50 Ohms)

EDIT:
I noticed you removed the part from post #6, where you weren't concerned about a short.
Please clarify whether this is no longer acceptable.

 

[Dannibou]

Taking the suggestions from Nigel and Pommie, the below should give what you require (+ an additional 50 Ohms)

Bingo. You got it Mick. So simple that I did not think about it. Congratulations and thanks
Danny

 

[Mickster]

Happy that works for you Danny, although most of the solution came from the combined input of Grossel, Nigel and Pommie.

You should thank them.

 

[JonSea]

I would put the resistors across the center switch terminals, and leave the short on the pair of outer terminals. When the switch is open, the resistor is in the circuit. When the switch connects the shorted terminals, the resistor is shorted out.

 

[AnalogKid]

Was thinking about Nigel approach, but there is a huge risk of making a short circuit when all switches is set to off.

Not if there is no switch across the 10 ohm resistor. Put that SPST switch in series with the string, and you have 5 combinations: open, 10, 20, 30, 40. Of course, having the 5th option be zero rather than open might be desirable, depending on where the resistor box is used.

ak

 

[Pommie]

If there is no switch across the 10 ohm resistor then the range becomes 10 to 150 in 20 ohm steps. That is not what the OP asked for.

However, you seem to be suggesting a different approach, how does your 5 combination work?

Mike.

 

[Nigel Goodwin]

Was thinking about Nigel approach, but there is a huge risk of making a short circuit when all switches is set to off.

As I mentioned, it would give from zero to 100 ohms, which 'might' be advantageous for him, we don't know, as his post was too vague.

As AnalogKid has already mentioned, don't fit a switch across one of the resistors, this will make that value the lowest setting.

 

[gophert]

As I mentioned, it would give from zero to 100 ohms, which 'might' be advantageous for him, we don't know, as his post was too vague.

As AnalogKid has already mentioned, don't fit a switch across one of the resistors, this will make that value the lowest setting.

Use rotary switches to insure you never have a short.

**broken link removed**

 

[Pommie]

As AnalogKid has already mentioned, don't fit a switch across one of the resistors, this will make that value the lowest setting.

Assuming no switch across the 10Ω then the values obtainable will be 10, 30, 50 etc. I.E. 20Ω steps.

Mike.
Edit, probably best keeping all 4 switches and add a 10Ω resistor in series so making the range 10 to 160 ohms.

 

[audioguru]

In my career and hobby, I have never used a resistor switch box and have never used EENIE, MEENIE, MINIE MOE to design a circuit. Instead I use datasheets and Ohms Law. Occasionally I use a trimpot to select a resistance to make a voltage, current or frequency that works best.

 

[Grossel]

. . . . . EENIE, MEENIE, MINIE MOE . . . . . .

Eh,what's that - a fairy tail device of some sort?

 

[Mickster]

Eh,what's that - a fairy tail device of some sort?

Picking at random.