Electrical failure/switch failure

[jcoloccia]

The type of load just has to do mostly with how the switch is pounded on when opening or closing the connection. An inductive load, for example, doesn't do didly to the switch when you close it, but when you open the circuit it'll try it's best to maintain an arc across the contacts. For this, you'd want a very fast acting switch with a large airgap between contacts.

So just to take this one example, you could have a switch rated at 20VDC for a resistave load but in reality could slowly kill itself switching a 5 or 10 volt inductive load. So basically, the heavier the contacts are, the more current the switch can handle. The quicker the contacts move and the larger the air gap, the more inductive load it can SWITCH.

The "lamp" rating just has to do with how it's rated when you're switching a lamp. Tungsten has it's own unique characteristics as it heats up. The current is initially extremely high but very quickly drops as it heats.

Anyhow, I wouldn't solder the switch rivets. If the rivets are coming loose, either the switches are defective or you're killing them with very high currents/voltages causing the plastic case to deform, or something like that. All soldering could do is hide the problem.

Personally, I'd sit back, sip a beer and scratch my head for 20 minutes wondering why my electrical system keeps killing my switches. Most likely, the switches are just defective from a bad batch (loose terminals leading to high temperatures and arcing from what you've said) but I'd go through the exercise all the same just to convince myself I hadn't done anything dumb during a moment of "just being human"

just my $.02.

[lucky333]

Originally Posted by jcoloccia

The type of load just has to do mostly with how the switch is pounded on when opening or closing the connection. An inductive load, for example, doesn't do didly to the switch when you close it, but when you open the circuit it'll try it's best to maintain an arc across the contacts. For this, you'd want a very fast acting switch with a large airgap between contacts.
..just my $.02.

John is right, even the lowly mechanical switch needs to be correctly specified for the application (voltage, current, type of load, environmental conditions, sealing, vibration.. and so on). Each of these factors will affect the design, construction and materials used in the switch. Spring tension, contact size, shape, material, hardness, plating and wiping action are but a few of the factors that go into the design of a switch. Some, like the chart in the aeroelectric link (a Microswitch TP series) are quite flexible. Other switches are specified only for DC (AT series) and others for AC only (TS series). When you go through a catalog, you'll see pages and pages of identical looking switches with different part numbers. There is a reason for that. Different loads present vastly different conditions to the contacts, closing AND opening, and the switch manufacturer will take the indended load and power supply into account when designing the switch.

Will an AC specified switch work reliably on DC? The answer is 'maybe'. In the FAQ section on their site, Carlingswitch says as a 'rule of thumb' you can use an AC rated switch in DC applications up to 30V. They should know. But they also have switches that are only specified only for one or the other. Does the same rule apply? Don't know. One series of 125VAC toggle switches may work just fine at 12VDC, even if its not specified. The problems can start when the manufacturer changes the design of the switch, has it made in another factory (adios Mexico, hello China) or it goes obsolete and the supplier replaces it with another 125VAC unit thats 'just the same' except not with DC loads but thats 'OK because its not specified for DC'.

For my money, I want to know that the manufacturer of the switch agrees with what I want to use it for. The only way to be sure is to consult the datasheet for the exact switch and make sure that its specified and rated for the load you are switching. That doesn't mean that you need 30 different switches, there are lots that handle a wide range of load types and voltages. If you are not sure, a short email to an applications engineer at the mfr is usually all it takes to clear things up.

From all of this, you can probably guess that I would also recommend buying the highest quality switches you can afford and watch out for the overseas stuff. The Japanese (Omron, Aromat et al) generally make great stuff even if it not made right in Japan. Chinese stuff is getting hard to avoid but at least try to stick with a known brand. Avoid no-name 'will-fits' at all costs.

As for the loose rivets, John is right again. Soldering is out. The switch was not designed to have loose rivets so if they do get loose, you have an application or quality problem. Maybe vibration or just a crappy switch. Soldering not only masks the problem but can introduce brand new ones from contact misalignment or flux wicking up into the switch.

Here's a tech blurb from Omron. Its about their snap switches but the same principles apply. Of particular note is the variety of contact configurations, gaps, materials etc and the DC switching performance (from Excellent to Inferior, in their words) for the different part numbers. All in a series of switches that are identical in external appearance. Different load types are also described.

Switch Info

OTTO T9 Toggles are fully specified (and expensive). Good tech info on their site.

McGill aint bad either - Note AC/DC specs.

Hope some of this helps.
John

[vlittle]

I'm an EE, but switch engineering is part mechanical design, part electrical.

The reason you can't translate AC ratings to DC is that AC currents all pass through zero at 120 times per second. Any arc across the contacts caused by switching loads off is thus automatically quenced.

If, however, you try to switch the same voltage, but the current is DC, then it's possible that the arc persists long enough to weld the contacts together.

At low voltages, however, the arcs have much less energy and will quench on their own... hence the rule of thumb that the 125VAC current rating is OK for 12VDC.

Switching inductive loads of any kind can really mess up a switch... hence the snubber diodes recommended on relay coils.

The load in question (strobe power supply) is not an inductive load, so it should not have damaged the switch unless it was already defective (loose tabs).

Any way, the problem is fixed and the strobes are running.

V