Micro Switches - Incorrect Schematics - Page 1
Forum section: electronics / general

author: SgtRock
author_rank: Unknown
timestamp: June 23, 2014, 02:54:01 pm
content: Greetings EEVBees:--I was rounding up and inspecting all of my used Micro Switches to sell on eBay, see pictures below, when I noticed a couple of things. Firstly, I had to discard about half of the switches because they had scorched contacts. My experience tells me that once contacts cannot be made to shine with a couple of strokes of a brass bristle brush, they will tend to spark and eventually fail, or go intermittent. This is often the cause of the loud pop heard from some older microwaves. People are selling these used on eBay at $5 a whack and more, without inspecting the inside. Russian Roulette!!--Secondly, please look at the schematic drawings on the switch top covers. They are all marked just the same, with a connection to NC, even though the bottom two, with the red mark are normally closed, and the other 9 are normally open. There is just no way the same diagram can be correct for both. So apparently every switch must be checked with a DMM. Since if you install according to the diagram, you could be in for a big surprise."All right, I'm ready to reform. Which one's got the orange juice"Alfred Hawthorne "Benny" Hill 1924 - 1992Best RegardsClear Ether

author: What_NZ
author_rank: Regular Contributor
timestamp: June 23, 2014, 09:15:10 pm
content: Well I wouldn't buy second hand microswitches for reuse on anything high current but on low current I suppose they would be ok and at $5?? I think it is too much. Anyway that's not really the point of my post.The diagrams are 100% correct. There is no surprise. The switches are different!For the switches (at the top) of your picture there is a spade in the NO position i.e when you connect a wire to the Common spade and a wire to the NO spade. The "circuit" is NO Normally OpenFor the two you marked in red (at the bottom) of your picture there is a spade in the NC position i.e when you connect a wire to the Common spade and a wire to the NC spade. The "circuit" is NC Normally Closed

author: AmmoJammo
author_rank: Frequent Contributor
timestamp: June 23, 2014, 09:55:46 pm
content: I'm confused, microswitches cost less than $4 new.

author: SgtRock
author_rank: Unknown
timestamp: June 23, 2014, 11:39:03 pm
content: Dear What_NZ:--Thanks for your help. I was aware that the switches are different, that is why I, in my naivety, questioned the fact that the cover plates are identical. Please look carefully at the two switch covers marked what01 and what02 in the below pictures again. Both show the line from C1 making contact with NC and neither making contact with NO. My question is. If you removed the red mark how would you tell from the diagrams (the diagrams not the spade lugs), which switch is NO?--To Ammo: Yes I agree micro switches go for less that $4. If you look at the below picture of an eBay page you will see that they also go, used, for about $5, and there are other listings for even more money. I hope that clears up the confusion. I am attempting to sell these 11 switches for a starting bid of $14.99 with Expedited Shipping included.--Thanks for the comments."I believe that at the end of the century the use of words and general educated opinion will have altered so much that one will be able to speak of machines thinking without expecting to be contradicted."Alan Turing 1912 - 1954Best RegardsClear Ether

author: What_NZ
author_rank: Regular Contributor
timestamp: June 24, 2014, 12:10:13 am
content: Quote from: SgtRock on June 23, 2014, 11:39:03 pmDon't think of the switch in the way of being NO or NC. Think of it as, What the writing next to the spade says.Example:If the switch has 3 spades i.e. spades at COM, NC and NO then it is capable of both NC and NO configurations.The default position (Normal) is always "with the button not being pressed"If you connected a multimeter probe (continuity mode or resistance mode) to the COM spade and the other probe to the NC spade then it would be a short circuit i.e. NC Normally ClosedIf you then connected the probe to the NO spade then it would be an open circuit i.e. NO Normally OpenSo the diagram is just an indication of the function. The COM, NC and or NO spade terminals are the function.To answer your question specifically from just looking at those two photo's - I wouldn't have a clue. If you showed me a picture with all the spade terminals shown as well, then I would know immediately.

author: tom66
author_rank: Super Contributor
timestamp: June 24, 2014, 12:23:06 am
content: Hmm, I would always interpret NC as being the undriven state - i.e. switch contacts in their default position. Is this not always the case...?

author: SeanB
author_rank: Super Contributor
timestamp: June 24, 2014, 12:23:39 am
content: Look carefully and you see the common is always the same position opposite common, but the NO or NC will always be opposite the marking. These switches are a speciality item to use in microwave ovens, where the NO switches are used in pairs as door interlock switches so that power is only connected when top and bottom of the door is latched. The NC switch is used in the middle of the door, and shorts the transformer when the door is open. This safety interlock prevents operation with the door open or if the NO switched weld shut from either old age or overcurrent. On old units the switches will be somewhat eroded, and if it has been blowing the mains fuse then the NC switch will either be welded shut or will have bad contact erosion.Why anybody would use an old one when the new ones are cheap ( at least here in SA) and readily available. I do keep them from old microwave ovens as they do fail, and the old one is a quick way to confirm it was the switch, though many ovens have issues with the plastic door moulding warping so that it blows a fuse every so often. I keep a box or two of the 10A ceramic fuses around as well, as you can easily go through a half dozen in checking an oven with dicey door switches. Do not use glass fuses, as they will explode. You also come across a resistor in a glass case that is used to reduce the short current so you only blow the fuse and not the breakers when the safety interlock works. It is marked with a red label. Funny is the lamp failing often also blows the fuse, so I keep both types of lamp spare.

author: What_NZ
author_rank: Regular Contributor
timestamp: June 24, 2014, 01:00:47 am
content: Quote from: SeanB on June 24, 2014, 12:23:39 amJust a side interest (maybe only related to Panasonic Microwaves) we told the engineers about the light failing and blowing the fuse. Their reasoning was it would stop the customer from attempting repair themselves and take it to a service centre. It became annoying to us when it blew the fuse tracks on the PCB also. Their solution (if I remember correctly) was to put more fusible track positions on the PCB.

author: What_NZ
author_rank: Regular Contributor
timestamp: June 24, 2014, 01:12:24 am
content: Maybe this will help too - Copied from the websitehttp://arcadecontrols.com/arcade_wiring.shtml

author: mamalala
author_rank: Unknown
timestamp: June 24, 2014, 01:21:20 am
content: Those switches are labelled just fine.They are not dual throw, but single throw. The markings "NC" and "NO" are there because the switch can be either Normalle Closed or Normally Open (Normally = no action on the lever/pin/whatever). The spade will be roughly aligned with either the NO or NC marking, and thus tell you what the switch is.A switch can not be NO and NC at the same time anyways, so it should be clear that the marking is to be read in conjunction with where the spade comes out.Greetings,Chris