This is my first post on this forum, so please bear with me. My question is about switches on portable power tools. My Bosch jigsaw, Porter-Cable router table and small table saw all have a switch that has contacts (like points in a car ignition) inside it. The contacts have all pitted and burned very quickly. What causes this? Is not enough current or voltage getting to the tool or too much (not likely)? Will putting a different switch on them that won’t burn out cause the motor to be damaged instead?
I’m sure there’s an electrical expert among the readers out there.
Thanks, Ben.
Replies
Ben,
Some burning and pitting of contacts is normal and, as long as the switch functions properly, the condition can be ignored. So the first question I would ask is have the switches actually failed to work or did you discover the condition of the contacts while doing some other maintenance on the tools? On most portable tools the the switches are permanently sealed so I'm guessing that the switches failed and you pried them open to see what was happening.
Simple on/off switches in portable power tools don't serve any protection function, so you could safely use a different switch without damaging the motors. The simplest solution would be to leave the tool's original switch locked on and control the power from a new switch box that the tool plugs into.
Dust and oil getting on the contacts are a well known cause of pitting and burning. A well designed switch should keep dust and dirt out, some manufacturers are better at this than others. I don't think that low voltage would ordinarily cause the switch problem but low voltage can be damaging to motors by causing them to overheat.
John W.
John,
Thanks for the info. The switches did fail and I opened them to see if I could fix them. I managed to get them working again, but only for a very short time. So I guess I can wire in better switches and not worry about damaging the motors.
I'm just curious why the contacts are in there in the first place if they are not to protect the motor from low voltage or amperage?
Ben.
Ben,
Contacts serve only one purpose in simple switches, they open and close to make and break the circuit. Over current protection can be incorporated in a switch, but that protection is provided by additional components in the switch that will trigger the contacts to open, the contacts themselves don't gauge the current.
John W.
There's something in the logic of this solution that isn't making sense to me. If the switch has failed because of pitting and corrosion, then by definition it's not properly conveying electricity across the contacts, right? So...closing the defective switch and using another switch to turn the machine on and off leaves you with a motor that's not getting the correct amount of current fed to it.
Seems like if you want to avoid the bad switch and install a different one (without actually replacing with a new tool-specific part), then the old switch should be removed from the picture to eliminate that weakness in the power supply.forestgirl -- you can take the girl out of the forest, but you can't take the forest out of the girl ;-)
I am going to pull out the old switches and wire in better switches that don't have those contacts.
Thanks for all the responses.
Ben.
Low voltage at the tool will tear up a swich.
You have a heavy enough of an extension cord? Light gauge cords or one that is to long is not good either. 120VAC at the service panel? (+/- 2/3 volts.) 110 / 115 will make a tool suffer in the long run.
Who ever invented work didn't know how to fish....
Yeah, I presumed the old switches were being replaced or bypassed.
John W.
Quote from #2 (that's you): "The simplest solution would be to leave the tool's original switch locked on and control the power from a new switch box that the tool plugs into." Seemed odd.
forestgirl -- you can take the girl out of the forest, but you can't take the forest out of the girl ;-)
Edited 9/3/2003 6:07:18 PM ET by forestgirl
Dear First Lady of the Forest: Hmmm I thought you once told me that you didn't know anything about electricity,just read your postings sure seems to me that you know a heck of alot more about it than your letin on there kido..LOL.......
ToolDoc
Hello FG,
I've worked extensively in the past in avionics and particularly weapons and control systems on fighters in the USN, somewhat more recently in repair and calibration of test equipment for utility companies and of course now in my computer shop. Switch pitting and failure is not at all uncommon. Depending on the current and voltage involved the slightest corrosion (impurity) on the contacts can cause the electrons to arc or jump across the plates for a split second as the contacts are moved in close proximity to one another - impatient little things. This causes a small pit in the plates which will become larger with more arcing as time goes along eventually causing the switch to fail. If you look at a typical wall switch you will sometimes see a small blue spark inside when you throw it which is caused by this arcing or look at spark plugs. Good quality switches will generally hold up longer and the better ones have protection (capacitors are often used for decoupling because they soften the voltage "push") but they all will eventually fail - especially on AC circuits (DC has more of a tendency to fuse contacts but in WW is generally going to be very low current so this won't pose a problem). Contacts are often able to be cleaned basically by sanding the plates and using a solvent. This will level out the pits and clean the carbon residue but this still leaves abrasions and is at best only a temporary fix.
My suggestion to remedy the failed switch short of replacing it would be to solder jumpers across the contacts on the switch thereby electrically removing it from the circuit and use some sort of an external control. Pitted switches do not conduct the current very well and that will deteriorate in time because they are not making solid contact. I don't want any sparks inside tools if I can avoid it and I also want the tool to provide full power.
Be safe,Ken
>If you look at a typical wall switch you will sometimes see a small blue spark inside when you throw it which is caused by this arcing<
Nothing like a twelve inch bolt of "blue lightning" in the cockpit while you are flying. Have seen switches or other parts of avionics junk fail and this happen a couple times. Course I used to drink coffee and eat fried chicken while I was flying so my maintainers would sometimes tell me that I was "part of the problem."
Hi Ed,
Nothing like a bit of turbulence and you have FOD IN the cockpit! :+) My favorites though were the officer fly boys in the USN who would complain on a VIDSMAF that the IFF system didn't work in the OFF position. hehehe ...
Be safe,Ken
I don't know how many hundreds of VIDS/MAFS I saw that were signed off as "Corrective Action: Educated the pilot on proper function/ operation of the system." This is why senior pilots get their junior copilots to write any avionics gripes.
Hope the hand is healing well. Good day. Ed
Ben....
seems to me your switches are having to deal with more current than they're designed for.... pitting is caused when the breaking of the circuit creates a momentary electrical arc..... the more current, the more damaging the arc..
3 probable causes...
1/ power cords don't have a high enough cross sectional area or are excessively long
2/ worn motor brushes
3/ (less likely) worn commutator
To prevent the damage re-occurring to your table saw and router table, I'd be inclined to add a no-volt release switch that's rated for higher current applications.
Mike Wallace
Stay safe....Have fun
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