DC vs AC Rated Switches

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J.L. Frusha

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Article on using diodes to protect switches...
 

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Dan Thomas

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The better way to minimize arcing on DC contacts is with a reverse-biased diode, just as you see across the coil of a relay, but the reason you see them on relay coils but not on most other applications is to protect the switch from the release of stored energy in the coil when power is removed.
That's part of it, but the coil in the relay or contactor generates a large voltage spike when its current is cut off and the field collapses. That spike can jump the relay's contacts before they open and enter the airplane's system and fry sensitive electronics that might be switched on. Newer boxes have suppression stuff in them but some older stuff didn't, and some aircraft have no diode on the contactor coil at all. Encoders are a particular problem with some airplanes that have no avionics bus switch. Can't turn them off before shutting off the master.

I used my 'scope once and measured a 600-volt spike off the coil in a 12-volt circuit.

There's an AD against ACS ignition switches. It demands inspection and lubrication of the switch's contacts every 2000 hours, and a suppression diode across the starter contactor. The spike burns the switch, and debris from the burning can interfere with the mag switching contacts.
 

Dan Thomas

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Welded switch contacts are only one risk, probably a minor one. Arc-burned contacts that give up at a critical time is the other and is more likely.
 

wsimpso1

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Article about AC switches.

What year was that article? Big fractions using AC only rated switches? I do not get that one. Aircraft Spruce and Wicks and B&C are our main suppliers, and all of the switches I found on their sites have ratings for AC and for DC circuits. Maybe there are some folks using hardware store stuff intended purely for AC, but our homebuilder suppliers seem to be supplying suitable stuff.

All that talk about 125VDC and above is specious at best. We usually run 14VDC and some run 28 VDC systems in our little airplanes. Of what pertinence is the 125VDC capacity to us?

In switches, you gotta know what the load is that is being switched. Volts, DC or AC, current, and if the load is inductive or not. Bob Nuckolls talks about ratings and reading the specs. Use the wrong switches and have trouble.

Billski
 
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wsimpso1

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So, why aren't folks using capacitors across the poles, like on a points/plug/condenser ignition, to prevent arcing across the contacts?

..., but..., wtf do I know?

Where an inductive circuit is being switched, well, a switch intended for inductive circuits is appropriate. They are out there. The biggest switched inductive loads in our airplanes are the contactors being used for Masters, Bus-Ties/Electric Cross Feeds, and starter circuits. As sold by B&C and by some others, they include a diode or two. If your source does not include a diode, all of the AeroElectric Connection circuits show the diode and spec number as well as polarity... Good stuff.

I suppose designing in a protective device in some of our inductive circuits might be useful. Besides the contactors, the circuits feeding the fuel pumps injectors, and ignition coils might be able to use some help of this type where they are switched circuits. On SDS, the pumps are powered through switches, but each of the injectors and coils are usually hooked to the bus through a fuse or breaker, and then they are connected and disconnected to ground inside the ECU boxes. The rest of my switches are just providing a signal voltage to a board, and the main circuitry is made or broken there.

Again, be educated on what your system requires and build accordingly.

Billski
 

Dan Thomas

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A capacitor across a switch can add further troubles. It is charged when the switch is open, and discharges through the switch when it closes. That adds amperage to the switch contacts right during any inrush caused by inductive or incandescent loads.

Across a coil a capacitor delays opening and closing of the contacts.
 

TiPi

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The key difference in breaking an AC circuit vs breaking a DC circuit is as follows:
The AC current goes through 0 (zero) amps twice per cycle. This is an enormous help in breaking the arc that burns between the opening contact surfaces. As mentioned already, fast-action switch contact opening is another requirement.
DC on the other hand is a continuous current that will form an arc until the ionised space between the 2 contact surfaces is wide enough for the arc to break. This will depend on current, voltage and contact shape. While there is an arc buring, this has the same effect as used for arc welding, just at a lower level.

Most quality switches will have an AC rating and a DC rating to account for the different arc breaking capabilities.

I have quite some experience in solar design and installation from the the early days (90s), when there were very few DC-rated switches available. Some installers used 240V AC-rated switches in 24 to 48V DC systems and some would not break the arc until the switch was a blobbering mess in the switchboard.
One way around that was to put 2 switches in series, operated simultaneously. That doubled the arc breaking capacity to break the DC arc.
 

