I’m building a low resistance decade box (basically). I need to get to resistances as low as 0.05 ohm.
The plan:
1 solderful breadboard: DKS-SOLDERBREAD-02 DIGIKEY STANDARD | Prototyping, Fabrication Products | DigiKey
Dip switches for switches: - NEED LOW CONTACT RESISTANCE
Resistors (to be turned on and off): 0.08 ohm, 0.13 ohm, 0.25 ohm, 0.5 ohm, 1 ohm, 0.5-10 ohm pot, 1-100 ohm pot.
All resistors are in parallel and have their own dip switch.
Only two pot pins are used to act as variable resistors.
The pots mostly act to dial in the resistance precisely.
With this setup (assuming no contact resistances) - I can do resistances from 0.5 ohm to 100 ohm with high accuracy just by using using the dip switch to turn on or off resistors.
Questions: Should I be worried about thermal effects at the lower resistances? And can I get a dip switch with contact resistances below 0.05 ohm? The ones I found all seem to have an initial contact resistance of 0.05 ohm, which effectively increases the minimum obtainable resistance by that much.
A slightly modified circuit representation is shown here.
Hello @alex.bernard,
Welcome to the DigiKey forum.
You nailed it! The contact’s resistance is the Achilles’ heel of your project. In addition, the wire length, contact resistance, resistor lead length will all impact your design. The external wire used to connect the decade box will also impact the results. Switch age and contact oxidation are also problems.
Thermal resistance is a small factor given all the other considerations.
Options:
- better performing DIP switch
- rotary switch with silver plate contacts
- MOSFET-based selection (assuming you are using the box in a DC system)
Sincerely,
Aaron
P.S. Are you familiar with 4-wire resistor measurements techniques? This is the best way to determine true resistance. Look for the option installed in high end bench multimeters.
1 Like
Hello alex.bernard,
I checked for some SPST through hole DIP switches, and most are 0.050 ohms initially.
I found a series that list the contact Resistance of <25m Ohms initial, which is the KT series from CIT. Click here for 2449-KT01EH-ND, which has a raised actuator. Click here for 2449-KT01RH-ND, which has a flush or recessed actuator.
Since you will be switching these frequently, I would suggest going with the raised actuator.
Click here for the datasheet.
Thanks Aaron.
I’ve got a great microohmmeter from keysight/agilent that can get 7 digit precision on the 4-wire resistance, so i’m good there. It is a DC system. I have no clue what a MOSFET is so I’ll look into that.
Would a rotary switch only have one of the circuits closed? I want to be able to really dial in the resistance which requires having each resistor turned on or off but in parralel with the others.
Thank you David,
I found this datasheet that implied <20 mOhm for the 90B configuration: https://www.digikey.ca/en/products/detail/grayhill-inc/76SC04T/2680734
It’s a complex circuitry it seems though.
Also should I be considering the force of the switch as an important parameter? I’m assuming a higher force = lower contact resistance?
I would choose a non shorting type so only one circuit would close. Otherwise you would briefly have two circuits with a shorting version.
Hi alex.bernard ,
I thought to point out regular deoxidation of contacts will help in low resistance applications. Pure silver is known to tarnish, although a better conductor when not contaminated - some switches use a silver alloy to reduce tarnish formation of silver, especially in low-voltage applications, although the alloy will add a slight increase in resistance but may offer more consistency.
Ref. Cleaning & Protecting Contacts, Switches, & Potentiometers
Hi @alex.bernard,
Is that HP/Keysight/Agilent? 
I know the instrument.
There are many different ways to construct a rotary switch. The variety is limited by physical size and available money. An off-the-shelf unit for your configuration would likely be six-pole single-throw. Generally, these are single connection break-before-make units. Here is an example with silver-over-nickel contacts, an initial 15 mΩ resistance with a lifetime limiting current capacity in the 100 mA range.
As a side note, you may be better off using a number of heavy toggle switches. Here is an example with an initial 10 mΩ resistance. There may be others so better suit the size and cost of your design.
Please let us know how you designed the circuit.
Sincerely,
Aaron
