General > General Technical Chat
Reinforce springiness of the brush contacts of a DMM rotary switch
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RoGeorge:
Can the brushes in a DMM rotary switch be "steel quenched"?
(not sure if that's the correct term, want them to press harder on the PCB contacts, and be more elastic)
I have a cheap clone of an my64 DMM (3.5digits) but with absolutely superb LCD visibility and instant power up. Other much better and more expensive DMMs I have are absolutely horrible in display readability, speed and battery consumption.
I don't know what happened with that type of crystal-clear 7 segments only type of LCD, and with the black blob die straight on PCB stile of ICs, or why they went extinct, but I digress.
The problem with this my64 clone I have is bad contacts in its rotary switch. Under the microscope, the contact springs and the PCB plots look clean. I suspect, the cause is that the mobile brushes lost their springiness, or didn't have enough of it.
Last time I've open it, some years ago, cleaning with izopropil 90% didn't fix it (contacts were clean already). Bending the brushes a little more, made it work a few more years, during which years, it slowly become more and more unreliable.
If iron can be hardened into steel by adding carbon then fast crystallization (for example, turn an iron nail red hot with a torch, then throwing it in a jar of water will harden the metal), would that work, too, for the brush contacts alloy?
Would prefer very much to keep this DMM, any other advice for how to fix the rotary contacts?
ebastler:
Beryllium copper is the "proper" material for spring contacts, although I don't know whether there is an easy way to confirm that this is what is actually used in your meter. Heat treatment and annealing are indeed used to strengthen beryllium copper: https://materion.com/about/new-at-materion/in-our-element-heat-treating-copper-beryllium-parts
Not sure whether it's a good idea to try and do that with the fully formed contacts in your meter though; it might be an effective way to ruin them too... Depending on how the contacts are arranged in the meter, maybe it is possible to insert a thin layer of springy/spongy material above them to help push them down towards the PCB?
coppercone2:
no you need to work harden that material (brass or better bronze). it hardens from that. its not heat treated to spring form. brass is the lowest quality choice, get bronze
you could bend like spring bronze sheet cuttings to proper shape and solder them in place with a big lap joint maybe. or remake the whole part by etching bronze sheet
they are manufactured by stamping it into place and I think you can get spring bronze/brass sheet too but it needs to be formed. I think agressive stamping gives it the correct parameters https://www.mcmaster.com/products/spring-tempered-bronze/
i imagine beating it into a die with hammer and special anvil might help (like repeditive)
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