AdrianS

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The key difference in breaking an AC circuit vs breaking a DC circuit is as follows:
The AC current goes through 0 (zero) amps twice per cycle. This is an enormous help in breaking the arc that burns between the opening contact surfaces. As mentioned already, fast-action switch contact opening is another requirement.
DC on the other hand is a continuous current that will form an arc until the ionised space between the 2 contact surfaces is wide enough for the arc to break. This will depend on current, voltage and contact shape. While there is an arc buring, this has the same effect as used for arc welding, just at a lower level.

Most quality switches will have an AC rating and a DC rating to account for the different arc breaking capabilities.

I have quite some experience in solar design and installation from the the early days (90s), when there were very few DC-rated switches available. Some installers used 240V AC-rated switches in 24 to 48V DC systems and some would not break the arc until the switch was a blobbering mess in the switchboard.
One way around that was to put 2 switches in series, operated simultaneously. That doubled the arc breaking capacity to break the DC arc.

I was involved in solar powered vehicles in the late 80's / early 90's.

We ended up designing and making our own master switches, as interrupting a high-current, high-voltage DC circuit is a nontrivial exercise.

The final design had a spring-loaded contactor: the switch pulled a pin which allowed the spring to separate the contacts (at speed) to a gap big enough to extinguish the arc.
Think old-fashioned mousetrap.

But at 12 or 24 volts, commodity switches should be fine. Just check the DC current rating.
 

trimtab

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So, why aren't folks using capacitors across the poles, like on a points/plug/condenser ignition, to prevent arcing across the contacts?

..., but..., wtf do I know?
DC rated switches *very* commonly use inductor+diode flyback circuits to increase contact life dramatically.

Using a cap is largely ineffective, bulky, expensive, and prone to failure. Nobody does that.

This is what most quality relays and switches do:
 

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AdrianS

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DC rated switches *very* commonly use inductor+diode flyback circuits to increase contact life dramatically.

Using a cap is largely ineffective, bulky, expensive, and prone to failure. Nobody does that.

This is what most quality relays and switches do:
Diodes do nothing to help a switch break a DC current. They are vital for inductive loads like relays, but no use for resistive loads.
 

Tom H

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One way around that was to put 2 switches in series, operated simultaneously. That doubled the arc breaking capacity to break the DC arc.
[/QUOTE]

Another method that I have seen is the DC rated contactor has a built-in permanent magnet that pulls/lengthens the arc that is formed when the switch is opened. Don't know if this method is used in smaller toggle type switches.
 
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What spurred the article and its preceding debate was homebuilders using "designer" rocker switches, etc. to dress up their panels that ceratinly looked cool on the outside, but ran wicked hot on the inside because their guts weren't robust enough for some 12VDC environments.
99% of the applications probably will experience no problems, but which application is that 1%? If you haven't done your homework and run the numbers to determine the safety margins of all your various applications, stick with DC rated switches in your homebuilts: there are lots of cool looking ones out there now to dress up your panel with.
 

BJC

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The final design had a spring-loaded contactor: the switch pulled a pin which allowed the spring to separate the contacts (at speed) to a gap big enough to extinguish the arc.
Think old-fashioned mousetrap.
High voltage high current AC breakers in a generation substation bus typically open fully in a couple of cycles (making a really loud noise) and blast the arc with sulfur hexafluoride to extinguish the arc. I don’t recall the exact time, but I do know that if they didn’t clear within 5 cycles, a breaker failure scheme (backup protection) would automatically open the next breakers in the bus.

Contrary to popular opinion, they need a power supply, typically 130 VDC, to operate.


BJC
 

trimtab

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Diodes do nothing to help a switch break a DC current. They are vital for inductive loads like relays, but no use for resistive loads.
A lot of loads in aircraft are inductive: the master contactor and other contactors, for example. Electric gyros.

The surge is pretty significant.

I was tracing down an issue in my master, for example. I scoped >70 A for several milliseconds. The first current probe was saturated. I replaced it temporarily with a snubbed version, and the current surge dropped to <7A. A replacement is priced exceptionally, and so I'm keeping the original of course, and replaced the master switch, since it lasted roughly 60 years, also at a jaw-dropping expense.

The difference in surge was that the control solenoid was snubbed in the temporary I used.
 
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