What makes for a 'good' rotary switch?

[Jeff_Birt]

After seeing a few teardown of the types of fancy rotary switches used for precision decade boxes and voltage references I have been thinking of what makes for a 'good' switch for this type of application. Some things that come to mind are:

1) Solid detent into positon, i.e. feel of switch
2) Low contact resistance
3) Contact material
4) Self wiping contacts
5) Break before make/make before break
6) Contact arrangement/number of contacts
7) Multilevel possible
Style: thumbwheel or standard

I have a machine shop at my disposal so I thought it would be fun to try and design a switch from scratch. What is your idea of the ideal rotary switch for precision instrumentation? Any links to switches you lust over?

[uncle_bob]

Hi

Often high end switches are purpose built. The best switch for a decade box is not the best switch for an RF attenuator. Neither one may be best for the front panel of a miniature radio.

Better to decide on an application first. Then design a switch that fits the needs (and costs) of that application. Solid gold is nice, but rarely makes it past the budget review.

Bob

[Messtechniker]

In some cases I use a fairly cheap rotary switch
to switch an array of SIL or DIP relays mainly because
the contacts of the DIL and SIP relays are sealed
thus protecting the contacts against oxidation corrosion.
In my opinion this is cheaper and provides more consistent results
compared to a low-milliohm gold plated rotary switch.

YMMV of course.

Yours Messtechniker

[dacman]

For low thermal (EMF) applications, gold clad switches, even in latching relays.  The coil in the relay can heat the contacts if held by current.  Also, the easier it is to turn, the lighter the detent can be.

[uncle_bob]

For low thermal (EMF) applications, gold clad switches, even in latching relays.  The coil in the relay can heat the contacts if held by current.  Also, the easier it is to turn, the lighter the detent can be.

Hi

... but to be *really* good, scrap this plating idea. Go for solid metal contacts. Gold if you can afford it, other things if you can't.

The issue is thermocouple junctions. Same / same / same throughout is the best idea. That way no nasty offsets. Of course if this is an AC (rather than DC) application none of that really matters much. Then other things come into play.

Bob

[calexanian]

we use about 20K a year of a specific PC mount rotary switch. That was enough quantity that GS switch http://www.tw-gs.com.tw made us a custom spec part. It is based on an industry standard part much like this http://www.mouser.com/ds/2/128/c5_6-2530.pdf we used Electroswitch for many years but the quality began to decline and the price went up. We went to GS and specified a PC mount 12 position switch with no stop with a metal D shaft, small signal non shorting gold plated contacts and standard 3/8" threaded mounting bushing and they did it without question. They are also very responsive with quality concerns and seem like they actually want out business, and all of this for a price less than a third of what we were paying before. We actually did have a QC problem about a year ago after about 4 years of no trouble. Some internal heat pressed internal tabs that held the wiper were breaking. It was not an extreme problem. It only showed itself while the customer was turning the switch during programming of the product and it would go back to normal when the switch was in detent, but none the less it was a problem. They traced the problem to their plastic resin supplier providing pellets with some recycled content and the thermal welding properties were being compromised as a result. The solution they came up with was to switch the spec to a transparent plastic which can not have any recycled or filler content or it would be cloudy or visible. They replaced all of our parts, told us to destroy the remaining ones from that batch, and compensated us for the QC time to screen our products and replace any failures from the field. When we confronted Electroswitch with our quality issues a few years earlier where the bodies of the switches were literally falling off of the switch wafer backs they told us some BS story of worn out tooling and a change of operator was causing the problem and basically told us they are still to spec and to pound sand, oh and our price was going up too. Within a few calls to a parts house we use we had new samples of the new part inbound and we never looked back.

[uncle_bob]

Hi

Ok, so there is a really good description of a "good" rotary switch. It's not as much about the switch as the quality of the supplier. It's also a pretty good walk through of a lot of the "why" people buy this switch rather than that switch. It's not only about contact resistance ....

Bob

[Jeff_Birt]

I appreciate all of the replies. It goes without saying that support from the manufacturer is as important as the product itself as when (not if) you have an issue they should be there to help solve it. This is not exactly the area I was interested in though.

For example, I have a ~16 year old Tektronix DMM that I inherited at my job. There are days I would  like to throw it against the wall as the selector switch is so awful. It does not firmly lock into position so the mode you set it to is a bit of a lottery. The contacts on the PCB are not gold plated or nickel plates but have something resembling carbon tracks. Even if you get it in the right mode it might decide to change on its own. Did I mention it is awful? (I did take it apart and clean it again a few weeks ago so it is back to being 'OK' now, but on the awful side of OK.) The problem with this switch is the lack of a firm detent and the poor choice of PCB contact trace coating/plating.

What I was after what aspect of mechanical design makes the switch 'good'. In this case 'good' being a generic qualitative measurement where the specifics are application based. For example for a high current switch you contacts are heavier (and probably a silver alloy), for a very low contact resistance you might want gold plating. For a high voltage switch application the isolation voltage is a concern.

So let's pick on application to make things easier. For a low contact resistance rotary switch what are the important aspects of the mechanical design. Perhaps a strange question on an electronics form but it is a large area where the two disciplines meet. I picked up a .1% 7 decade box from EBay last evening and I am interested to see how the switches are made.

