Advertisement

Blog

This Little Switch Went to Market

In my last article (The Diabolical & the X-Ray), I got the chance to talk about remote-controlled switches and diabolical devices. This time I want to talk about selecting a mechanical switch. I will limit this to basic snap-action switches and their specifications.

Switches with a small form factor, typically 0.5 x 0.5 inches, can usually handle up to 3 amps. The smaller the switch size, the shorter the travel is. That means lower voltage rating (contact spacing when open) and the smaller the amount of current that can be handled (contact size). Another specification many of us are concerned with the operating force of the switch. To maintain a good contact, strong springs are needed.

If you are looking for light-weight switch action then you will be limited in both size and current. In snap-action switches, the useful current capacity range is from a few milliamps up to perhaps 30 amps. Switches are also rated with different specs for DC voltage and AC voltage. There should be details for the normally open (NO) position, typically called insulation resistance. You can see 100 MΩ or higher value. This rating is at a stated voltage higher than normal operating conditions.

Reliability is very key in mechanical switches. It not unusual to have a mechanical life of more than one million operations. The electric specification can be equal to the mechanical life. Typically though it is lower — sometimes only one-tenth the mechanical life. I have seen some limited-production switches with a mechanical life of 25 million cycles. Price and delivery could set any project back, especially if your stipulations are higher than the design requirements. But in some cases, there may be just pennies in the difference between a basic switch and a more reliable version.

Another detail you should be concerned with is environmental qualifications. You should understand what conditions the switch will be used in and the conditions you might encounter during maintenance, storage, or shipping. Water is not the only item that can get into a switch. Contaminants such as oils and greases can stop a switch from functioning properly.

Another specification is outflow (although you'll see other terms that mean the same thing). This refers to contaminants coming from the switch and getting into a medical or similar delicate environment. The potential of heating in the switch elements could make the outflow of switch lubricant an issue in some instances. Contaminant leakages in and out are usually the same specification.

I will wrap up with a few last details. Actuators can range from being small pin plungers, to hinge-style levers, to panel-mounted roller plungers. The total travel is the total movement of switch without damage occurring (operating travel plus pre-travel). Pre-travel is the distance of the plunger button (actuator) movement from rest position to the point of operation. Pre-travel is often specified as a maximum amount.

The maximum operating frequency is the fastest rate at which the switch can be operated. There is typically a difference between mechanical and electrical speeds. Mechanical action is faster. Approvals for the switch are definitely a factor especially in interchangeability with other switches. UL94B flammability rating, IP50 terminal type, and IP67 lead wiring and splash rating. Check your requirement regarding UL, CSA, VDE, RoHS, and other foreign designators.

Related posts:

12 comments on “This Little Switch Went to Market

  1. RedDerek
    August 28, 2013

    Recently I need to switch up to 10A in a small form factor. The switch could only handle 100 mA. Solution was to use the switch to turn on a MOSFET that could handle the higher current.

    Second challenge was that all I had was a 1.5V battery and to get the MOSFET to fully turn on I used an LED driver to boost the votlage. Benefit was that I was able to show the switch turning on as the LED turned on as well.

  2. PZman
    August 28, 2013

    I love the idea with the LED driver, don't know why I didn't think of that. Maybe cause I love switches & relays in general. Gold Star to you, RedDerek.

  3. jkvasan
    August 29, 2013

    Peter,

    Nice post.

    I was just wondering if a hybrid switch (switch+triac) is possible to make. This way, the switch need not have a high current carrying capacity and can look cute. The triac could take all the load.

    I also see some limitations in this approach.

    1. Possibly the size may not become small as the housing needs to include the triac.

    2. Triac needs zero cross triggering which may need additional circuitry.

    3. May not be suitable for all types of loads.

    First of all, Is this possible and available?

  4. Vishal Prajapati
    August 29, 2013

    @JK, it is a nice idea to have Hybrid switch. I haven't found anywhere this type of switch readily available.

     

    Your concerns for specs and limitations are correct.

     

    Apart from that you also need to think about snubber circuit. You will most likely not be able to integrate it inside the switch. As snubber has to be designed according to load characteristic.

  5. samicksha
    August 29, 2013

    You are right @Jayram, In a typical triac, the gate threshold current is generally a few milliampères, but we have to take into account that,the higher the temperature, the higher the reverse currents in the blocked junctions.

  6. jkvasan
    August 29, 2013

    @VP,

    Protective circuitry needs to be incorporated. i would list the following

    1. Snubber

    2. Transient Voltage Suppressors – in case of inductive loads

    3. Zero cross detection and triggering circuitry.

     

  7. Vishal Prajapati
    August 29, 2013

    All this circuits will take lot of space and I don't think it will fit in the switch form. Probably it can become a module but it will be probably 5 times bigger than normal switch. And also you can design general snubber circuit which can fit all type of loads adequetely.

  8. Brad_Albing
    September 3, 2013

    @Peter – just an additional note on the low current considerations (cf. high current). When switching low currents (e.g., audio signals, logic signals) of a few mA with a toggle switch, it's best to use switches with gold plating or gold flash on the contacts. Conventional silver plating will build up a thin layer of oxide or sulfide (airborne contaminant) which is of course not especially conductive. Copper contacts will also build up an oxide layer. At higher applied voltages, tiny arcing thru these layers will occur, and if the current flow is high enough, the oxide layer will be vaporized.

    Sometimes, slide switches with silver plating will be OK. The wiping action will clear the contacts.

  9. PZman
    September 27, 2013

    Brad, I was wondering about the wipe action on silver based switch contacts. If the wiping does not clear it what is your and others recommendation. In rebuilding tube amps I come across these switches with noise. The switches have been discontinued long ago. I have always debated that a ultrasonic bath will clear them but for how long? I used to use Freon based cleaners, thats over. I remember using a pink fluid but I don't remember what it was and is it still available. Thanks all!

  10. PZman
    September 27, 2013

    JK, don't forget about the G-forces in the switch. A toggle and a spring loaded pushbutton can have a high load due to the speed or the action that the internal contacts make. I have had small components mounted on the rear of a switch and after a year or so the leads on the cap went intermitten.

  11. Brad_Albing
    September 27, 2013

    @PZ – yep – those slide switches do get noisy. As the silver wears away, that is the inevitable result. If it were me, and if it's physically practical, I'd install small C&K (or similar) toggle switches with the gold flash contacts.

  12. jkvasan
    September 28, 2013

    PZman,

    Valid point. At the very moment, mechanical action and current surging in can create havoc , as they are almost simultaenous. All these need to be factored in. It is not just an electronic design, I agree fully,

Leave a Reply

This site uses Akismet to reduce spam. Learn how your comment data is processed.