Electrical Component Question

[Cajunman06]

I was wondering around the hardware store the other day, daydreaming, and started looking through the switches and such. Now I have a question:

Is there a difference in, for example, a rocker switch or momentary switch that only states VAC specs as opposed to one that lists DC specs? What I mean is can I use one that is rated for VAC on a DC circuit?

C

 

[sandman]

Not really!

DC and AC work differently.

With AC the voltage actually goes to 0 volts 60 times a sec so there is minimal arcing inside the switch.

DC has a constant voltage so you have a nice healthy arc every time you use a DC switch.

There was an interesting article on DC switches years ago in AOPA magazine (Aircraft Owners and Pilots Assoc.) The big problem with some aircraft is the fact that they use 24 volt DC systems and you have a large number of switch failures.

Think of it this way...there are very few AC arc welders on the market!

For the best reliability on high DC voltages you really need to use a relay...and replace the relay occasionally when the contact points get iffy!

sandman

 

[davel]

Typically, the DC voltage rating of a switch will be 1/4 of the VAC rating. The concern is that an arc will sustain itself within the switch. On AC, the voltage passes through zero 120 times a second and any arc will quench as the voltage falls through zero. That doesn't happen for DC, and therefore a switch with a given contact spacing will have a lower DC rating than AC.

 

[Cajunman06]

OK, so bottom line is I should NOT use switches that don't list DC ratings on DC circuits, correct?

C

 

[Micromeister]

Just about every device manufactured has a rating for both. some of the really cheap-o imports may not but even then if you look at the produce as having about 1/4 the AC rating there fine, I'd still look for a DC rated device if you have the choice. Like Sandman said the make/break arc's are a lot hotter in DC circuits.

 

[mge55389]

I'm not sure exactly what type of project your working on; but I'm guessing it's a launch controller. The best type of switch to use should have a DC rating. If you are using say a toggle light switch for a pad selection switch you will be okay. All switches have a current rating on them. Most generic residental light switches have a 15A rating. Since an Estes igniters and copperheads draw 3 Amps or less you should be fine. I would go to the automotive section for a Normally Open momentary pushbutton with a DC rating for the launch button. The launch button is the only switch drawing an arc when pressed and released. Sure you get a few milliamps from your continuity light when you select your pad, but that won't do much damage. The pad selector switch acts the same as a safety disconnect switch for a large motor. They are not designed to handle the heat created by the arc caused when opening the switch under load, but handle the running current just fine. I built a 4 pad launcher with toggle switches back in high school 12+ yrs ago and it still works great!

 

[UhClem]

Copperheads and Estes solar ignitors draw less than 3 Amps?

Not in a 12 volt system they don't. Both have resistances of less than 1 ohm. I = E/R.


As for the switches that are not switched under load not needing a high rating, I can show you a pair of 3 Amp (@125VAC) rated switches that were never switched under load. They sacrificed themselves to protect the ignitor.

 

[mge55389]

Well I guess you can't believe everything that you read. I dug out some igniters and my Fluke 29 multimeter. Estes igniters :around 1 ohm. Copperhead igniters : checked 3 = all about 2.4 ohms

Estes
I= E/R I = 12V / 1 ohm I = 12 Amps

Copperhead
I= 12V / 2.4 ohms I = 5 amps

Another factor to consider is the resistance of the wire used for your leads. This will affect the total resistance on long runs.

As for your switch failure, it was more than likely happened because you used a 3 Amp switch. Those switches are only good for low current control applications. You normally can NEVER use an electrical component at 100% of its rating. In my example I was switching what I thought was 3 amps ( I guess it was about 12 amps) with a 15 amp switch. This was at 80% of its rating. No problem. It is not uncommon to design an AC circuit up to 80% of its rating as this is permitted by the NEC.
20 Amp and 30 Amp toggle switches are also easliy available at any Home Depot, Menards, etc.