[dom0]

In some cases I use a fairly cheap rotary switch
to switch an array of SIL or DIP relays mainly because
the contacts of the DIL and SIP relays are sealed
thus protecting the contacts against oxidation corrosion.
In my opinion this is cheaper and provides more consistent results
compared to a low-milliohm gold plated rotary switch.

This approach has other advantages as well.

- It's simple to make reasonably complex switching matrices without additional logic (for example range-switching with a single rotary switch across more than one attenuator / amplifier stage).
- You can go the other way and make it programmable without having to revise the analog design.
- It keeps signals local and requires no detour to the switch (unless you are using a cam switch -- typical example Tek 7k plugins - awful to maintain if they became dirty/dusty).
- For low {voltage, frequency, current} requirements relays can have many suppliers that are virtually interchangeable

[dom0]

As for low resistance rotary switches, an old but well-working mechanism are these:



(Image from http://amplifier.cd/Test_Equipment/other/Tettex.html )

Similar switches can be found in many older / better decade resistor boxes.

Detents on these are sometimes made with bearing ball(s) loaded with a spring against a "toothed plate" mounted to the switch shaft (don't have a better word).

The switch pictured:

Thermal EMF <20 nV
Contact resistance <0.4 m?
Insulation resistance >1 T?
Current >20 A

If I remember correctly this kind of switch was invented in the 40s in the netherlands(?) specifically for decade resistors.

[Jeff_Birt]

The circular contact has a button of a different material on the surface. Perhaps gold? It is similar to how high current switches/contactors will use a button of silver.

The moving contact 'leaf' is very interesting. It appears to be a lamination with the top 2-3 leaves not making contact, I'm wondering if they are beryllium copper which is used to make springs.

Thanks so much for the photo, it is very interesting.

[calexanian]

multi layer contacts like that were very common before the more modern plating came along.

[metrologist]

This is great! One of my lathes is missing it's reversing On/Off switch and I wanted to build one. It will control a single phase 110/220 AC motor. I am eager to see any plans and share ideas.

[ajo518]

I would agree with others its all about application. One thing I believe to be universal though is that it needs to have the correct feel/ detent. I think this is something that has been lost in lots of mass produced items.
If you are going to have a rotary switch then you always want to know where it is.

The leaf design is cool and looks like it should be robust.

[uncle_bob]

I would agree with others its all about application. One thing I believe to be universal though is that it needs to have the correct feel/ detent. I think this is something that has been lost in lots of mass produced items.
If you are going to have a rotary switch then you always want to know where it is.

The leaf design is cool and looks like it should be robust.

Hi

I agree that feel is an important parameter. Even that is application specific. The amount of detent in a Wheatstone Bridge rotary switch is a bit different than the "clunk" of an enable / disable switch on a large machine. In the first case, you are moving it all the time as you do stuff, major detents lead to fatigue. In the second case, you very much do not want to switch things by accident. The force is a feedback to ask you "do you really want to do this?".

Bob

[SeanB]

The circular contact has a button of a different material on the surface. Perhaps gold? It is similar to how high current switches/contactors will use a button of silver.

The moving contact 'leaf' is very interesting. It appears to be a lamination with the top 2-3 leaves not making contact, I'm wondering if they are beryllium copper which is used to make springs.

Thanks so much for the photo, it is very interesting.

That will be either a bronze alloy or a brass, selected because it is machinable to make the contact, and a good electrical conductor. the wiper is made from Beryllium copper sheet, with the multistrip construction used to make the contact a self wiping spring on the contact.

Very low noise when at rest, and most importantly contact resistance stays relatively constant in operation provided it is kept clean by being operated every so often to wipe any film off the contact surfaces. Con of these is the power rating is poor for the size, at least if you want to keep the contact wear down to low levels. If you want high power then you change the wiper from copper strip to a carbon brush with a high copper content, to get a long life high power contact, but which will have a higher contact resistance than copper, but capable of handling very high currents.

[Jeff_Birt]

I have worked with high current switches and contactors that could handle 600A or so and I never saw one with a carbon brush. The contactors usually have silver contacts as it self cleans very nicely when switching.

I received the Ohmite decade box and snapped a picture of the switches. The switches have a ceramic body and the contacts has a silver colored plating (not sure what material it is will look up the part number this evening.) Fours of the 1K resistors on this unit were burnt but the switch contacts look fine, no discoloring, etc.

[CM800]

Love the way that switch is designed to it lifts up the contact from the current connection, breaking it, then goes down to hit the second one on top of it squarely by following a wheel over a 'wiggly line'

[Jeff_Birt]

It do lift up and then come back down on the next contact. It does not have a firm detent into position though, you feel it drop into position and you have to turn it a bit more and you feel it stop and the number shows up in the window.

[uncle_bob]

It do lift up and then come back down on the next contact. It does not have a firm detent into position though, you feel it drop into position and you have to turn it a bit more and you feel it stop and the number shows up in the window.

Hi

I would take a close look at the lubrication and the structure of the switch. I'd bet that "as designed" it did have a fairly solid detent. The springs or some other part of the assembly may not have held up for the (who knows how many) decades since it was built and the use it was put through.

Bob